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Zhang 4 Intended status: Standards Track J. Drake 5 Expires: January 17, 2019 E. Rosen, Ed. 6 Juniper Networks, Inc. 7 J. Rabadan 8 Nokia 9 A. Sajassi 10 Cisco Systems 11 July 16, 2018 13 EVPN Optimized Inter-Subnet Multicast (OISM) Forwarding 14 draft-ietf-bess-evpn-irb-mcast-01 16 Abstract 18 Ethernet VPN (EVPN) provides a service that allows a single Local 19 Area Network (LAN), comprising a single IP subnet, to be divided into 20 multiple "segments". Each segment may be located at a different 21 site, and the segments are interconnected by an IP or MPLS backbone. 22 Intra-subnet traffic (either unicast or multicast) always appears to 23 the endusers to be bridged, even when it is actually carried over the 24 IP or MPLS backbone. When a single "tenant" owns multiple such LANs, 25 EVPN also allows IP unicast traffic to be routed between those LANs. 26 This document specifies new procedures that allow inter-subnet IP 27 multicast traffic to be routed among the LANs of a given tenant, 28 while still making intra-subnet IP multicast traffic appear to be 29 bridged. These procedures can provide optimal routing of the inter- 30 subnet multicast traffic, and do not require any such traffic to 31 leave a given router and then reenter that same router. These 32 procedures also accommodate IP multicast traffic that needs to travel 33 to or from systems that are outside the EVPN domain. 35 Status of This Memo 37 This Internet-Draft is submitted in full conformance with the 38 provisions of BCP 78 and BCP 79. 40 Internet-Drafts are working documents of the Internet Engineering 41 Task Force (IETF). Note that other groups may also distribute 42 working documents as Internet-Drafts. The list of current Internet- 43 Drafts is at https://datatracker.ietf.org/drafts/current/. 45 Internet-Drafts are draft documents valid for a maximum of six months 46 and may be updated, replaced, or obsoleted by other documents at any 47 time. It is inappropriate to use Internet-Drafts as reference 48 material or to cite them other than as "work in progress." 49 This Internet-Draft will expire on January 17, 2019. 51 Copyright Notice 53 Copyright (c) 2018 IETF Trust and the persons identified as the 54 document authors. All rights reserved. 56 This document is subject to BCP 78 and the IETF Trust's Legal 57 Provisions Relating to IETF Documents 58 (https://trustee.ietf.org/license-info) in effect on the date of 59 publication of this document. Please review these documents 60 carefully, as they describe your rights and restrictions with respect 61 to this document. Code Components extracted from this document must 62 include Simplified BSD License text as described in Section 4.e of 63 the Trust Legal Provisions and are provided without warranty as 64 described in the Simplified BSD License. 66 Table of Contents 68 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 4 69 1.1. Background . . . . . . . . . . . . . . . . . . . . . . . 4 70 1.1.1. Segments, Broadcast Domains, and Tenants . . . . . . 4 71 1.1.2. Inter-BD (Inter-Subnet) IP Traffic . . . . . . . . . 5 72 1.1.3. EVPN and IP Multicast . . . . . . . . . . . . . . . . 6 73 1.1.4. BDs, MAC-VRFS, and EVPN Service Models . . . . . . . 7 74 1.2. Need for EVPN-aware Multicast Procedures . . . . . . . . 7 75 1.3. Additional Requirements That Must be Met by the Solution 8 76 1.4. Terminology . . . . . . . . . . . . . . . . . . . . . . . 10 77 1.5. Model of Operation: Overview . . . . . . . . . . . . . . 12 78 1.5.1. Control Plane . . . . . . . . . . . . . . . . . . . . 12 79 1.5.2. Data Plane . . . . . . . . . . . . . . . . . . . . . 14 80 2. Detailed Model of Operation . . . . . . . . . . . . . . . . . 17 81 2.1. Supplementary Broadcast Domain . . . . . . . . . . . . . 17 82 2.2. Detecting When a Route is About/For/From a Particular BD 18 83 2.3. Use of IRB Interfaces at Ingress PE . . . . . . . . . . . 21 84 2.4. Use of IRB Interfaces at an Egress PE . . . . . . . . . . 23 85 2.5. Announcing Interest in (S,G) . . . . . . . . . . . . . . 23 86 2.6. Tunneling Frames from Ingress PE to Egress PEs . . . . . 24 87 2.7. Advanced Scenarios . . . . . . . . . . . . . . . . . . . 25 88 3. EVPN-aware Multicast Solution Control Plane . . . . . . . . . 26 89 3.1. Supplementary Broadcast Domain (SBD) and Route Targets . 26 90 3.2. Advertising the Tunnels Used for IP Multicast . . . . . . 27 91 3.2.1. Constructing Routes for the SBD . . . . . . . . . . . 27 92 3.2.2. Ingress Replication . . . . . . . . . . . . . . . . . 28 93 3.2.3. Assisted Replication . . . . . . . . . . . . . . . . 29 94 3.2.4. BIER . . . . . . . . . . . . . . . . . . . . . . . . 29 95 3.2.5. Inclusive P2MP Tunnels . . . . . . . . . . . . . . . 31 96 3.2.5.1. Using the BUM Tunnels as IP Multicast Inclusive 97 Tunnels . . . . . . . . . . . . . . . . . . . . . 31 98 3.2.5.2. Using Wildcard S-PMSI A-D Routes to Advertise 99 Inclusive Tunnels Specific to IP Multicast . . . 32 100 3.2.6. Selective Tunnels . . . . . . . . . . . . . . . . . . 33 101 3.3. Advertising SMET Routes . . . . . . . . . . . . . . . . . 34 102 4. Constructing Multicast Forwarding State . . . . . . . . . . . 36 103 4.1. Layer 2 Multicast State . . . . . . . . . . . . . . . . . 36 104 4.1.1. Constructing the OIF List . . . . . . . . . . . . . . 37 105 4.1.2. Data Plane: Applying the OIF List to an (S,G) Frame . 38 106 4.1.2.1. Eligibility of an AC to Receive a Frame . . . . . 38 107 4.1.2.2. Applying the OIF List . . . . . . . . . . . . . . 38 108 4.2. Layer 3 Forwarding State . . . . . . . . . . . . . . . . 40 109 5. Interworking with non-OISM EVPN-PEs . . . . . . . . . . . . . 41 110 5.1. IPMG Designated Forwarder . . . . . . . . . . . . . . . . 43 111 5.2. Ingress Replication . . . . . . . . . . . . . . . . . . . 44 112 5.2.1. Ingress PE is non-OISM . . . . . . . . . . . . . . . 45 113 5.2.2. Ingress PE is OISM . . . . . . . . . . . . . . . . . 47 114 5.3. P2MP Tunnels . . . . . . . . . . . . . . . . . . . . . . 48 115 6. Traffic to/from Outside the EVPN Tenant Domain . . . . . . . 48 116 6.1. Layer 3 Interworking via EVPN OISM PEs . . . . . . . . . 48 117 6.1.1. General Principles . . . . . . . . . . . . . . . . . 48 118 6.1.2. Interworking with MVPN . . . . . . . . . . . . . . . 51 119 6.1.2.1. MVPN Sources with EVPN Receivers . . . . . . . . 53 120 6.1.2.1.1. Identifying MVPN Sources . . . . . . . . . . 53 121 6.1.2.1.2. Joining a Flow from an MVPN Source . . . . . 54 122 6.1.2.2. EVPN Sources with MVPN Receivers . . . . . . . . 56 123 6.1.2.2.1. General procedures . . . . . . . . . . . . . 56 124 6.1.2.2.2. Any-Source Multicast (ASM) Groups . . . . . . 57 125 6.1.2.2.3. Source on Multihomed Segment . . . . . . . . 58 126 6.1.2.3. Obtaining Optimal Routing of Traffic Between MVPN 127 and EVPN . . . . . . . . . . . . . . . . . . . . 59 128 6.1.2.4. Selecting the MEG SBD-DR . . . . . . . . . . . . 59 129 6.1.3. Interworking with 'Global Table Multicast' . . . . . 60 130 6.1.4. Interworking with PIM . . . . . . . . . . . . . . . . 60 131 6.1.4.1. Source Inside EVPN Domain . . . . . . . . . . . . 61 132 6.1.4.2. Source Outside EVPN Domain . . . . . . . . . . . 62 133 6.2. Interworking with PIM via an External PIM Router . . . . 63 134 7. Using an EVPN Tenant Domain as an Intermediate (Transit) 135 Network for Multicast traffic . . . . . . . . . . . . . . . . 64 136 8. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 66 137 9. Security Considerations . . . . . . . . . . . . . . . . . . . 67 138 10. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 67 139 11. References . . . . . . . . . . . . . . . . . . . . . . . . . 67 140 11.1. Normative References . . . . . . . . . . . . . . . . . . 67 141 11.2. Informative References . . . . . . . . . . . . . . . . . 69 142 Appendix A. Integrated Routing and Bridging . . . . . . . . . . 71 143 Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 76 145 1. Introduction 147 1.1. Background 149 Ethernet VPN (EVPN) [RFC7432] provides a Layer 2 VPN (L2VPN) 150 solution, which allows IP backbone provider to offer ethernet service 151 to a set of customers, known as "tenants". 153 In this section (as well as in [EVPN-IRB]), we provide some essential 154 background information on EVPN. 156 The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", 157 "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and 158 "OPTIONAL" in this document are to be interpreted as described in BCP 159 14 [RFC2119] [RFC8174] when, and only when, they appear in all 160 capitals, as shown here. 162 1.1.1. Segments, Broadcast Domains, and Tenants 164 One of the key concepts of EVPN is the Broadcast Domain (BD). A BD 165 is essentially an emulated ethernet. Each BD belongs to a single 166 tenant. A BD typically consists of multiple ethernet "segments", and 167 each segment may be attached to a different EVPN Provider Edge 168 (EVPN-PE) router. EVPN-PE routers are often referred to as "Network 169 Virtualization Endpoints" or NVEs. However, this document will use 170 the term "EVPN-PE", or, when the context is clear, just "PE". 172 In this document, we use the term "segment" to mean the same as 173 "Ethernet Segment" or "ES" in [RFC7432]. 175 Attached to each segment are "Tenant Systems" (TSes). A TS may be 176 any type of system, physical or virtual, host or router, etc., that 177 can attach to an ethernet. 179 When two TSes are on the same segment, traffic between them does not 180 pass through an EVPN-PE. When two TSes are on different segments of 181 the same BD, traffic between them does pass through an EVPN-PE. 183 When two TSes, say TS1 and TS2 are on the same BD, then: 185 o If TS1 knows the MAC address of TS2, TS1 can send unicast ethernet 186 frames to TS2. TS2 will receive the frames unaltered. 188 o If TS1 broadcasts an ethernet frame, TS2 will receive the 189 unaltered frame. 191 o If TS1 multicasts an ethernet frame, TS2 will receive the 192 unaltered frame, as long as TS2 has been provisioned to receive 193 ethernet multicasts. 195 When we say that TS2 receives an unaltered frame from TS1, we mean 196 that the frame still contains TS1's MAC address, and that no 197 alteration of the frame's payload (and consequently, no alteration of 198 the payload's IP header) has been made. 200 EVPN allows a single segment to be attached to multiple PE routers. 201 This is known as "EVPN multi-homing". Suppose a given segment is 202 attached to both PE1 and PE2, and suppose PE1 receives a frame from 203 that segment. It may be necessary for PE1 to send the frame over the 204 backbone to PE2. EVPN has procedures to ensure that such a frame 205 cannot be sent by PE2 back to its originating segment. This is 206 particularly important for multicast, because a frame arriving at PE1 207 from a given segment will already have been seen by all the systems 208 on that segment that need to see it. If the frame were sent back to 209 the originating segment by PE2, receivers on that segment would 210 receive the packet twice. Even worse, the frame might be sent back 211 to PE1, which could cause an infinite loop. 213 1.1.2. Inter-BD (Inter-Subnet) IP Traffic 215 If a given tenant has multiple BDs, the tenant may wish to allow IP 216 communication among these BDs. Such a set of BDs is known as an 217 "EVPN Tenant Domain" or just a "Tenant Domain". 219 If tenant systems TS1 and TS2 are not in the same BD, then they do 220 not receive unaltered ethernet frames from each other. In order for 221 TS1 to send traffic to TS2, TS1 encapsulates an IP datagram inside an 222 ethernet frame, and uses ethernet to send these frames to an IP 223 router. The router decapsulates the IP datagram, does the IP 224 processing, and re-encapsulates the datagram for ethernet. The MAC 225 source address field now has the MAC address of the router, not of 226 TS1. The TTL field of the IP datagram should be decremented by 227 exactly 1, even if the frame needs to be sent from one PE to another. 228 The structure of the provider's IP backbone is thus hidden from the 229 tenants. 231 EVPN accommodates the need for inter-BD communication within a Tenant 232 Domain by providing an integrated L2/L3 service for unicast IP 233 traffic. EVPN's Integrated Routing and Bridging (IRB) functionality 234 is specified in [EVPN-IRB]. Each BD in a Tenant Domain is assumed to 235 be a single IP subnet, and each IP subnet within a a given Tenant 236 Domain is assumed to be a single BD. EVPN's IRB functionality allows 237 IP traffic to travel from one BD to another, and ensures that proper 238 IP processing (e.g., TTL decrement) is done. 240 A brief overview of IRB, including the notion of an "IRB interface", 241 can be found in Appendix A. As explained there, an IRB interface is 242 a sort of virtual interface connecting an L3 routing instance to a 243 BD. A BD may have multiple attachment circuits (ACs) to a given PE, 244 where each AC connects to a different ethernet segment of the BD. 245 However, these ACs are not visible to the L3 routing function; from 246 the perspective of an L3 routing instance, a PE has just one 247 interface to each BD, viz., the IRB interface for that BD. 249 The "L3 routing instance" depicted in Appendix A is associated with a 250 single Tenant Domain, and may be thought of as an IP-VRF for that 251 Tenant Domain. 253 1.1.3. EVPN and IP Multicast 255 [EVPN-IRB] and [EVPN_IP_Prefix] cover inter-subnet (inter-BD) IP 256 unicast forwarding, but they do not cover inter-subnet IP multicast 257 forwarding. 259 [RFC7432] covers intra-subnet (intra-BD) ethernet multicast. The 260 intra-subnet ethernet multicast procedures of [RFC7432] are used for 261 ethernet Broadcast traffic, for ethernet unicast traffic whose MAC 262 Destination Address field contains an Unknown address, and for 263 ethernet traffic whose MAC Destination Address field contains an 264 ethernet Multicast MAC address. These three classes of traffic are 265 known collectively as "BUM traffic" (Broadcast/Unknown-Unicast/ 266 Multicast), and the procedures for handling BUM traffic are known as 267 "BUM procedures". 269 [IGMP-Proxy] extends the intra-subnet ethernet multicast procedures 270 by adding procedures that are specific to, and optimized for, the use 271 of IP multicast within a subnet. However,that document does not 272 cover inter-subnet IP multicast. 274 The purpose of this document is to specify procedures for EVPN that 275 provide optimized IP multicast functionality within an EVPN tenant 276 domain. This document also specifies procedures that allow IP 277 multicast packets to be sourced from or destined to systems outside 278 the Tenant Domain. We refer to the entire set of these procedures as 279 "OISM" (Optimized Inter-Subnet Multicast) procedures. 281 In order to support the OISM procedures specified in this document, 282 an EVPN-PE MUST also support [EVPN-IRB] and [IGMP-Proxy]. (However, 283 certain of the procedures in [IGMP-Proxy] are modified when OISM is 284 supported.) 286 1.1.4. BDs, MAC-VRFS, and EVPN Service Models 288 [RFC7432] defines the notion of "MAC-VRF". A MAC-VRF contains one or 289 more "Bridge Tables" (see section 3 of [RFC7432] for a discussion of 290 this terminology), each of which represents a single Broadcast 291 Domain. 293 In the IRB model (outlined in Appendix A) a L3 routing instance has 294 one IRB interface per BD, NOT one per MAC-VRF. This document does 295 not distinguish between a "Broadcast Domain" and a "Bridge Table", 296 and will use the terms interchangeably (or will use the acronym "BD" 297 to refer to either). The way the BDs are grouped into MAC-VRFs is 298 not relevant to the procedures specified in this document. 300 Section 6 of [RFC7432] also defines several different EVPN service 301 models: 303 o In the "vlan-based service", each MAC-VRF contains one "bridge 304 table", where the bridge table corresponds to a particular Virtual 305 LAN (VLAN). (See section 3 of [RFC7432] for a discussion of this 306 terminology.) Thus each VLAN is treated as a BD. 308 o In the "vlan bundle service", each MAC-VRF contains one bridge 309 table, where the bridge table corresponds to a set of VLANs. Thus 310 a set of VLANs are treated as constituting a single BD. 312 o In the "vlan-aware bundle service", each MAC-VRF may contain 313 multiple bridge tables, where each bridge table corresponds to one 314 BD. If a MAC-VRF contains several bridge tables, then it 315 corresponds to several BDs. 317 The procedures of this document are intended to work for all these 318 service models. 320 1.2. Need for EVPN-aware Multicast Procedures 322 Inter-subnet IP multicast among a set of BDs can be achieved, in a 323 non-optimal manner, without any specific EVPN procedures. For 324 instance, if a particular tenant has n BDs among which he wants to 325 send IP multicast traffic, he can simply attach a conventional 326 multicast router to all n BDs. Or more generally, as long as each BD 327 has at least one IP multicast router, and the IP multicast routers 328 communicate multicast control information with each other, 329 conventional IP multicast procedures will work normally, and no 330 special EVPN functionality is needed. 332 However, that technique does not provide optimal routing for 333 multicast. In conventional multicast routing, for a given multicast 334 flow, there is only one multicast router on each BD that is permitted 335 to send traffic of that flow to the BD. If that BD has receivers for 336 a given flow, but the source of the flow is not on that BD, then the 337 flow must pass through that multicast router. This leads to the 338 "hair-pinning" problem described (for unicast) in Appendix A. 340 For example, consider an (S,G) flow that is sourced by a TS S and 341 needs to be received by TSes R1 and R2. Suppose S is on a segment of 342 BD1, R1 is on a segment of BD2, but both are attached to PE1. 343 Suppose also that the tenant has a multicast router, attached to a 344 segment of BD1 and to a segment of BD2. However, the segments to 345 which that router is attached are both attached to PE2. Then the 346 flow from S to R would have to follow the path: 347 S-->PE1-->PE2-->Tenant Multicast Router-->PE2-->PE1-->R1. Obviously, 348 the path S-->PE1-->R would be preferred. 350 Now suppose that there is a second receiver, R2. R2 is attached to a 351 third BD, BD3. However, it is attached to a segment of BD3 that is 352 attached to PE1. And suppose also that the Tenant Multicast Router 353 is attached to a segment of BD3 that attaches to PE2. In this case, 354 the Tenant Multicast Router will make two copies of the packet, one 355 for BD2 and one for BD3. PE2 will send both copies back to PE1. Not 356 only is the routing sub-optimal, but PE2 sends multiple copies of the 357 same packet to PE1. This is a further sub-optimality. 359 This is only an example; many more examples of sub-optimal multicast 360 routing can easily be given. To eliminate sub-optimal routing and 361 extra copies, it is necessary to have a multicast solution that is 362 EVPN-aware, and that can use its knowledge of the internal structure 363 of a Tenant Domain to ensure that multicast traffic gets routed 364 optimally. The procedures of this document allow us to avoid all 365 such sub-optimalities when routing inter-subnet multicasts within a 366 Tenant Domain. 368 1.3. Additional Requirements That Must be Met by the Solution 370 In addition to providing optimal routing of multicast flows within a 371 Tenant Domain, the EVPN-aware multicast solution is intended to 372 satisfy the following requirements: 374 o The solution must integrate well with the procedures specified in 375 [IGMP-Proxy]. That is, an integrated set of procedures must 376 handle both intra-subnet multicast and inter-subnet multicast. 378 o With regard to intra-subnet multicast, the solution MUST maintain 379 the integrity of multicast ethernet service. This means: 381 * If a source and a receiver are on the same subnet, the MAC 382 source address (SA) of the multicast frame sent by the source 383 will not get rewritten. 385 * If a source and a receiver are on the same subnet, no IP 386 processing of the ethernet payload is done. The IP TTL is not 387 decremented, the header checksum is not changed, no 388 fragmentation is done, etc. 390 o On the other hand, if a source and a receiver are on different 391 subnets, the frame received by the receiver will not have the MAC 392 Source address of the source, as the frame will appear to have 393 come from a multicast router. Also, proper processing of the IP 394 header is done, e.g., TTL decrement by 1, header checksum 395 modification, possibly fragmentation, etc. 397 o If a Tenant Domain contains several BDs, it MUST be possible for a 398 multicast flow (even when the multicast group address is an "any 399 source multicast" (ASM) address), to have sources in one of those 400 BDs and receivers in one or more of the other BDs, without 401 requiring the presence of any system performing PIM Rendezvous 402 Point (RP) functions ([RFC7761]). Multicast throughout a Tenant 403 Domain must not require the tenant systems to be aware of any 404 underlying multicast infrastructure. 406 o Sometimes a MAC address used by one TS on a particular BD is also 407 used by another TS on a different BD. Inter-subnet routing of 408 multicast traffic MUST NOT make any assumptions about the 409 uniqueness of a MAC address across several BDs. 411 o If two EVPN-PEs attached to the same Tenant Domain both support 412 the OISM procedures, each may receive inter-subnet multicasts from 413 the other, even if the egress PE is not attached to any segment of 414 the BD from which the multicast packets are being sourced. It 415 MUST NOT be necessary to provision the egress PE with knowledge of 416 the ingress BD. 418 o There must be a procedure that that allows EVPN-PE routers 419 supporting OISM procedures to send/receive multicast traffic to/ 420 from EVPN-PE routers that support only [RFC7432], but that do not 421 support the OISM procedures or even the procedures of [EVPN-IRB]. 422 However, when interworking with such routers (which we call 423 "non-OISM PE routers"), optimal routing may not be achievable. 425 o It MUST be possible to support scenarios in which multicast flows 426 with sources inside a Tenant Domain have "external" receivers, 427 i.e., receivers that are outside the domain. It must also be 428 possible to support scenarios where multicast flows with external 429 sources (sources outside the Tenant Domain) have receivers inside 430 the domain. 432 This presupposes that unicast routes to multicast sources outside 433 the domain can be distributed to EVPN-PEs attached to the domain, 434 and that unicast routes to multicast sources within the domain can 435 be distributed outside the domain. 437 Of particular importance are the scenario in which the external 438 sources and/or receivers are reachable via L3VPN/MVPN, and the 439 scenario in which external sources and/or receivers are reachable 440 via IP/PIM. 442 The solution for external interworking MUST allow for deployment 443 scenarios in which EVPN does not need to export a host route for 444 every multicast source. 446 o The solution for external interworking must not presuppose that 447 the same tunneling technology is used within both the EVPN domain 448 and the external domain. For example, MVPN interworking must be 449 possible when MVPN is using MPLS P2MP tunneling, and EVPN is using 450 Ingress Replication or VXLAN tunneling. 452 o The solution must not be overly dependent on the details of a 453 small set of use cases, but must be adaptable to new use cases as 454 they arise. (That is, the solution must be robust.) 456 1.4. Terminology 458 In this document we make frequent use of the following terminology: 460 o OISM: Optimized Inter-Subnet Multicast. EVPN-PEs that follow the 461 procedures of this document will be known as "OISM" PEs. EVPN-PEs 462 that do not follow the procedures of this document will be known 463 as "non-OISM" PEs. 465 o IP Multicast Packet: An IP packet whose IP Destination Address 466 field is a multicast address that is not a link-local address. 467 (Link-local addresses are IPv4 addresses in the 224/8 range and 468 IPv6 address in the FF02/16 range.) 470 o IP Multicast Frame: An ethernet frame whose payload is an IP 471 multicast packet (as defined above). 473 o (S,G) Multicast Packet: An IP multicast packet whose IP Source 474 Address field contains S and whose IP Destination Address field 475 contains G. 477 o (S,G) Multicast Frame: An IP multicast frame whose payload 478 contains S in its IP Source Address field and G in its IP 479 Destination Address field. 481 o Broadcast Domain (BD): an emulated ethernet, such that two systems 482 on the same BD will receive each other's link-local broadcasts. 484 Note that EVPN supports service models in which a single EVPN 485 Instance (EVI) contains only one BD, and service models in which a 486 single EVI contains multiple BDs. Both types of service model are 487 supported by this draft. In all models, a given BD belongs to 488 only one EVI. 490 o Designated Forwarder (DF). As defined in [RFC7432], an ethernet 491 segment may be multi-homed (attached to more than one PE). An 492 ethernet segment may also contain multiple BDs, of one or more 493 EVIs. For each such EVI, one of the PEs attached to the segment 494 becomes that EVI's DF for that segment. Since a BD may belong to 495 only one EVI, we can speak unambiguously of the BD's DF for a 496 given segment. 498 When the text makes it clear that we are speaking in the context 499 of a given BD, we will frequently use the term "a segment's DF" to 500 mean the given BD's DF for that segment. 502 o AC: Attachment Circuit. An AC connects the bridging function of 503 an EVPN-PE to an ethernet segment of a particular BD. ACs are not 504 visible at the router (L3) layer. 506 If a given ethernet segment, attached to a given PE, contains n 507 BDs, we will say that the PE has n ACs to that segment. 509 o L3 Gateway: An L3 Gateway is a PE that connects an EVPN tenant 510 domain to an external multicast domain by performing both the OISM 511 procedures and the Layer 3 multicast procedures of the external 512 domain. 514 o PEG (PIM/EVPN Gateway): A L3 Gateway that connects an EVPN Tenant 515 Domain to an external multicast domain whose Layer 3 multicast 516 procedures are those of PIM ([RFC7761]). 518 o MEG (MVPN/EVPN Gateway): A L3 Gateway that connects an EVPN Tenant 519 Domain to an external multicast domain whose Layer 3 multicast 520 procedures are those of MVPN ([RFC6513], [RFC6514]). 522 o IPMG (IP Multicast Gateway): A PE that is used for interworking 523 OISM EVPN-PEs with non-OISM EVPN-PEs. 525 o DR (Designated Router): A PE that has special responsibilities for 526 handling multicast on a given BD. 528 o FHR (First Hop Router): The FHR is a PIM router ([RFC7761]) with 529 special responsibilities. It is the first multicast router to see 530 (S,G) packets from source S, and if G is an "Any Source Multicast 531 (ASM)" group, the FHR is responsible for sending PIM Register 532 messages to the PIM Rendezvous Point for group G. 534 o LHR (Last Hop Router): The LHR is a PIM router ([RFC7761]) with 535 special responsibilities. Generally it is attached to a LAN, and 536 it determines whether there are any hosts on the LAN that need to 537 receive a given multicast flow. If so, it creates and sends the 538 PIM Join messages that are necessary to draw the flow. 540 o EC (Extended Community). A BGP Extended Communities attribute 541 ([RFC4360], [RFC7153]) is a BGP path attribute that consists of 542 one or more extended communities. 544 o RT (Route Target): A Route Target is a particular kind of BGP 545 Extended Community. A BGP Extended Community consists of a type 546 field, a sub-type field, and a value field. Certain type/sub-type 547 combinations indicate that a particular Extended Community is an 548 RT. RT1 and RT2 are considered to be the same RT if and only if 549 they have the same type, same sub-type, and same value fields. 551 o Use of the "C-" prefix. In many documents on VPN multicast, the 552 prefix "C-" appears before any address or wildcard that refers to 553 an address or addresses in a tenant's address space, rather than 554 to an address of addresses in the address space of the backbone 555 network. This document omits the "C-" prefix in many cases where 556 it is clear from the context that the reference is to the tenant's 557 address space. 559 This document also assumes familiarity with the terminology of 560 [RFC4364], [RFC6514], [RFC7432], [RFC7761], [IGMP-Proxy], 561 [EVPN_IP_Prefix] and [EVPN-BUM]. 563 1.5. Model of Operation: Overview 565 1.5.1. Control Plane 567 In this section, and in the remainder of this document, we assume the 568 reader is familiar with the procedures of IGMP/MLD (see [RFC2236] and 569 [RFC2710]), by which hosts announce their interest in receiving 570 particular multicast flows. 572 Consider a Tenant Domain consisting of a set of k BDs: BD1, ..., BDk. 573 To support the OISM procedures, each Tenant Domain must also be 574 associated with a "Supplementary Broadcast Domain" (SBD). An SBD is 575 treated in the control plane as a real BD, but it does not have any 576 ACs. The SBD has several uses; these will be described later in this 577 document (see Section 2.1 and Section 3). 579 Each PE that attaches to one or more of the BDs in a given tenant 580 domain will be provisioned to recognize that those BDs are part of 581 the same Tenant Domain. Note that a given PE does not need to be 582 configured with all the BDs of a given Tenant Domain. In general, a 583 PE will only be attached to a subset of the BDs in a given Tenant 584 Domain, and will be configured only with that subset of BDs. 585 However, each PE attached to a given Tenant Domain must be configured 586 with the SBD for that Tenant Domain. 588 Suppose a particular segment of a particular BD is attached to PE1. 589 [RFC7432] specifies that PE1 must originate an Inclusive Multicast 590 Ethernet Tag (IMET) route for that BD, and that the IMET route must 591 be propagated to all other PEs attached to the same BD. If the given 592 segment contains a host that has interest in receiving a particular 593 multicast flow, either an (S,G) flow or a (*,G) flow, PE1 will learn 594 of that interest by participating in the IGMP/MLD procedures, as 595 specified in [IGMP-Proxy]. In this case, we will say that: 597 o PE1 is interested in receiving the flow; 599 o The AC attaching the interested host to PE1 is also said to be 600 interested in the flow; 602 o The BD containing an AC that is interested in a particular flow is 603 also said to be interested in that flow. 605 Once PE1 determines that it has an AC that is interested in receiving 606 a particular flow or set of flows, it originates one or more 607 Selective Multicast Ethernet Tag (SMET) route to advertise that 608 interest. 610 Note that each IMET or SMET route is "for" a particular BD. The 611 notion of a route being "for" a particular BD is explained in 612 Section 2.2. 614 When OISM is being supported, the procedures of [IGMP-Proxy], are 615 modified as follows: 617 o The IMET route originated by a particular PE for a particular BD 618 is distributed to all other PEs attached to the Tenant Domain 619 containing that BD, even to those PEs that are not attached to 620 that particular BD. 622 o The SMET routes originated by a particular PE are originated on a 623 per-Tenant-Domain basis, rather than on a per-BD basis. That is, 624 the SMET routes are considered to be for the Tenant Domain's SBD, 625 rather than for any of its ordinary BDs. These SMET routes are 626 distributed to all the PEs attached to the Tenant Domain. 628 In this way, each PE attached to a given Tenant Domain learns, 629 from each other PE attached to the same Tenant Domain, the set of 630 flows that are of interest to each of those other PEs. 632 An OISM PE that is provisioned with several BDs in the same Tenant 633 Domain MUST originate an IMET route for each such BD. To indicate 634 its support of [IGMP-Proxy], it SHOULD attach the EVPN Multicast 635 Flags Extended Community to each such IMET route, but it MUST attach 636 the EC to at least one such IMET route. 638 Suppose PE1 is provisioned with both BD1 and BD2, and is provisioned 639 to consider them to be part of the same Tenant Domain. It is 640 possible that PE1 will receive from PE2 both an IMET route for BD1 641 and an IMET route for BD2. If either of these IMET routes has the 642 EVPN Multicast Flags Extended Community, PE1 MUST assume that PE2 is 643 supporting the procedures of [IGMP-Proxy] for ALL BDs in the Tenant 644 Domain. 646 If a PE supports OISM functionality, it indicates that by setting the 647 "OISM-supported" flag in the Multicast Flags Extended Community that 648 it attaches to some or all of its IMET routes. An OISM PE SHOULD 649 attach this EC with the OISM-supported flag set to all the IMET 650 routes it originates. However, if PE1 imports IMET routes from PE2, 651 and at least one of PE2's IMET routes indicates that PE2 is an OISM 652 PE, PE1 MUST assume that PE2 is following OISM procedures. 654 1.5.2. Data Plane 656 Suppose PE1 has an AC to a segment in BD1, and PE1 receives from that 657 AC an (S,G) multicast frame (as defined in Section 1.4). 659 There may be other ACs of PE1 on which TSes have indicated an 660 interest (via IGMP/MLD) in receiving (S,G) multicast packets. PE1 is 661 responsible for sending the received multicast packet out those ACs. 662 There are two cases to consider: 664 o Intra-Subnet Forwarding: In this case, an attachment AC with 665 interest in (S,G) is connected to a segment that is part of the 666 source BD, BD1. If the segment is not multi-homed, or if PE1 is 667 the Designated Forwarder (DF) (see [RFC7432]) for that segment, 668 PE1 sends the multicast frame on that AC without changing the MAC 669 SA. The IP header is not modified at all; in particular, the TTL 670 is not decremented. 672 o Inter-Subnet Forwarding: An AC with interest in (S,G) is connected 673 to a segment of BD2, where BD2 is different than BD1. If PE1 is 674 the DF for that segment (or if the segment is not multi-homed), 675 PE1 decapsulates the IP multicast packet, performs any necessary 676 IP processing (including TTL decrement), then re-encapsulates the 677 packet appropriately for BD2. PE1 then sends the packet on the 678 AC. Note that after re-encapsulation, the MAC SA will be PE1's 679 MAC address on BD2. The IP TTL will have been decremented by 1. 681 In addition, there may be other PEs that are interested in (S,G) 682 traffic. Suppose PE2 is such a PE. Then PE1 tunnels a copy of the 683 IP multicast frame (with its original MAC SA, and with no alteration 684 of the payload's IP header) to PE2. The tunnel encapsulation 685 contains information that PE2 can use to associate the frame with an 686 "apparent source BD". If the actual source BD of the frame is BD1, 687 then: 689 o If PE2 is attached to BD1, the tunnel encapsulation used to send 690 the frame to PE2 will cause PE2 to identify BD1 as the apparent 691 source BD. 693 o If PE2 is not attached to BD1, the tunnel encapsulation used to 694 send the frame to PE2 will cause PE2 to identify the SBD as the 695 apparent source BD. 697 Note that the tunnel encapsulation used for a particular BD will have 698 been advertised in an IMET route or S-PMSI route ([EVPN-BUM]) for 699 that BD. That route carries a PMSI Tunnel attribute, which specifies 700 how packets originating from that BD are encapsulated. This 701 information enables the PE receiving a tunneled packet to identify 702 the apparent source BD as stated above. See Section 3.2 for more 703 details. 705 When PE2 receives the tunneled frame, it will forward it on any of 706 its ACs that have interest in (S,G). 708 If PE2 determines from the tunnel encapsulation that the apparent 709 source BD is BD1, then 711 o For those ACs that connect PE2 to BD1, the intra-subnet forwarding 712 procedure described above is used, except that it is now PE2, not 713 PE1, carrying out that procedure. Unmodified EVPN procedures from 715 [RFC7432] are used to ensure that a packet originating from a 716 multi-homed segment is never sent back to that segment. 718 o For those ACs that do not connect to BD1, the inter-subnet 719 forwarding procedure described above is used, except that it is 720 now PE2, not PE1, carrying out that procedure. 722 If the tunnel encapsulation identifies the apparent source BD as the 723 SBD, PE2 applies the inter-subnet forwarding procedures described 724 above to all of its ACs that have interest in the flow. 726 These procedures ensure that an IP multicast frame travels from its 727 ingress PE to all egress PEs that are interested in receiving it. 728 While in transit, the frame retains its original MAC SA, and the 729 payload of the frame retains its original IP header. Note that in 730 all cases, when an IP multicast packet is sent from one BD to 731 another, these procedures cause its TTL to be decremented by 1. 733 So far we have assumed that an IP multicast packet arrives at its 734 ingress PE over an AC that belongs to one of the BDs in a given 735 Tenant Domain. However, it is possible for a packet to arrive at its 736 ingress PE in other ways. Since an EVPN-PE supporting IRB has an 737 IP-VRF, it is possible that the IP-VRF will have a "VRF interface" 738 that is not an IRB interface. For example, there might be a VRF 739 interface that is actually a physical link to an external ethernet 740 switch, or to a directly attached host, or to a router. When an 741 EVPN-PE, say PE1, receives a packet through such means, we will say 742 that the packet has an "external" source (i.e., a source "outside the 743 Tenant Domain"). There are also other scenarios in which a multicast 744 packet might have an external source, e.g., it might arrive over an 745 MVPN tunnel from an L3VPN PE. In such cases, we will still refer to 746 PE1 as the "ingress EVPN-PE". 748 When an EVPN-PE, say PE1, receives an externally sourced multicast 749 packet, and there are receivers for that packet inside the Tenant 750 Domain, it does the following: 752 o Suppose PE1 has an AC in BD1 that has interest in (S,G). Then PE1 753 encapsulates the packet for BD1, filling in the MAC SA field with 754 PE1's own MAC address on BD1. It sends the resulting frame on the 755 AC. 757 o Suppose some other EVPN-PE, say PE2, has interest in (S,G). PE1 758 encapsulates the packet for ethernet, filling in the MAC SA field 759 with PE1's own MAC address on the SBD. PE1 then tunnels the 760 packet to PE2. The tunnel encapsulation will identify the 761 apparent source BD as the SBD. Since the apparent source BD is 762 the SBD, PE2 will know to treat the frame as an inter-subnet 763 multicast. 765 When ingress replication is used to transmit IP multicast frames from 766 an ingress EVPN-PE to a set of egress PEs, then of course the ingress 767 PE has to send multiple copies of the frame. Each copy is the 768 original ethernet frame; decapsulation and IP processing take place 769 only at the egress PE. 771 If a Point-to-Multipoint (P2MP) tree or BIER ([EVPN-BIER]) is used to 772 transmit an IP multicast frame from an ingress PE to a set of egress 773 PEs, then the ingress PE only has to send one copy of the frame to 774 each of its next hops. Again, each egress PE receives the original 775 frame and does any necessary IP processing. 777 2. Detailed Model of Operation 779 The model described in Section 1.5.2 can be expressed more precisely 780 using the notion of "IRB interface" (see Appendix A). For a given 781 Tenant Domain: 783 o A given PE has one IRB for each BD to which it is attached. This 784 IRB interface connects L3 routing to that BD. When IP multicast 785 packets are sent or received on the IRB interfaces, the semantics 786 of the interface is modified from the semantics described in 787 Appendix A. See Section 2.3 for the details of the modification. 789 o Each PE also has an IRB interface that connects L3 routing to the 790 SBD. The semantics of this interface is different than the 791 semantics of the IRB interface to the real BDs. See Section 2.3. 793 In this section we assume that PIM is not enabled on the IRB 794 interfaces. In general, it is not necessary to enable PIM on the IRB 795 interfaces unless there are PIM routers on one of the Tenant Domain's 796 BDs, or unless there is some other scenario requiring a Tenant 797 Domain's L3 routing instance to become a PIM adjacency of some other 798 system. These cases will be discussed in Section 7. 800 2.1. Supplementary Broadcast Domain 802 Suppose a given Tenant Domain contains three BDs (BD1, BD2, BD3) and 803 two PEs (PE1, PE2). PE1 attaches to BD1 and BD2, while PE2 attaches 804 to BD2 and BD3. 806 To carry out the procedures described above, all the PEs attached to 807 the Tenant Domain must be provisioned with the SBD for that tenant 808 domain. A Route Target (RT) must be associated with the SBD, and 809 provisioned on each of those PEs. We will refer to that RT as the 810 "SBD-RT". 812 A Tenant Domain is also configured with an IP-VRF ([EVPN-IRB]), and 813 the IP-VRF is associated with an RT. This RT MAY be the same as the 814 SBD-RT. 816 Suppose an (S,G) multicast frame originating on BD1 has a receiver on 817 BD3. PE1 will transmit the packet to PE2 as a frame, and the 818 encapsulation will identify the frame's source BD as BD1. Since PE2 819 is not provisioned with BD1, it will treat the packet as if its 820 source BD were the SBD. That is, a packet can be transmitted from 821 BD1 to BD3 even though its ingress PE is not configured for BD3, and/ 822 or its egress PE is not configured for BD1. 824 EVPN supports service models in which a given EVPN Instance (EVI) can 825 contain only one BD. It also supports service models in which a 826 given EVI can contain multiple BDs. No matter which service model is 827 being used for a particular tenant, it is highly RECOMMENDED that an 828 EVI containing only the SBD be provisioned for that tenant. 830 If, for some reason, it is not feasible to provision an EVI that 831 contains only the SBD, it is possible to put the SBD in an EVI that 832 contains other BDs. However, in that case, the SBD-RT MUST be 833 different than the RT associated with any other BD. Otherwise the 834 procedures of this document (as detailed in Sections 2.2 and 3.1) 835 will not produce correct results. 837 2.2. Detecting When a Route is About/For/From a Particular BD 839 In this document, we frequently say that a particular multicast route 840 is "about" a particular BD, or is "from" a particular BD, or is "for" 841 a particular BD or is "related to" a particular BD or "is associated 842 with" a particular BD. These terms are used interchangeably. 843 Subsequent sections of this document explain when various routes must 844 be originated for particular BDs. In this section, we explain how 845 the PE originating a route marks the route to indicate which BD it is 846 about. We also explain how a PE receiving the route determines which 847 BD the route is about. 849 In EVPN, each BD is assigned a Route Target (RT). An RT is a BGP 850 extended community that can be attached to the BGP routes used by the 851 EVPN control plane. In some EVPN service models, each BD is assigned 852 a unique RT. In other service models, a set of BDs (all in the same 853 EVI) may be assigned the same RT. The RT that is assigned to the SBD 854 is called the "SBD-RT". 856 In those service models that allow a set of BDs to share a single RT, 857 each BD is assigned a non-zero Tag ID. The Tag ID appears in the 858 Network Layer Reachability Information (NLRI) of many of the BGP 859 routes that are used by the EVPN control plane. 861 A given route may be about the SBD, or about an "ordinary BD" (a BD 862 that is not the SBD). An RT that has been assigned to an ordinary BD 863 will be known as an "ordinary BD-RT". 865 When constructing an IMET, SMET, S-PMSI ([EVPN-BUM]), or Leaf 866 ([EVPN-BUM]) route that is about a given BD, the following rules 867 apply: 869 o If the route is about an ordinary BD, say BD1, then 871 * the route MUST carry the ordinary BD-RT associated with BD1, 872 and 874 * the route MUST NOT carry any RT that is associated with an 875 ordinary BD other than BD1. 877 o If the route is about the SBD, the route MUST carry the SBD-RT, 878 and MUST NOT carry any RT that is associated with any other BD. 880 o As detailed in subsequent sections, under certain circumstances a 881 route that is about BD1 may carry both the RT of BD1 and also the 882 SBD-RT. 884 When receiving an IMET, SMET, S-PMSI or Leaf route, it is necessary 885 for the receiving PE to determine the BD to which the route belongs. 886 This is done by examining the RTs carried by the route, as well as 887 the Tag ID field of the route's NLRI. There are several cases to 888 consider. Some of these cases are error cases that arise when the 889 route has not been properly constructed. 891 When one of the error cases is detected, the route MUST be regarded 892 as a malformed route, and the "treat-as-withdraw" procedure of 893 [RFC7606] MUST be applied. Note though that these error cases are 894 only detectable by EVPN procedures at the receiving PE; BGP 895 procedures at intermediate nodes will generally not detect the 896 existence of such error cases, and in general SHOULD NOT attempt to 897 do so. 899 Case 1: The receiving PE recognizes more than one of the route's RTs 900 as being an SBD-RT (i.e., the route carries SBD-RTs of more 901 than one Tenant Domain). 903 This is an error case; the route has not been properly 904 constructed. 906 Case 2: The receiving PE recognizes one of the route's RTs as being 907 associated with an ordinary BD, and recognizes one of the 908 route's other RTs as being associated with a different 909 ordinary BD. 911 This is an error case; the route has not been properly 912 constructed. 914 Case 3: The receiving PE recognizes one of the route's RTs as being 915 associated with an ordinary BD in a particular Tenant 916 Domain, and recognizes another of the route's RTs as being 917 associated with the SBD of a different Tenant Domain. 919 This is an error case; the route has not been properly 920 constructed. 922 Case 4: The receiving PE does not recognize any of the route's RTs 923 as being associated with an ordinary BD in any of its tenant 924 domains, but does recognize one of the RTs as the SBD-RT of 925 one of its Tenant Domains. 927 In this case, receiving PE associates the route with the SBD 928 of that Tenant Domain. This association is made even if the 929 Tag ID field of the route's NLRI is not the Tag ID of the 930 SBD. 932 This is a normal use case where either (a) the route is for 933 a BD to which the receiving PE is not attached, or (b) the 934 route is for the SBD. In either case, the receiving PE 935 associates the route with the SBD. 937 Case 5: The receiving PE recognizes exactly one of the RTs as an 938 ordinary BD-RT that is associated with one of the PE's EVIs, 939 say EVI-1. The receiving PE also recognizes one of the RTs 940 as being the SBD-RT of the Tenant Domain containing EVI-1. 942 In this case, the route is associated with the BD in EVI-1 943 that is identified (in the context of EVI-1) by the Tag ID 944 field of the route's NLRI. (If EVI-1 contains only a single 945 BD, the Tag ID is likely to be zero.) 947 This is the case where the route is for a BD to which the 948 receiving PE is attached, but the route also carries the 949 SBD-RT. In this case, the receiving PE associates the route 950 with the ordinary BD, not with the SBD. 952 N.B.: According to the above rules, the mapping from BD to RT is a 953 many-to-one or one-to-one mapping. A route that an EVPN-PE 954 originates for a particular BD carries that BD's RT, and an EVPN-PE 955 that receives the route associates it with a BD as described above. 956 However, RTs are not used only to help identify the BD to which a 957 route belongs; they may also used by BGP to determine the path along 958 which the route is distributed, and to determine which PEs receive 959 the route. There may be cases where it is desirable to originate a 960 route about a particular BD, but have that route distributed to only 961 some of the EVPN-PEs attached to that BD. Or one might want the 962 route distributed to some intermediate set of systems, where it might 963 be modified or replaced before being propagated further. Such 964 situations are outside the scope of this document. 966 Additionally, there may be situations where it is desirable to 967 exchange routes among two or more different Tenant Domains ("EVPN 968 Extranet"). Such situations are outside the scope of this document. 970 2.3. Use of IRB Interfaces at Ingress PE 972 When an (S,G) multicast frame is received from an AC belonging to a 973 particular BD, say BD1: 975 1. The frame is sent unchanged to other EVPN-PEs that are interested 976 in (S,G) traffic. The encapsulation used to send the frame to 977 the other EVPN-PEs depends on the tunnel type being used for 978 multicast transmission. (For our purposes, we consider Ingress 979 Replication (IR), Assisted Replication (AR) and BIER to be 980 "tunnel types", even though IR, AR and BIER do not actually use 981 P2MP tunnels.) At the egress PE, the apparent source BD of the 982 frame can be inferred from the tunnel encapsulation. If the 983 egress PE is not attached to the actual source BD, it will infer 984 that the apparent source BD is the SBD. 986 Note that the the inter-PE transmission of a multicast frame 987 among EVPN-PEs of the same Tenant Domain does NOT involve the IRB 988 interfaces, as long as the multicast frame was received over an 989 AC attached to one of the Tenant Domain's BDs. 991 2. The frame is also sent up the IRB interface that attaches BD1 to 992 the Tenant Domain's L3 routing instance in this PE. That is, the 993 L3 routing instance, behaving as if it were a multicast router, 994 receives the IP multicast frames that arrive at the PE from its 995 local ACs. The L3 routing instance decapsulates the frame's 996 payload to extract the IP multicast packet, decrements the IP 997 TTL, adjusts the header checksum, and does any other necessary IP 998 processing (e.g., fragmentation). 1000 3. The L3 routing instance keeps track of which BDs have local 1001 receivers for (S,G) traffic. (A "local receiver" is a TS, 1002 reachable via a local AC, that has expressed interest in (S,G) 1003 traffic.) If the L3 routing instance has an IRB interface to 1004 BD2, and it knows that BD2 has a LOCAL receiver interested in 1005 (S,G) traffic, it encapsulates the packet in an ethernet header 1006 for BD2, putting its own MAC address in the MAC SA field. Then 1007 it sends the packet down the IRB interface to BD2. 1009 If a packet is sent from the L3 routing instance to a particular BD 1010 via the IRB interface (step 3 in the above list), and if the BD in 1011 question is NOT the SBD, the packet is sent ONLY to LOCAL ACs of that 1012 BD. If the packet needs to go to other PEs, it has already been sent 1013 to them in step 1. Note that this is a change in the IRB interface 1014 semantics from what is described in [EVPN-IRB] and Figure 2. 1016 If a given locally attached segment is multi-homed, existing EVPN 1017 procedures ensure that a packet is not sent by a given PE to that 1018 segment unless the PE is the DF for that segment. Those procedures 1019 also ensure that a packet is never sent by a PE to its segment of 1020 origin. Thus EVPN segment multi-homing is fully supported; duplicate 1021 delivery to a segment or looping on a segment are thereby prevented, 1022 without the need for any new procedures to be defined in this 1023 document. 1025 What if an IP multicast packet is received from outside the tenant 1026 domain? For instance, perhaps PE1's IP-VRF for a particular tenant 1027 domain also has a physical interface leading to an external switch, 1028 host, or router, and PE1 receives an IP multicast packet or frame on 1029 that interface. Or perhaps the packet is from an L3VPN, or a 1030 different EVPN Tenant Domain. 1032 Such a packet is first processed by the L3 routing instance, which 1033 decrements TTL and does any other necessary IP processing. Then the 1034 packet is sent into the Tenant Domain by sending it down the IRB 1035 interface to the SBD of that Tenant Domain. This requires 1036 encapsulating the packet in an ethernet header. The MAC SA field 1037 will contain the PE's own MAC on the SBD. 1039 An IP multicast packet sent by the L3 routing instance down the IRB 1040 interface to the SBD is treated as if it had arrived from a local AC, 1041 and steps 1-3 are applied. Note that the semantics of sending a 1042 packet down the IRB interface to the SBD are thus slightly different 1043 than the semantics of sending a packet down other IRB interfaces. IP 1044 multicast packets sent down the SBD's IRB interface may be 1045 distributed to other PEs, but IP multicast packets sent down other 1046 IRB interfaces are distributed only to local ACs. 1048 If a PE sends a link-local multicast packet down the SBD IRB 1049 interface, that packet will be distributed (as an ethernet frame) to 1050 other PEs of the Tenant Domain, but will not appear on any of the 1051 actual BDs. 1053 2.4. Use of IRB Interfaces at an Egress PE 1055 Suppose an egress EVPN-PE receives an (S,G) multicast frame from the 1056 frame's ingress EVPN-PE. As described above, the packet will arrive 1057 as an ethernet frame over a tunnel from the ingress PE, and the 1058 tunnel encapsulation will identify the source BD of the ethernet 1059 frame. 1061 We define the notion of the frame's "apparent source BD" as follows. 1062 If the egress PE is attached to the actual source BD, the actual 1063 source BD is the apparent source BD. If the egress PE is not 1064 attached to the actual source BD, the SBD is the apparent source BD. 1066 The egress PE now takes the following steps: 1068 1. If the egress PE has ACs belonging to the apparent source BD of 1069 the frame, it sends the frame unchanged to any ACs of that BD 1070 that have interest in (S,G) packets. The MAC SA of the frame is 1071 not modified, and the IP header of the frame's payload is not 1072 modified in any way. 1074 2. The frame is also sent to the L3 routing instance by being sent 1075 up the IRB interface that attaches the L3 routing instance to the 1076 apparent source BD. Steps 2 and 3 of Section 2.3 are then 1077 applied. 1079 2.5. Announcing Interest in (S,G) 1081 [IGMP-Proxy] defines procedures used by an egress PE to announce its 1082 interest in a multicast flow or set of flows. If an egress PE 1083 determines it has LOCAL receivers in a particular BD, say BD1, that 1084 are interested in a particular set of flows, it originates one or 1085 more SMET routes for BD1. Each SMET route specifies a particular 1086 (S,G) or (*,G) flow. By originating an SMET route for BD1, a PE is 1087 announcing "I have receivers for (S,G) or (*,G) in BD1". Such an 1088 SMET route carries the Route Target (RT) for BD1, ensuring that it 1089 will be distributed to all PEs that are attached to BD1. 1091 The OISM procedures for originating SMET routes differ slightly from 1092 those in [IGMP-Proxy]. In most cases, the SMET routes are considered 1093 to be for the SBD, rather than for the BD containing local receivers. 1094 These SMET routes carry the SBD-RT, and do not carry any ordinary BD- 1095 RT. Details on the processing of SMET routes can be found in 1096 Section 3.3. 1098 Since the SMET routes carry the SBD-RT, every ingress PE attached to 1099 a particular Tenant Domain will learn of all other PEs (attached to 1100 the same Tenant Domain) that have interest in a particular set of 1101 flows. Note that a PE that receives a given SMET route does not 1102 necessarily have any BDs (other than the SBD) in common with the PE 1103 that originates that SMET route. 1105 If all the sources and receivers for a given (*,G) are in the Tenant 1106 Domain, inter-subnet "Any Source Multicast" traffic will be properly 1107 routed without requiring any Rendezvous Points, shared trees, or 1108 other complex aspects of multicast routing infrastructure. Suppose, 1109 for example, that: 1111 o PE1 has a local receiver, on BD1, for (*,G) 1113 o PE2 has a local source, on BD2, for (*,G). 1115 PE1 will originate an SMET(*,G) route for the SBD, and PE2 will 1116 receive that route, even if PE2 is not attached to BD1. PE2 will 1117 thus know to forward (S,G) traffic to PE1. PE1 does not need to do 1118 any "source discovery". (This does assume that source S does not 1119 send the same (S,G) datagram on two different BDs, and that the 1120 Tenant Domain does not contain two or more sources with the same IP 1121 address S. The use of multicast sources that have IP "anycast" 1122 addresses is outside the scope of this document.) 1124 If some PE attached to the Tenant Domain does not support [IGMP- 1125 Proxy], it will be assumed to be interested in all flows. Whether a 1126 particular remote PE supports [IGMP-Proxy] is determined by the 1127 presence of the Multicast Flags Extended Community in its IMET route; 1128 this is specified in [IGMP-Proxy]. 1130 2.6. Tunneling Frames from Ingress PE to Egress PEs 1132 [RFC7432] specifies the procedures for setting up and using "BUM 1133 tunnels". A BUM tunnel is a tunnel used to carry traffic on a 1134 particular BD if that traffic is (a) broadcast traffic, or (b) 1135 unicast traffic with an unknown MAC DA, or (c) ethernet multicast 1136 traffic. 1138 This document allows the BUM tunnels to be used as the default 1139 tunnels for transmitting IP multicast frames. It also allows a 1140 separate set of tunnels to be used, instead of the BUM tunnels, as 1141 the default tunnels for carrying IP multicast frames. Let's call 1142 these "IP Multicast Tunnels". 1144 When the tunneling is done via Ingress Replication or via BIER, this 1145 difference is of no significance. However, when P2MP tunnels are 1146 used, there is a significant advantage to having separate IP 1147 multicast tunnels. 1149 Other things being equal, it is desirable for an ingress PE to 1150 transmit a copy of a given (S,G) multicast frame on only one P2MP 1151 tunnel. All egress PEs interested in (S,G) packets then have to join 1152 that tunnel. If the source BD and PE for an (S,G) frame are BD1 an 1153 PE1 respectively, and if PE2 has receivers on BD2 for (S,G), then PE2 1154 must join the P2MP LSP on which PE1 transmits the (S,G) frame. PE2 1155 must join this P2MP LSP even if PE2 is not attached to the source BD 1156 (BD1). If PE1 were transmitting the multicast frame on its BD1 BUM 1157 tunnel, then PE2 would have to join the BD1 BUM tunnel, even though 1158 PE2 has no BD1 attachment circuits. This would cause PE2 to pull all 1159 the BUM traffic from BD1, most of which it would just have to 1160 discard. Thus we RECOMMEND that the default IP multicast tunnels be 1161 distinct from the BUM tunnels. 1163 Notwithstanding the above, link local IP multicast traffic MUST 1164 always be carried on the BUM tunnels, and ONLY on the BUM tunnels. 1165 Link local IP multicast traffic consists of IPv4 traffic with a 1166 destination address prefix of 224/8 and IPv6 traffic with a 1167 destination address prefix of FF02/16. In this document, the terms 1168 "IP multicast packet" and "IP multicast frame" are defined in 1169 Section 1.4 so as to exclude the link-local traffic. 1171 Note that it is also possible to use "selective tunnels" to carry 1172 particular multicast flows (see Section 3.2). When an (S,G) frame is 1173 transmitted on a selective tunnel, it is not transmitted on the BUM 1174 tunnel or on the default IP Multicast tunnel. 1176 2.7. Advanced Scenarios 1178 There are some deployment scenarios that require special procedures: 1180 1. Some multicast sources or receivers are attached to PEs that 1181 support [RFC7432], but do not support this document or 1182 [EVPN-IRB]. To interoperate with these "non-OISM PEs", it is 1183 necessary to have one or more gateway PEs that interface the 1184 tunnels discussed in this document with the BUM tunnels of the 1185 legacy PEs. This is discussed in Section 5. 1187 2. Sometimes multicast traffic originates from outside the EVPN 1188 domain, or needs to be sent outside the EVPN domain. This is 1189 discussed in Section 6. An important special case of this, 1190 integration with MVPN, is discussed in Section 6.1.2. 1192 3. In some scenarios, one or more of the tenant systems is a PIM 1193 router, and the Tenant Domain is used for as a transit network 1194 that is part of a larger multicast domain. This is discussed in 1195 Section 7. 1197 3. EVPN-aware Multicast Solution Control Plane 1199 3.1. Supplementary Broadcast Domain (SBD) and Route Targets 1201 As discussed in Section 2.1, every Tenant Domain is associated with a 1202 single Supplementary Broadcast Domain (SBD). Recall that a Tenant 1203 Domain is defined to be a set of BDs that can freely send and receive 1204 IP multicast traffic to/from each other. If an EVPN-PE has one or 1205 more ACs in a BD of a particular Tenant Domain, and if the EVPN-PE 1206 supports the procedures of this document, that EVPN-PE MUST be 1207 provisioned with the SBD of that Tenant Domain. 1209 At each EVPN-PE attached to a given Tenant Domain, there is an IRB 1210 interface leading from the L3 routing instance of that Tenant Domain 1211 to the SBD. However, the SBD has no ACs. 1213 Each SBD is provisioned with a Route Target (RT). All the EVPN-PEs 1214 supporting a given SBD are provisioned with that RT as an import RT. 1215 That RT MUST NOT be the same as the RT associated with any other BD. 1217 We will use the term "SBD-RT" to denote the RT has has been assigned 1218 to the SBD. Routes carrying this RT will be propagated to all 1219 EVPN-PEs in the same Tenant Domain as the originator. 1221 Section 2.2 specifies the rules by which an EVPN-PE that receives a 1222 route determines whether a received route "belongs to" a particular 1223 ordinary BD or SBD. 1225 Section 2.2 also specifies additional rules that must be following 1226 when constructing routes that belong to a particular BD, including 1227 the SBD. 1229 The SBD SHOULD be in an EVPN Instance (EVI) of its own. Even if the 1230 SBD is not in an EVI of its own, the SBD-RT MUST be different than 1231 the RT associated with any other BD. This restriction is necessary 1232 in order for the rules of Sections 2.2 and 3.1 to work correctly. 1234 Note that an SBD, just like any other BD, is associated on each 1235 EVPN-PE with a MAC-VRF. Per [RFC7432], each MAC-VRF is associated 1236 with a Route Distinguisher (RD). When constructing a route that is 1237 "about" an SBD, an EVPN-PE will place the RD of the associated 1238 MAC-VRF in the "Route Distinguisher" field of the NLRI. (If the 1239 Tenant Domain has several MAC-VRFs on a given PE, the EVPN-PE has a 1240 choice of which RD to use.) 1242 If Assisted Replication (AR, see [EVPN-AR]) is used, each 1243 AR-REPLICATOR for a given Tenant Domain must be provisioned with the 1244 SBD of that Tenant Domain, even if the AR-REPLICATOR does not have 1245 any L3 routing instance. 1247 3.2. Advertising the Tunnels Used for IP Multicast 1249 The procedures used for advertising the tunnels that carry IP 1250 multicast traffic depend upon the type of tunnel being used. If the 1251 tunnel type is neither Ingress Replication, Assisted Replication, nor 1252 BIER, there are procedures for advertising both "inclusive tunnels" 1253 and "selective tunnels". 1255 When IR, AR or BIER are used to transmit IP multicast packets across 1256 the core, there are no P2MP tunnels. Once an ingress EVPN-PE 1257 determines the set of egress EVPN-PEs for a given flow, the IMET 1258 routes contain all the information needed to transport packets of 1259 that flow to the egress PEs. 1261 If AR is used, the ingress EVPN-PE is also an AR-LEAF and the IMET 1262 route coming from the selected AR-REPLICATOR contains the information 1263 needed. The AR-REPLICATOR will behave as an ingress EVPN-PE when 1264 sending a flow to the egress EVPN-PEs. 1266 If the tunneling technique requires P2MP tunnels to be set up (e.g., 1267 RSVP-TE P2MP, mLDP, PIM), some of the tunnels may be selective 1268 tunnels and some may be inclusive tunnels. 1270 Selective P2MP tunnels are always advertised by the ingress PE using 1271 S-PMSI A-D routes ([EVPN-BUM]). 1273 For inclusive tunnels, there is a choice between using a BD's 1274 ordinary "BUM tunnel" [RFC7432] as the default inclusive tunnel for 1275 carrying IP multicast traffic, or using a separate IP multicast 1276 tunnel as the default inclusive tunnel for carrying IP multicast. In 1277 the former case, the inclusive tunnel is advertised in an IMET route. 1278 In the latter case, the inclusive tunnel is advertised in a (C-*,C-*) 1279 S-PMSI A-D route ([EVPN-BUM]). Details may be found in subsequent 1280 sections. 1282 3.2.1. Constructing Routes for the SBD 1284 There are situations in which an EVPN-PE needs to originate IMET, 1285 SMET, and/or SPMSI routes for the SBD. Throughout this document, we 1286 will refer to such routes respectively as "SBD-IMET routes", 1287 "SBD-SMET routes", and "SBD-SPMSI routes". Subsequent sections 1288 detail the conditions under which these routes need to be originated. 1290 When an EVPN-PE needs to originate an SBD-IMET, SBD-SMET, or 1291 SBD-SPMSI route, it constructs the route as follows: 1293 o the RD field of the route's NLRI is set to the RD of the MAC-VRF 1294 that is associated with the SBD; 1296 o the SBD-RT is attached to the route; 1298 o the "Tag ID" field of the route's NLRI is set to the Tag ID that 1299 has been assigned to the SBD. This is most likely 0 if a 1300 VLAN-based or VLAN-bundle service is being used, but non-zero if a 1301 VLAN-aware bundle service is being used. 1303 3.2.2. Ingress Replication 1305 When Ingress Replication (IR) is used to transport IP multicast 1306 frames of a given Tenant Domain, each EVPN-PE attached to that Tenant 1307 Domain MUST originate an SBD-IMET route (see Section 3.2.1). 1309 The SBD-IMET route MUST carry a PMSI Tunnel attribute (PTA), and the 1310 MPLS label field of the PTA MUST specify a downstream-assigned MPLS 1311 label that maps uniquely (in the context of the originating EVPN-PE) 1312 to the SBD. 1314 Following the procedures of [RFC7432], an EVPN-PE MUST also originate 1315 an IMET route for each BD to which it is attached. Each of these 1316 IMET routes carries a PTA specifying a downstream-assigned label that 1317 maps uniquely, in the context of the originating EVPN-PE, to the BD 1318 in question. These IMET routes need not carry the SBD-RT. 1320 When an ingress EVPN-PE needs to use IR to send an IP multicast frame 1321 from a particular source BD to an egress EVPN-PE, the ingress PE 1322 determines whether the egress PE has originated an IMET route for 1323 that BD. If so, that IMET route contains the MPLS label that the 1324 egress PE has assigned to the source BD. The ingress PE uses that 1325 label when transmitting the packet to the egress PE. Otherwise, the 1326 ingress PE uses the label that the egress PE has assigned to the SBD 1327 (in the SBD-IMET route originated by the egress). 1329 Note that the set of IMET routes originated by a given egress PE, and 1330 installed by a given ingress PE, may change over time. If the egress 1331 PE withdraws its IMET route for the source BD, the ingress PE MUST 1332 stop using the label carried in that IMET route, and instead MUST use 1333 the label carried in the SBD-IMET route from that egress PE. 1334 Implementors must also take into account that an IMET route from a 1335 particular PE for a particular BD may arrive after that PE's SBD-IMET 1336 route. 1338 3.2.3. Assisted Replication 1340 When Assisted Replication is used to transport IP multicast frames of 1341 a given Tenant Domain, each EVPN-PE (including the AR-REPLICATOR) 1342 attached to the Tenant Domain MUST originate an SBD-IMET route (see 1343 Section 3.2.1). 1345 An AR-REPLICATOR attached to a given Tenant Domain is considered to 1346 be an EVPN-PE of that Tenant Domain. It is attached to all the BDs 1347 in the Tenant Domain, but it has no IRB interfaces. 1349 As with Ingress Replication, the SBD-IMET route carries a PTA where 1350 the MPLS label field specifies the downstream-assigned MPLS label 1351 that identifies the SBD. However, the AR-REPLICATOR and AR-LEAF 1352 EVPN-PEs will set the PTA's flags differently, as per [EVPN-AR]. 1354 In addition, each EVPN-PE originates an IMET route for each BD to 1355 which it is attached. As in the case of Ingress Replication, these 1356 routes carry the downstream-assigned MPLS labels that identify the 1357 BDs and do not carry the SBD-RT. 1359 When an ingress EVPN-PE, acting as AR-LEAF, needs to send an IP 1360 multicast frame from a particular source BD to an egress EVPN-PE, the 1361 ingress PE determines whether there is any AR-REPLICATOR that 1362 originated an IMET route for that BD. After the AR-REPLICATOR 1363 selection (if there are more than one), the AR-LEAF uses the label 1364 contained in the IMET route of the AR-REPLICATOR when transmitting 1365 packets to it. The AR-REPLICATOR receives the packet and, based on 1366 the procedures specified in [EVPN-AR], transmits the packets to the 1367 egress EVPN-PEs using the labels contained in the IMET routes 1368 received from the egress PEs. 1370 If an ingress AR-LEAF for a given BD has not received any IMET route 1371 for that BD from an AR-REPLICATOR, the ingress AR-LEAF follows the 1372 procedures in Section 3.2.2. 1374 3.2.4. BIER 1376 When BIER is used to transport multicast packets of a given Tenant 1377 Domain, and a given EVPN-PE attached to that Tenant Domain is a 1378 possible ingress EVPN-PE for traffic originating outside that Tenant 1379 Domain, the given EVPN-PE MUST originate an SBD-IMET route, (see 1380 Section 3.2.1). 1382 In addition, IMET routes that are originated for other BDs in the 1383 Tenant Domain MUST carry the SBD-RT. 1385 Each IMET route (including but not limited to the SBD-IMET route) 1386 MUST carry a PMSI Tunnel attribute (PTA). The MPLS label field of 1387 the PTA MUST specify an upstream-assigned MPLS label that maps 1388 uniquely (in the context of the originating EVPN-PE) to the BD for 1389 which the route is originated. 1391 Suppose an ingress EVPN-PE, PE1, needs to use BIER to tunnel an IP 1392 multicast frame to a set of egress EVPN-PEs. And suppose the frame's 1393 source BD is BD1. The frame is encapsulated as follows: 1395 o A four-octet MPLS label stack entry ([RFC3032]) is prepended to 1396 the frame. The Label field is set to the upstream-assigned label 1397 that PE1 has assigned to BD1. 1399 o The resulting MPLS packet is then encapsulated in a BIER 1400 encapsulation ([RFC8296], [EVPN-BIER]). The BIER BitString is set 1401 to identify the egress EVPN-PEs. The BIER "proto" field is set to 1402 the value for "MPLS packet with upstream-assigned label at top of 1403 stack". 1405 Note: It is possible that the packet being tunneled from PE1 1406 originated outside the Tenant Domain. In this case, the actual 1407 source BD (BD1) is considered to be the SBD, and the 1408 upstream-assigned label it carries will be the label that PE1 1409 assigned to the SBD, and advertised in its SBD-IMET route. 1411 Suppose an egress PE, PE2, receives such a BIER packet. The BFIR-id 1412 field of the BIER header allows PE2 to determine that the ingress PE 1413 is PE1. There are then two cases to consider: 1415 1. PE2 has received and installed an IMET route for BD1 from PE1. 1417 In this case, the BIER packet will be carrying the 1418 upstream-assigned label that is specified in the PTA of that IMET 1419 route. This enables PE2 to determine the "apparent source BD" 1420 (as defined in Section 2.4). 1422 2. PE2 has not received and installed an IMET route for BD1 from 1423 PE1. 1425 In this case, PE2 will not recognize the upstream-assigned label 1426 carried in the BIER packet. PE2 MUST discard the packet. 1428 Further details on the use of BIER to support EVPN can be found in 1429 [EVPN-BIER]. 1431 3.2.5. Inclusive P2MP Tunnels 1433 3.2.5.1. Using the BUM Tunnels as IP Multicast Inclusive Tunnels 1435 The procedures in this section apply only when 1437 (a) it is desired to use the BUM tunnels to carry IP multicast 1438 traffic across the backbone, and 1440 (b) the BUM tunnels are P2MP tunnels (i.e., neither IR, AR, nor BIER 1441 are being used to transport the BUM traffic). 1443 In this case, an IP multicast frame (whether inter-subnet or 1444 intra-subnet) will be carried across the backbone in the BUM tunnel 1445 belonging to its source BD. Each EVPN-PE attached to a given Tenant 1446 Domain needs to join the BUM tunnels for every BD in the Tenant 1447 Domain, even those BDs to which the EVPN-PE is not locally attached. 1448 This ensures that an IP multicast packet from any source BD can reach 1449 all PEs attached to the Tenant Domain. 1451 Note that this will cause all the BUM traffic from a given BD in a 1452 Tenant Domain to be sent to all PEs that attach to that Tenant 1453 Domain, even the PEs that don't attach to the given BD. To avoid 1454 this, it is RECOMMENDED that the BUM tunnels not be used as IP 1455 Multicast inclusive tunnels, and that the procedures of 1456 Section 3.2.5.2 be used instead. 1458 If a PE is a possible ingress EVPN-PE for traffic originating outside 1459 the Tenant Domain, the PE MUST originate an SBD-IMET route (see 1460 Section 3.2.1). This route MUST carry a PTA specifying the P2MP 1461 tunnel used for transmitting IP multicast packets that originate 1462 outside the tenant domain. All EVPN-PEs of the Tenant Domain MUST 1463 join the tunnel specified in the PTA of an SBD-IMET route: 1465 o If the tunnel is an RSVP-TE P2MP tunnel, the originator of the 1466 route MUST use RSVP-TE P2MP procedures to add each PE of the 1467 Tenant Domain to the tunnel, even PEs that have not originated an 1468 SBD-IMET route. 1470 o If the tunnel is an mLDP or PIM tunnel, each PE importing the 1471 SBD-IMET route MUST add itself to the tunnel, using mLDP or PIM 1472 procedures, respectively. 1474 Whether or not a PE originates an SBD-IMET route, it will of course 1475 originate an IMET route for each BD to which it is attached. Each of 1476 these IMET routes MUST carry the SBD-RT, as well as the RT for the BD 1477 to which it belongs. 1479 If a received IMET route is not the SBD-IMET route, it will also be 1480 carrying the RT for its source BD. The route's NLRI will carry the 1481 Tag ID for the source BD. From the RT and the Tag ID, any PE 1482 receiving the route can determine the route's source BD. 1484 If the MPLS label field of the PTA contains zero, the specified P2MP 1485 tunnel is used only to carry frames of a single source BD. 1487 If the MPLS label field of the PTA does not contain zero, it MUST 1488 contain an upstream-assigned MPLS label that maps uniquely (in the 1489 context of the originating EVPN-PE) to the source BD (or, in the case 1490 of an SBD-IMET route, to the SBD). The tunnel may then be used to 1491 carry frames of multiple source BDs. The apparent source BD of a 1492 particular packet is inferred from the label carried by the packet. 1494 IP multicast traffic originating outside the Tenant Domain is 1495 transmitted with the label corresponding to the SBD, as specified in 1496 the ingress EVPN-PE's SBD-IMET route. 1498 3.2.5.2. Using Wildcard S-PMSI A-D Routes to Advertise Inclusive 1499 Tunnels Specific to IP Multicast 1501 The procedures of this section apply when (and only when) it is 1502 desired to transmit IP multicast traffic on an inclusive tunnel, but 1503 not on the same tunnel used to transmit BUM traffic. 1505 However, these procedures do NOT apply when the tunnel type is 1506 Ingress Replication or BIER, EXCEPT in the case where it is necessary 1507 to interwork between non-OISM PEs and OISM PEs, as specified in 1508 Section 5. 1510 Each EVPN-PE attached to the given Tenant Domain MUST originate an 1511 SBD-SPMSI A-D route. The NLRI of that route MUST contain (C-*,C-*) 1512 (see [RFC6625]). Additional rules for constructing that route are 1513 given in Section 3.2.1. 1515 In addition, an EVPN-PE MUST originate an S-PMSI A-D route containing 1516 (C-*,C-*) in its NLRI for each of the other BDs, in the given Tenant 1517 Domain, to which it is attached. All such routes MUST carry the 1518 SBD-RT. This ensures that those routes are imported by all EVPN-PEs 1519 attached to the Tenant Domain. 1521 A PE receiving these routes follows the procedures of Section 2.2 to 1522 determine which BD the route is for. 1524 If the MPLS label field of the PTA contains zero, the specified 1525 tunnel is used only to carry frames of a single source BD. 1527 If the MPLS label field of the PTA does not contain zero, it MUST 1528 specify an upstream-assigned MPLS label that maps uniquely (in the 1529 context of the originating EVPN-PE) to the source BD. The tunnel may 1530 be used to carry frames of multiple source BDs, and the apparent 1531 source BD for a particular packet is inferred from the label carried 1532 by the packet. 1534 The EVPN-PE advertising these S-PMSI A-D route routes is specifying 1535 the default tunnel that it will use (as ingress PE) for transmitting 1536 IP multicast packets. The upstream-assigned label allows an egress 1537 PE to determine the apparent source BD of a given packet. 1539 3.2.6. Selective Tunnels 1541 An ingress EVPN-PE for a given multicast flow or set of flows can 1542 always assign the flow to a particular P2MP tunnel by originating an 1543 S-PMSI A-D route whose NLRI identifies the flow or set of flows. The 1544 NLRI of the route could be (C-*,C-G), or (C-S,C-G). The S-PMSI A-D 1545 route MUST carry the SBD-RT, so that it is imported by all EVPN-PEs 1546 attached to the Tenant Domain. 1548 An S-PMSI A-D route is "for" a particular source BD. It MUST carry 1549 the RT associated with that BD, and it MUST have the Tag ID for that 1550 BD in its NLRI. 1552 When an EVPN-PE imports an S-PMSI A-D route, it applies the rules of 1553 Section 2.2 to associate the route with a particular BD. 1555 Each such route MUST contain a PTA, as specified in Section 3.2.5.2. 1557 An egress EVPN-PE interested in the specified flow or flows MUST join 1558 the specified tunnel. Procedures for joining the specified tunnel 1559 are specific to the tunnel type. (Note that if the tunnel type is 1560 RSVP-TE P2MP LSP, the Leaf Information Required (LIR) flag of the PTA 1561 SHOULD NOT be set. An ingress OISM PE knows which OISM EVPN PEs are 1562 interested in any given flow, and hence can add them to the RSVP-TE 1563 P2MP tunnel that carries such flows.) 1565 If the PTA does not specify a non-zero MPLS label, the apparent 1566 source BD of any packets that arrive on that tunnel is considered to 1567 be the BD associated with the route that carries the PTA. If the PTA 1568 does specify a non-zero MPLS label, the apparent source BD of any 1569 packets that arrive on that tunnel carrying the specified label is 1570 considered to be the BD associated with the route that carries the 1571 PTA. 1573 It should be noted that when either IR or BIER is used, there is no 1574 need for an ingress PE to use S-PMSI A-D routes to assign specific 1575 flows to selective tunnels. The procedures of Section 3.3, along 1576 with the procedures of Section 3.2.2, Section 3.2.3, or 1577 Section 3.2.4, provide the functionality of selective tunnels without 1578 the need to use S-PMSI A-D routes. 1580 3.3. Advertising SMET Routes 1582 [IGMP-Proxy] allows an egress EVPN-PE to express its interest in a 1583 particular multicast flow or set of flows by originating an SMET 1584 route. The NLRI of the SMET route identifies the flow or set of 1585 flows as (C-*,C-*) or (C-*,C-G) or (C-S,C-G). 1587 Each SMET route belongs to a particular BD. The Tag ID for the BD 1588 appears in the NLRI of the route, and the route carries the RT 1589 associated that that BD. From this pair, other EVPN-PEs 1590 can identify the BD to which a received SMET route belongs. 1591 (Remember though that the route may be carrying multiple RTs.) 1593 There are three cases to consider: 1595 o Case 1: It is known that no BD of a Tenant Domain contains a 1596 multicast router. 1598 In this case, an egress PE advertises its interest in a flow or 1599 set of flows by originating an SMET route that belongs to the SBD. 1600 We refer to this as an SBD-SMET route. The SBD-SMET route carries 1601 the SBD-RT, and has the Tag ID for the SBD in its NLRI. SMET 1602 routes for the individual BDs are not needed, because there is no 1603 need for a PE that receives an SMET route to send a corresponding 1604 IGMP Join message out any of its ACs. 1606 o Case 2: It is known that more than one BD of a Tenant Domain may 1607 contain a multicast router. 1609 This is very like Case 1. An egress PE advertises its interest in 1610 a flow or set of flows by originating an SBD-SMET route. The 1611 SBD-SMET route carries the SBD-RT, and has the Tag ID for the SBD 1612 in its NLRI. 1614 In this case, it is important to be sure that SMET routes for the 1615 individual BDs are not originated. Suppose, for example, that PE1 1616 had local receivers for a given flow on both BD1 and BD2, and that 1617 it originated SMET routes for both those BDs. Then PEs receiving 1618 those SMET routes might send IGMP Joins on both those BDs. This 1619 could cause externally sourced multicast traffic to enter the 1620 Tenant Domain at both BDs, which could result in duplication of 1621 data. 1623 N.B.: If it is possible that more than one BD contains a tenant 1624 multicast router, then in order to receive multicast data 1625 originating from outside EVPN, the PEs MUST follow the procedures 1626 of Section 6. 1628 o Case 3: It is known that only a single BD of a Tenant Domain 1629 contains a multicast router. 1631 Suppose that an egress PE is attached to a BD on which there might 1632 be a tenant multicast router. (The tenant router is not 1633 necessarily on a segment that is attached to that PE.) And 1634 suppose that the PE has one or more ACs attached to that BD which 1635 are interested in a given multicast flow. In this case, IN 1636 ADDITION to the SMET route for the SBD, the egress PE MAY 1637 originate an SMET route for that BD. This will enable the ingress 1638 PE(s) to send IGMP/MLD messages on ACs for the BD, as specified in 1639 [IGMP-Proxy]. As long as that is the only BD on which there is a 1640 tenant multicast router, there is no possibility of duplication of 1641 data. 1643 This document does not specify procedures for dynamically determining 1644 which of the three cases applies to a given deployment; the PEs of a 1645 given Tenant Domain MUST be provisioned to know which case applies. 1647 As detailed in [IGMP-Proxy], an SMET route carries a Multicast Flags 1648 EC containing flags indicating whether it is to result in the 1649 propagation of IGMP v1, v2, or v3 messages on the ACs of the BD to 1650 which the SMET route belongs. These flags SHOULD be set to zero in 1651 an SBD-SMET route. 1653 Note that a PE only needs to originate the set of SBD-SMET routes 1654 that are needed to pull in all the traffic in which it is interested. 1655 Suppose PE1 has ACs attached to BD1 that are interested in (C-*,C-G) 1656 traffic, and ACs attached to BD2 that are interested in (C-S,C-G) 1657 traffic. A single SBD-SMET route specifying (C-*,C-G) will pull in 1658 all the necessary flows. 1660 As another example, suppose the ACs attached to BD1 are interested in 1661 (C-*,C-G) but not in (C-S,C-G), while the ACs attached to BD2 are 1662 interested in (C-S,C-G). A single SBD-SMET route specifying 1663 (C-*,C-G) will pull in all the necessary flows. 1665 In other words, to determine the set of SBD-SMET routes that have to 1666 be sent for a given C-G, the PE has to merge the IGMP/MLD state for 1667 all the BDs (of the given Tenant Domain) to which it is attached. 1669 Per [IGMP-Proxy], importing an SMET route for a particular BD will 1670 cause IGMP/MLD state to be instantiated for the IRB interface to that 1671 BD. This applies as well when the BD is the SBD. 1673 However, traffic that originates in one of the actual BDs of a 1674 particular Tenant Domain MUST NOT be sent down the IRB interface that 1675 connects the L3 routing instance of that Tenant Domain to the SBD. 1676 That would cause duplicate delivery of traffic, since such traffic 1677 will have already been distributed throughout the Tenant Domain. 1678 Therefore, when setting up the IGMP/MLD state based on SBD-SMET 1679 routes, care must be taken to ensure that the IRB interface to the 1680 SBD is not added to the Outgoing Interface (OIF) list if the traffic 1681 originates within the Tenant Domain. 1683 There are some multicast scenarios that make use of "anycast 1684 sources". For example, two different sources may share the same 1685 anycast IP address, say S1, and each may transmit an (S1,G) multicast 1686 flow. In such a scenario, the two (S1,G) flows are typically 1687 identical. Ordinary PIM procedures will cause only one the flows to 1688 be delivered to each receiver that has expressed interest in either 1689 (*,G) or (S1,G). However, the OISM procedures described in this 1690 document will result in both of the (S1,G) flows being distributed in 1691 the Tenant Domain, and duplicate delivery will result. Therefore, if 1692 there are receivers for (*,G) in a given Tenant Domain, there MUST 1693 NOT be anycast sources for G within that Tenant Domain. (This 1694 restriction can be lifted by defining additional procedures; however 1695 that is outside the scope of this document.) 1697 4. Constructing Multicast Forwarding State 1699 4.1. Layer 2 Multicast State 1701 An EVPN-PE maintains "layer 2 multicast state" for each BD to which 1702 it is attached. 1704 Let PE1 be an EVPN-PE, and BD1 be a BD to which it is attached. At 1705 PE1, BD1's layer 2 multicast state for a given (C-S,C-G) or (C-*,C-G) 1706 governs the disposition of an IP multicast packet that is received by 1707 BD1's layer 2 multicast function on an EVPN-PE. 1709 An IP multicast (S,G) packet is considered to have been received by 1710 BD1's layer 2 multicast function in PE1 in the following cases: 1712 o The packet is the payload of an ethernet frame received by PE1 1713 from an AC that attaches to BD1. 1715 o The packet is the payload of an ethernet frame whose apparent 1716 source BD is BD1, and which is received by the PE1 over a tunnel 1717 from another EVPN-PE. 1719 o The packet is received from BD1's IRB interface (i.e., has been 1720 transmitted by PE1's L3 routing instance down BD1's IRB 1721 interface). 1723 According to the procedures of this document, all transmission of IP 1724 multicast packets from one EVPN-PE to another is done at layer 2. 1725 That is, the packets are transmitted as ethernet frames, according to 1726 the layer 2 multicast state. 1728 Each layer 2 multicast state (S,G) or (*,G) contains a set "output 1729 interfaces" (OIF list). The disposition of an (S,G) multicast frame 1730 received by BD1's layer 2 multicast function is determined as 1731 follows: 1733 o The OIF list is taken from BD1's layer 2 (S,G) state, or if there 1734 is no such (S,G) state, then from BD1's (*,G) state. (If neither 1735 state exists, the OIF list is considered to be null.) 1737 o The rules of Section 4.1.2 are applied to the OIF list. This will 1738 generally result in the frame being transmitted to some, but not 1739 all, elements of the OIF list. 1741 Note that there is no RPF check at layer 2. 1743 4.1.1. Constructing the OIF List 1745 In this document, we have extended the procedures of [IGMP-Proxy] so 1746 that IMET and SMET routes for a particular BD are distributed not 1747 just to PEs that attach to that BD, but to PEs that attach to any BD 1748 in the Tenant Domain. In this way, each PE attached to a given 1749 Tenant Domain learns, from each other PE attached to the same Tenant 1750 Domain, the set of flows that are of interest to each of those other 1751 PEs. (If some PE attached to the Tenant Domain does not support 1752 [IGMP-Proxy], it will be assumed to be interested in all flows. 1753 Whether a particular remote PE supports [IGMP-Proxy] is determined by 1754 the presence of an Extended Community in its IMET route; this is 1755 specified in [IGMP-Proxy].) If a set of remote PEs are interested in 1756 a particular flow, the tunnels used to reach those PEs are added to 1757 the OIF list of the multicast states corresponding to that flow. 1759 An EVPN-PE may run IGMP/MLD procedures on each of its ACs, in order 1760 to determine the set of flows of interest to each AC. (An AC is said 1761 to be interested in a given flow if it connects to a segment that has 1762 tenant systems interested in that flow.) If IGMP/MLD procedures are 1763 not being run on a given AC, that AC is considered to be interested 1764 in all flows. For each BD, the set of ACs interested in a given flow 1765 is determined, and the ACs of that set are added to the OIF list of 1766 that BD's multicast state for that flow. 1768 The OIF list for each multicast state must also contain the IRB 1769 interface for the BD to which the state belongs. 1771 Implementors should note that the OIF list of a multicast state will 1772 change from time to time as ACs and/or remote PEs either become 1773 interested in, or lose interest in, particular multicast flows. 1775 4.1.2. Data Plane: Applying the OIF List to an (S,G) Frame 1777 When an (S,G) multicast frame is received by the layer 2 multicast 1778 function of a given EVPN-PE, say PE1, its disposition depends (a) the 1779 way it was received, (b) upon the OIF list of the corresponding 1780 multicast state (see Section 4.1.1), (c) upon the "eligibility" of an 1781 AC to receive a given frame (see Section 4.1.2.1 and (d) upon its 1782 apparent source BD (see Section 3.2 for information about determining 1783 the apparent source BD of a frame received over a tunnel from another 1784 PE). 1786 4.1.2.1. Eligibility of an AC to Receive a Frame 1788 A given (S,G) multicast frame is eligible to be transmitted by a 1789 given PE, say PE1, on a given AC, say AC1, only if one of the 1790 following conditions holds: 1792 1. ESI labels are being used, PE1 is the DF for the segment to which 1793 AC1 is connected, and the frame did not originate from that same 1794 segment (as determined by the ESI label), or 1796 2. The ingress PE for the frame is a remote PE, say PE2, local bias 1797 is being used, and PE2 is not connected to the same segment as 1798 AC1. 1800 4.1.2.2. Applying the OIF List 1802 Assume a given (S,G) multicast frame has been received by a given PE, 1803 say PE1. PE1 determines the apparent source BD of the frame, finds 1804 the layer 2 (S,G) state for that BD (or the (*,G) state if there is 1805 no (S,G) state), and takes the OIF list from that state. (Note that 1806 if PE1 is not attached to the actual source BD, the apparent source 1807 BD will be the SBD.) 1808 Suppose PE1 has determined the frame's apparent source BD to be BD1 1809 (which may or may not be the SBD.) There are the following cases to 1810 consider: 1812 1. The frame was received by PE1 from a local AC, say AC1, that 1813 attaches to BD1. 1815 a. The frame MUST be sent out all local ACs of BD1 that appear 1816 in the OIF list, except for AC1 itself. 1818 b. The frame MUST also be delivered to any other EVPN-PEs that 1819 have interest in it. This is achieved as follows: 1821 i. If (a) AR is being used, and (b) PE1 is an AR-LEAF, and 1822 (c) the OIF list is non-null, PE1 MUST send the frame 1823 to the AR-REPLICATOR. 1825 ii. Otherwise the frame MUST be sent on all tunnels in the 1826 OIF list. 1828 c. The frame MUST be sent to the local L3 routing instance by 1829 being sent up the IRB interface of BD1. It MUST NOT be sent 1830 up any other IRB interfaces. 1832 2. The frame was received by PE1 over a tunnel from another PE. 1833 (See Section 3.2 for the rules to determine the apparent source 1834 BD of a packet received from another PE. Note that if PE1 is not 1835 attached to the source BD, it will regard the SBD as the apparent 1836 source BD.) 1838 a. The frame MUST be sent out all local ACs in the OIF list that 1839 connect to BD1 and that are eligible (per Section 4.1.2.1) to 1840 receive the frame. 1842 b. The frame MUST be sent up the IRB interface of the apparent 1843 source BD. (Note that this may be the SBD.) The frame MUST 1844 NOT be sent up any other IRB interfaces. 1846 c. If PE1 is not an AR-REPLICATOR, it MUST NOT send the frame to 1847 any other EVPN-PEs. However, if PE1 is an AR-REPLICATOR, it 1848 MUST send the frame to all tunnels in the OIF list, except 1849 for the tunnel over which the frame was received. 1851 3. The frame was received by PE1 from the BD1 IRB interface (i.e., 1852 the frame has been transmitted by PE1's L3 routing instance down 1853 the BD1 IRB interface), and BD1 is NOT the SBD. 1855 a. The frame MUST be sent out all local ACs in the OIF list that 1856 are eligible (per Section 4.1.2.1 to receive the frame. 1858 b. The frame MUST NOT be sent to any other EVPN-PEs. 1860 c. The frame MUST NOT be sent up any IRB interfaces. 1862 4. The frame was received from the SBD IRB interface (i.e., has been 1863 transmitted by PE1's L3 routing instance down the SBD IRB 1864 interface). 1866 a. The frame MUST be sent on all tunnels in the OIF list. This 1867 causes the frame to be delivered to any other EVPN-PEs that 1868 have interest in it. 1870 b. The frame MUST NOT be sent on any local ACs. 1872 c. The frame MUST NOT be sent up any IRB interfaces. 1874 4.2. Layer 3 Forwarding State 1876 If an EVPN-PE is performing IGMP/MLD procedures on the ACs of a given 1877 BD, it processes those messages at layer 2 to help form the layer 2 1878 multicast state. If also sends those messages up that BD's IRB 1879 interface to the L3 routing instance of a particular tenant domain. 1880 This causes layer 2 (C-S,C-G) or (C-*,C-G) L3 state to be created/ 1881 updated. 1883 A layer 3 multicast state has both an Input Interface (IIF) and an 1884 OIF list. 1886 To set the IIF of an (C-S,C-G) state, the EVPN-PE must determine the 1887 source BD of C-S. This is done by looking up S in the local 1888 MAC-VRF(s) of the given Tenant Domain. 1890 If the source BD is present on the PE, the IIF is set to the IRB 1891 interface that attaches to that BD. Otherwise the IIF is set to the 1892 SBD IRB interface. 1894 For (C-*,C-G) states, traffic can arrive from any BD, so the IIF 1895 needs to be set to a wildcard value meaning "any IRB interface". 1897 The OIF list of these states includes one or more of the IRB 1898 interfaces of the Tenant Domain. In general, maintenance of the OIF 1899 list does not require any EVPN-specific procedures. However, there 1900 is one EVPN-specific rule: 1902 If the IIF is one of the IRB interfaces (or the wild card meaning 1903 "any IRB interface"), then the SBD IRB interface MUST NOT be added 1904 to the OIF list. Traffic originating from within a particular 1905 EVPN Tenant Domain must not be sent down the SBD IRB interface, as 1906 such traffic has already been distributed to all EVPN-PEs attached 1907 to that Tenant Domain. 1909 Please also see Section 6.1.1, which states a modification of this 1910 rule for the case where OISM is interworking with external Layer 3 1911 multicast routing. 1913 5. Interworking with non-OISM EVPN-PEs 1915 It is possible that a given Tenant Domain will be attached to both 1916 OISM PEs and non-OISM PEs. Inter-subnet IP multicast should be 1917 possible and fully functional even if not all PEs attaching to a 1918 Tenant Domain can be upgraded to support OISM functionality. 1920 Note that the non-OISM PEs are not required to have IRB support, or 1921 support for [IGMP-Proxy]. It is however advantageous for the 1922 non-OISM PEs to support [IGMP-Proxy]. 1924 In this section, we will use the following terminology: 1926 o PE-S: the ingress PE for an (S,G) flow. 1928 o PE-R: an egress PE for an (S,G) flow. 1930 o BD-S: the source BD for an (S,G) flow. PE-S must have one or more 1931 ACs attached BD-S, at least one of which attaches to host S. 1933 o BD-R: a BD that contains a host interested in the flow. The host 1934 is attached to PE-R via an AC that belongs to BD-R. 1936 To allow OISM PEs to interwork with non-OISM PEs, a given Tenant 1937 Domain needs to contain one or more "IP Multicast Gateways" (IPMGs). 1938 An IPMG is an OISM PE with special responsibilities regarding the 1939 interworking between OISM and non-OISM PEs. 1941 If a PE is functioning as an IPMG, it MUST signal this fact by 1942 setting the "IPMG" flag in the Multicast Flags EC that it attaches to 1943 its IMET routes. An IPMG SHOULD attach this EC with the IPMG flag 1944 set to all IMET routes it originates. However, if PE1 imports any 1945 IMET route from PE2 that has the EC present with the "IPMG" flag set, 1946 then the PE1 will assume that PE2 is an IPMG. 1948 An IPMG Designated Forwarder (IPMG-DF) selection procedure is used to 1949 ensure that, at any given time, there is exactly one active IPMG-DF 1950 for any given BD. Details of the IPMG-DF selection procedure are in 1951 Section 5.1. The IPMG-DF for a given BD, say BD-S, has special 1952 functions to perform when it receives (S,G) frames on that BD: 1954 o If the frames are from a non-OISM PE-S: 1956 * The IPMG-DF forwards them to OISM PEs that do not attach to 1957 BD-S but have interest in (S,G). 1959 Note that OISM PEs that do attach to BD-S will have received 1960 the frames on the BUM tunnel from the non-OISM PE-S. 1962 * The IPMG-DF forwards them to non-OISM PEs that have interest in 1963 (S,G) on ACs that do not belong to BD-S. 1965 Note that if a non-OISM PE has multiple BDs other than BD-S 1966 with interest in (S,G), it will receive one copy of the frame 1967 for each such BD. This is necessary because the non-OISM PEs 1968 cannot move IP multicast traffic from one BD to another. 1970 o If the frames are from an OISM PE, the IPMG-DF forwards them to 1971 non-OISM PEs that have interest in (S,G) on ACs that do not belong 1972 to BD-S. 1974 If a non-OISM PE has interest in (S,G) on an AC belonging to BD-S, 1975 it will have received a copy of the (S,G) frame, encapsulated for 1976 BD-S, from the OISM PE-S. (See Section 3.2.2.) If the non-OISM 1977 PE has interest in (S,G) on one or more ACs belonging to 1978 BD-R1,...,BD-Rk where the BD-Ri are distinct from BD-S, the 1979 IPMG-DF needs to send it a copy of the frame for BD-Ri. 1981 If an IPMG receives a frame on a BD for which it is not the IPMG-DF, 1982 it just follows normal OISM procedures. 1984 This section specifies several sets of procedures: 1986 o the procedures that the IPMG-DF for a given BD needs to follow 1987 when receiving, on that BD, an IP multicast frame from a non-OISM 1988 PE; 1990 o the procedures that the IPMG-DF for a given BD needs to follow 1991 when receiving, on that BD, an IP multicast frame from an OISM PE; 1993 o the procedures that an OISM PE needs to follow when receiving, on 1994 a given BD, an IP multicast frame from a non-OISM PE, when the 1995 OISM PE is not the IPMG-DF for that BD. 1997 To enable OISM/non-OISM interworking in a given Tenant Domain, the 1998 Tenant Domain MUST have some EVPN-PEs that can function as IPMGs. An 1999 IPMG must be configured with the SBD. It must also be configured 2000 with every BD of the Tenant Domain that exists on any of the non-OISM 2001 PEs of that domain. (Operationally, it may be simpler to configure 2002 the IPMG with all the BDs of the Tenant Domain.) 2004 A non-OISM PE of course only needs to be configured with BDs for 2005 which it has ACs. An OISM PE that is not an IPMG only needs to be 2006 configured with the SBD and with the BDs for which it has ACs. 2008 An IPMG MUST originate a wildcard SMET route (with (C-*,C-*) in the 2009 NLRI) for each BD in the Tenant Domain. This will cause it to 2010 receive all the IP multicast traffic that is sourced in the Tenant 2011 Domain. Note that non-OISM nodes that do not support [IGMP-Proxy] 2012 will send all the multicast traffic from a given BD to all PEs 2013 attached to that BD, even if those PEs do not originate an SMET 2014 route. 2016 The interworking procedures vary somewhat depending upon whether 2017 packets are transmitted from PE to PE via Ingress Replication (IR) or 2018 via Point-to-Multipoint (P2MP) tunnels. We do not consider the use 2019 of BIER in this section, due to the low likelihood of there being a 2020 non-OISM PE that supports BIER. 2022 5.1. IPMG Designated Forwarder 2024 Every PE that is eligible for selection as an IPMG-DF for a 2025 particular BD originates both an IMET route for that BD and an 2026 SBD-IMET route. As stated in Section 5, these SBD-IMET routes carry 2027 a Multicast Flags EC with the IPMG Flag set. 2029 These SBD-IMET routes SHOULD also carry a DF Election EC. The DF 2030 Election EC and its use is specified in ([DF-Election-Framework]). 2031 When the route is originated, the AC-DF bit in the DF Election EC 2032 SHOULD be set to zero. This bit is not used when selecting an 2033 IPMSG-DF, i.e., it MUST be ignored by the receiver of an SBD-IMET 2034 route. 2036 In the context of a given Tenant Domain, to select the IPMG-DF for a 2037 particular BD, say BD1, the IPMGs of the Tenant Domain perform the 2038 following procedure: 2040 o From the set of received SBD-IMET routes for the given tenant 2041 domain, determine the candidate set of PEs that support IPMG 2042 functionality for that domain. 2044 o Eliminate from that candidate set any PEs from which an IMET route 2045 for BD1 has not been received. 2047 o Select a DF Election algorithm as specified in 2048 [DF-Election-Framework]. Some of the possible algorithms can be 2049 found, e.g., in [DF-Election-Framework], [RFC7432], and 2050 [EVPN-DF-WEIGHTED]. 2052 o Apply the DF Election Algorithm (see [DF-Election-Framework]) to 2053 the candidate set of PEs. The "winner' becomes the IPMG-DF for 2054 BD1. 2056 Note that even if a given PE supports MEG (Section 6.1.2) and/or PEG 2057 (Section 6.1.4) functionality, as well as IPMG functionality, its 2058 SBD-IMET routes carry only one DF Election EC. 2060 5.2. Ingress Replication 2062 The procedures of this section are used when Ingress Replication is 2063 used to transmit packets from one PE to another. 2065 When a non-OISM PE-S transmits a multicast frame from BD-S to another 2066 PE, PE-R, PE-S will use the encapsulation specified in the BD-S IMET 2067 route that was originated by PE-R. This encapsulation will include 2068 the label that appears in the "MPLS label" field of the PMSI Tunnel 2069 attribute (PTA) of the IMET route. If the tunnel type is VXLAN, the 2070 "label" is actually a Virtual Network Identifier (VNI); for other 2071 tunnel types, the label is an MPLS label. In either case, we will 2072 speak of the transmitted frames as carrying a label that was assigned 2073 to a particular BD by the PE-R to which the frame is being 2074 transmitted. 2076 To support OISM/non-OISM interworking, an OISM PE-R MUST originate, 2077 for each of its BDs, both an IMET route and an S-PMSI (C-*,C-*) A-D 2078 route. Note that even when IR is being used, interworking between 2079 OISM and non-OISM PEs requires the OISM PEs to follow the rules of 2080 Section 3.2.5.2, as modified below. 2082 Non-OISM PEs will not understand S-PMSI A-D routes. So when a 2083 non-OISM PE-S transmits an IP multicast frame with a particular 2084 source BD to an IPMG, it encapsulates the frame using the label 2085 specified in that IPMG's BD-S IMET route. (This is just the 2086 procedure of [RFC7432].) 2088 The (C-*,C-*) S-PMSI A-D route originated by a given OISM PE will 2089 have a PTA that specifies IR. 2091 o If MPLS tunneling is being used, the MPLS label field SHOULD 2092 contain a non-zero value, and the LIR flag SHOULD be zero. (The 2093 case where the MPLS label field is zero or the LIR flag is set is 2094 outside the scope of this document.) 2096 o If the tunnel encapsulation is VXLAN, the MPLS label field MUST 2097 contain a non-zero value, and the LIR flag MUST be zero. 2099 When an OISM PE-S transmits an IP multicast frame to an IPMG, it will 2100 use the label specified in that IPMG's (C-*,C-*) S-PMSI A-D route. 2102 When a PE originates both an IMET route and a (C-*,C-*) S-PMSI A-D 2103 route, the values of the MPLS label field in the respective PTAs must 2104 be distinct. Further, each MUST map uniquely (in the context of the 2105 originating PE) to the route's BD. 2107 As a result, an IPMG receiving an MPLS-encapsulated IP multicast 2108 frame can always tell by the label whether the frame's ingress PE is 2109 an OISM PE or a non-OISM PE. When an IPMG receives a VXLAN- 2110 encapsulated IP multicast frame it may need to determine the identity 2111 of the ingress PE from the outer IP encapsulation; it can then 2112 determine whether the ingress PE is an OISM PE or a non-OISM PE by 2113 looking the IMET route from that PE. 2115 Suppose an IPMG receives an IP multicast frame from another EVPN-PE 2116 in the Tenant Domain, and the IPMG is not the IPMG-DF for the frame's 2117 source BD. Then the IPMG performs only the ordinary OISM functions; 2118 it does not perform the IPMG-specific functions for that frame. In 2119 the remainder of this section, when we discuss the procedures applied 2120 by an IPMG when it receives an IP multicast frame, we are presuming 2121 that the source BD of the frame is a BD for which the IPMG is the 2122 IPMG-DF. 2124 We have two basic cases to consider: (1) a frame's ingress PE is a 2125 non-OISM node, and (2) a frame's ingress PE is an OISM node. 2127 5.2.1. Ingress PE is non-OISM 2129 In this case, a non-OISM PE, PE-S, has received an (S,G) multicast 2130 frame over an AC that is attached to a particular BD, BD-S. By 2131 virtue of normal EVPN procedures, PE-S has sent a copy of the frame 2132 to every PE-R (both OISM and non-OISM) in the Tenant Domain that is 2133 attached to BD-S. If the non-OISM node supports [IGMP-Proxy], only 2134 PEs that have expressed interest in (S,G) receive the frame. The 2135 IPMG will have expressed interest via a (C-*,C-*) SMET route and thus 2136 receives the frame. 2138 Any OISM PE (including an IPMG) receiving the frame will apply normal 2139 OISM procedures. As a result it will deliver the frame to any of its 2140 local ACs (in BD-S or in any other BD) that have interest in (S,G). 2142 An OISM PE that is also the IPMG-DF for a particular BD, say BD-S, 2143 has additional procedures that it applies to frames received on BD-S 2144 from non-OISM PEs: 2146 1. When the IPMG-DF for BD-S receives an (S,G) frame from a 2147 non-OISM node, it MUST forward a copy of the frame to every OISM 2148 PE that is NOT attached to BD-S but has interest in (S,G). The 2149 copy sent to a given OISM PE-R must carry the label that PE-R 2150 has assigned to the SBD in an S-PMSI A-D route. The IPMG MUST 2151 NOT do any IP processing of the frame's IP payload. TTL 2152 decrement and other IP processing will be done by PE-R, per the 2153 normal OISM procedures. There is no need for the IPMG to 2154 include an ESI label in the frame's tunnel encapsulation, 2155 because it is already known that the frame's source BD has no 2156 presence on PE-R. There is also no need for the IPMG to modify 2157 the frame's MAC SA. 2159 2. In addition, when the IPMG-DF for BD-S receives an (S,G) frame 2160 from a non-OISM node, it may need to forward copies of the frame 2161 to other non-OISM nodes. Before it does so, it MUST decapsulate 2162 the (S,G) packet, and do the IP processing (e.g., TTL 2163 decrement). Suppose PE-R is a non-OISM node that has an AC to 2164 BD-R, where BD-R is not the same as BD-S, and that AC has 2165 interest in (S,G). The IPMG must then encapsulate the (S,G) 2166 packet (after the IP processing has been done) in an ethernet 2167 header. The MAC SA field will have the MAC address of the 2168 IPMG's IRB interface to BD-R. The IPMG then sends the frame to 2169 PE-R. The tunnel encapsulation will carry the label that PE-R 2170 advertised in its IMET route for BD-R. There is no need to 2171 include an ESI label, as the source and destination BDs are 2172 known to be different. 2174 Note that if a non-OISM PE-R has several BDs (other than BD-S) 2175 with local ACs that have interest in (S,G), the IPMG will send 2176 it one copy for each such BD. This is necessary because the 2177 non-OISM PE cannot move packets from one BD to another. 2179 There may be deployment scenarios in which every OISM PE is 2180 configured with every BD that is present on any non-OISM PE. In such 2181 scenarios, the procedures of item 1 above will not actually result in 2182 the transmission of any packets. Hence if it is known a priori that 2183 this deployment scenario exists for a given tenant domain, the 2184 procedures of item 1 above can be disabled. 2186 5.2.2. Ingress PE is OISM 2188 In this case, an OISM PE, PE-S, has received an (S,G) multicast frame 2189 over an AC that attaches to a particular BD, BD-S. 2191 By virtue of receiving all the IMET routes about BD-S, PE-S will know 2192 all the PEs attached to BD-S. By virtue of normal OISM procedures: 2194 o PE-S will send a copy of the frame to every OISM PE-R (including 2195 the IPMG) in the Tenant Domain that is attached to BD-S and has 2196 interest in (S,G). The copy sent to a given PE-R carries the 2197 label that that the PE-R has assigned to BD-S in its (C-*,C-*) 2198 S-PMSI A-D route. 2200 o PE-S will also transmit a copy of the (S,G) frame to every OISM 2201 PE-R that has interest in (S,G) but is not attached to BD-S. The 2202 copy will contain the label that the PE-R has assigned to the SBD. 2203 (As in Section 5.2.1, an IPMG is assumed to have indicated 2204 interest in all multicast flows.) 2206 o PE-S will also transmit a copy of the (S,G) frame to every 2207 non-OISM PE-R that is attached to BD-S. It does this using the 2208 label advertised by that PE-R in its IMET route for BD-S. 2210 The PE-Rs follow their normal procedures. An OISM PE that receives 2211 the (S,G) frame on BD-S applies the OISM procedures to deliver the 2212 frame to its local ACs, as necessary. A non-OISM PE that receives 2213 the (S,G) frame on BD-S delivers the frame only to its local BD-S 2214 ACs, as necessary. 2216 Suppose that a non-OISM PE-R has interest in (S,G) on a BD, BD-R, 2217 that is different than BD-S. If the non-OISM PE-R is attached to 2218 BD-S, the OISM PE-S will send forward it the original (S,G) multicast 2219 frame, but the non-OISM PE-R will not be able to send the frame to 2220 ACs that are not in BD-S. If PE-R is not even attached to BD-S, the 2221 OISM PE-S will not send it a copy of the frame at all, because PE-R 2222 is not attached to the SBD. In these cases, the IPMG needs to relay 2223 the (S,G) multicast traffic from OISM PE-S to non-OISM PE-R. 2225 When the IPMG-DF for BD-S receives an (S,G) frame from an OISM PE-S, 2226 it has to forward it to every non-OISM PE-R that that has interest in 2227 (S,G) on a BD-R that is different than BD-S. The IPMG MUST 2228 decapsulate the IP multicast packet, do the IP processing, re- 2229 encapsulate it for BD-R (changing the MAC SA to the IPMG's own MAC 2230 address on BD-R), and send a copy of the frame to PE-R. Note that a 2231 given non-OISM PE-R will receive multiple copies of the frame, if it 2232 has multiple BDs on which there is interest in the frame. 2234 5.3. P2MP Tunnels 2236 When IR is used to distribute the multicast traffic among the 2237 EVPN-PEs, the procedures of Section 5.2 ensure that there will be no 2238 duplicate delivery of multicast traffic. That is, no egress PE will 2239 ever send a frame twice on any given AC. If P2MP tunnels are being 2240 used to distribute the multicast traffic, it is necessary have 2241 additional procedures to prevent duplicate delivery. 2243 At the present time, it is not clear that there will be a use case in 2244 which OISM nodes need to interwork with non-OISM nodes that use P2MP 2245 tunnels. If it is determined that there is such a use case, 2246 procedures for it will be included in a future revision of this 2247 document. 2249 6. Traffic to/from Outside the EVPN Tenant Domain 2251 In this section, we discuss scenarios where a multicast source 2252 outside a given EVPN Tenant Domain sends traffic to receivers inside 2253 the domain (as well as, possibly, to receivers outside the domain). 2254 This requires the OISM procedures to interwork with various layer 3 2255 multicast routing procedures. 2257 We assume in this section that the Tenant Domain is not being used as 2258 an intermediate transit network for multicast traffic; that is, we do 2259 not consider the case where the Tenant Domain contains multicast 2260 routers that will receive traffic from sources outside the domain and 2261 forward the traffic to receivers outside the domain. The transit 2262 scenario is considered in Section 7. 2264 We can divide the non-transit scenarios into two classes: 2266 1. One or more of the EVPN PE routers provide the functionality 2267 needed to interwork with layer 3 multicast routing procedures. 2269 2. A single BD in the Tenant Domain contains external multicast 2270 routers ("tenant multicast routers"), and those tenant multicast 2271 routers are used to interwork, on behalf of the entire Tenant 2272 Domain, with layer 3 multicast routing procedures. 2274 6.1. Layer 3 Interworking via EVPN OISM PEs 2276 6.1.1. General Principles 2278 Sometimes it is necessary to interwork an EVPN Tenant Domain with an 2279 external layer 3 multicast domain (the "external domain"). This is 2280 needed to allow EVPN tenant systems to receive multicast traffic from 2281 sources ("external sources") outside the EVPN Tenant Domain. It is 2282 also needed to allow receivers ("external receivers") outside the 2283 EVPN Tenant Domain to receive traffic from sources inside the Tenant 2284 Domain. 2286 In order to allow interworking between an EVPN Tenant Domain and an 2287 external domain, one or more OISM PEs must be "L3 Gateways". An L3 2288 Gateway participates both in the OISM procedures and in the L3 2289 multicast routing procedures of the external domain. 2291 An L3 Gateway that has interest in receiving (S,G) traffic must be 2292 able to determine the best route to S. If an L3 Gateway has interest 2293 in (*,G), it must be able to determine the best route to G's RP. In 2294 these interworking scenarios, the L3 Gateway must be running a layer 2295 3 unicast routing protocol. Via this protocol, it imports unicast 2296 routes (either IP routes or VPN-IP routes) from routers other than 2297 EVPN PEs. And since there may be multicast sources inside the EVPN 2298 Tenant Domain, the EVPN PEs also need to export, either as IP routes 2299 or as VPN-IP routes (depending upon the external domain), unicast 2300 routes to those sources. 2302 When selecting the best route to a multicast source or RP, an L3 2303 Gateway might have a choice between an EVPN route and an IP/VPN-IP 2304 route. When such a choice exists, the L3 Gateway SHOULD always 2305 prefer the EVPN route. This will ensure that when traffic originates 2306 in the Tenant Domain and has a receiver in the Tenant Domain, the 2307 path to that receiver will remain within the EVPN Tenant Domain, even 2308 if the source is also reachable via a routed path. This also 2309 provides protection against sub-optimal routing that might occur if 2310 two EVPN PEs export IP/VPN-IP routes and each imports the other's IP/ 2311 VPN-IP routes. 2313 Section 4.2 discusses the way layer 3 multicast states are 2314 constructed by OISM PEs. These layer 3 multicast states have IRB 2315 interfaces as their IIF and OIF list entries, and are the basis for 2316 interworking OISM with other layer 3 multicast procedures such as 2317 MVPN or PIM. From the perspective of the layer 3 multicast 2318 procedures running in a given L3 Gateway, an EVPN Tenant Domain is a 2319 set of IRB interfaces. 2321 When interworking an EVPN Tenant Domain with an external domain, the 2322 L3 Gateway's layer 3 multicast states will not only have IRB 2323 interfaces as IIF and OIF list entries, but also other "interfaces" 2324 that lead outside the Tenant Domain. For example, when interworking 2325 with MVPN, the multicast states may have MVPN tunnels as well as IRB 2326 interfaces as IIF or OIF list members. When interworking with PIM, 2327 the multicast states may have PIM-enabled non-IRB interfaces as IIF 2328 or OIF list members. 2330 As long as a Tenant Domain is not being used as an intermediate 2331 transit network for IP multicast traffic, it is not necessary to 2332 enable PIM on its IRB interfaces. 2334 In general, an L3 Gateway has the following responsibilities: 2336 o It exports, to the external domain, unicast routes to those 2337 multicast sources in the EVPN Tenant Domain that are locally 2338 attached to the L3 Gateway. 2340 o It imports, from the external domain, unicast routes to multicast 2341 sources that are in the external domain. 2343 o It executes the procedures necessary to draw externally sourced 2344 multicast traffic that is of interest to locally attached 2345 receivers in the EVPN Tenant Domain. When such traffic is 2346 received, the traffic is sent down the IRB interfaces of the BDs 2347 on which the locally attached receivers reside. 2349 One of the L3 Gateways in a given Tenant Domain becomes the "DR" for 2350 the SBD. (See Section 6.1.2.4.) This L3 gateway has the following 2351 additional responsibilities: 2353 o It exports, to the external domain, unicast routes to multicast 2354 sources that in the EVPN Tenant Domain that are not locally 2355 attached to any L3 gateway. 2357 o It imports, from the external domain, unicast routes to multicast 2358 sources that are in the external domain. 2360 o It executes the procedures necessary to draw externally sourced 2361 multicast traffic that is of interest to receivers in the EVPN 2362 Tenant Domain that are not locally attached to an L3 gateway. 2363 When such traffic is received, the traffic is sent down the SBD 2364 IRB interface. OISM procedures already described in this document 2365 will then ensure that the IP multicast traffic gets distributed 2366 throughout the Tenant Domain to any EVPN PEs that have interest in 2367 it. Thus to an OISM PE that is not an L3 gateway the externally 2368 sourced traffic will appear to have been sourced on the SBD. 2370 In order for this to work, some special care is needed when an L3 2371 gateway creates or modifies a layer 3 (*,G) multicast state. Suppose 2372 group G has both external sources (sources outside the EVPN Tenant 2373 Domain) and internal sources (sources inside the EVPN tenant domain). 2374 Section 4.2 states that when there are internal sources, the SBD IRB 2375 interface must not be added to the OIF list of the (*,G) state. 2376 Traffic from internal sources will already have been delivered to all 2377 the EVPN PEs that have interest in it. However, if the OIF list of 2378 the (*,G) state does not contain its SBD IRB interface, then traffic 2379 from external sources will not get delivered to other EVPN PEs. 2381 One way of handling this is the following. When a L3 gateway 2382 receives (S,G) traffic from other than an IRB interface, and the 2383 traffic corresponds to a layer 3 (*,G) state, the L3 gateway can 2384 create (S,G) state. The IIF will be set to the external interface 2385 over which the traffic is expected. The OIF list will contain the 2386 SBD IRB interface, as well as the IRB interfaces of any other BDs 2387 attached to the PEG DR that have locally attached receivers with 2388 interest in the (S,G) traffic. The (S,G) state will ensure that the 2389 external traffic is sent down the SBD IRB interface. The following 2390 text will assume this procedure; however other implementation 2391 techniques may also be possible. 2393 If a particular BD is attached to several L3 Gateways, one of the L3 2394 Gateways becomes the DR for that BD. (See Section 6.1.2.4.) If the 2395 interworking scenario requires FHR functionality, it is generally the 2396 DR for a particular BD that is responsible for performing that 2397 functionality on behalf of the source hosts on that BD. (E.g., if 2398 the interworking scenario requires that PIM Register messages be sent 2399 by a FHR, the DR for a given BD would send the PIM Register messages 2400 for sources on that BD.) Note though that the DR for the SBD does 2401 not perform FHR functionality on behalf of external sources. 2403 An optional alternative is to have each L3 gateway perform FHR 2404 functionality for locally attached sources. Then the DR would only 2405 have to perform FHR functionality on behalf of sources that are 2406 locally attached to itself AND sources that are not attached to any 2407 L3 gateway. 2409 N.B.: If it is possible that more than one BD contains a tenant 2410 multicast router, then a PE receiving an SMET route for that BD MUST 2411 NOT reconstruct IGMP Join Reports from the SMET route, and MUST NOT 2412 transmit any such IGMP Join Reports on its local ACs attaching to 2413 that BD. Otherwise, multicast traffic may be duplicated. 2415 6.1.2. Interworking with MVPN 2417 In this section, we specify the procedures necessary to allow EVPN 2418 PEs running OISM procedures to interwork with L3VPN PEs that run BGP- 2419 based MVPN ([RFC6514]) procedures. More specifically, the procedures 2420 herein allow a given EVPN Tenant Domain to become part of an L3VPN/ 2421 MVPN, and support multicast flows where either: 2423 o The source of a given multicast flow is attached to an ethernet 2424 segment whose BD is part of an EVPN Tenant Domain, and one or more 2425 receivers of the flow are attached to the network via L3VPN/MVPN. 2426 (Other receivers may be attached to the network via EVPN.) 2428 o The source of a given multicast flow is attached to the network 2429 via L3VPN/MVPN, and one or more receivers of the flow are attached 2430 to an ethernet segment that is part of an EVPN tenant domain. 2431 (Other receivers may be attached via L3VPN/MVPN.) 2433 In this interworking model, existing L3VPN/MVPN PEs are unaware that 2434 certain sources or receivers are part of an EVPN Tenant Domain. The 2435 existing L3VPN/MVPN nodes run only their standard procedures and are 2436 entirely unaware of EVPN. Interworking is achieved by having some or 2437 all of the EVPN PEs function as L3 Gateways running L3VPN/MVPN 2438 procedures, as detailed in the following sub-sections. 2440 In this section, we assume that there are no tenant multicast routers 2441 on any of the EVPN-attached ethernet segments. (There may of course 2442 be multicast routers in the L3VPN.) Consideration of the case where 2443 there are tenant multicast routers is deferred till Section 7.) 2445 To support MVPN/EVPN interworking, we introduce the notion of an 2446 MVPN/EVPN Gateway, or MEG. 2448 A MEG is an L3 Gateway (see Section 6.1.1), hence is both an OISM PE 2449 and an L3VPN/MVPN PE. For a given EVPN Tenant Domain it will have an 2450 IP-VRF. If the Tenant Domain is part of an L3VPN/MVPN, the IP-VRF 2451 also serves as an L3VPN VRF ([RFC4364]). The IRB interfaces of the 2452 IP-VRF are considered to be "VRF interfaces" of the L3VPN VRF. The 2453 L3VPN VRF may also have other local VRF interfaces that are not EVPN 2454 IRB interfaces. 2456 The VRF on the MEG will import VPN-IP routes ([RFC4364]) from other 2457 L3VPN Provider Edge (PE) routers. It will also export VPN-IP routes 2458 to other L3VPN PE routers. In order to do so, it must be 2459 appropriately configured with the Route Targets used in the L3VPN to 2460 control the distribution of the VPN-IP routes. These Route Targets 2461 will in general be different than the Route Targets used for 2462 controlling the distribution of EVPN routes, as there is no need to 2463 distribute EVPN routes to L3VPN-only PEs and no reason to distribute 2464 L3VPN/MVPN routes to EVPN-only PEs. 2466 Note that the RDs in the imported VPN-IP routes will not necessarily 2467 conform to the EVPN rules (as specified in [RFC7432]) for creating 2468 RDs. Therefore a MEG MUST NOT expect the RDs of the VPN-IP routes to 2469 be of any particular format other than what is required by the L3VPN/ 2470 MVPN specifications. 2472 The VPN-IP routes that a MEG exports to L3VPN are subnet routes and/ 2473 or host routes for the multicast sources that are part of the EVPN 2474 tenant domain. The exact set of routes that need to be exported is 2475 discussed in Section 6.1.2.2. 2477 Each IMET route originated by a MEG SHOULD carry a Multicast Flags 2478 Extended Community with the "MEG" flag set, indicating that the 2479 originator of the IMET route is a MEG. However, PE1 will consider 2480 PE2 to be a MEG if PE1 imports at least one IMET route from PE2 that 2481 carries the Multicast Flags EC with the MEG flag set. 2483 All the MEGs of a given Tenant Domain attach to the SBD of that 2484 domain, and one of them is selected to be the SBD's Designated Router 2485 (the "MEG SBD-DR") for the domain. The selection procedure is 2486 discussed in Section 6.1.2.4. 2488 In this model of operation, MVPN procedures and EVPN procedures are 2489 largely independent. In particular, there is no assumption that MVPN 2490 and EVPN use the same kind of tunnels. Thus no special procedures 2491 are needed to handle the common scenarios where, e.g., EVPN uses 2492 VXLAN tunnels but MVPN uses MPLS P2MP tunnels, or where EVPN uses 2493 Ingress Replication but MVPN uses MPLS P2MP tunnels. 2495 Similarly, no special procedures are needed to prevent duplicate data 2496 delivery on ethernet segments that are multi-homed. 2498 The MEG does have some special procedures (described below) for 2499 interworking between EVPN and MVPN; these have to do with selection 2500 of the Upstream PE for a given multicast source, with the exporting 2501 of VPN-IP routes, and with the generation of MVPN C-multicast routes 2502 triggered by the installation of SMET routes. 2504 6.1.2.1. MVPN Sources with EVPN Receivers 2506 6.1.2.1.1. Identifying MVPN Sources 2508 Consider a multicast source S. It is possible that a MEG will import 2509 both an EVPN unicast route to S and a VPN-IP route (or an ordinary IP 2510 route), where the prefix length of each route is the same. In order 2511 to draw (S,G) multicast traffic for any group G, the MEG SHOULD use 2512 the EVPN route rather than the VPN-IP or IP route to determine the 2513 "Upstream PE" (see section 5 of [RFC6513]). 2515 Doing so ensures that when an EVPN tenant system desires to receive a 2516 multicast flow from another EVPN tenant system, the traffic from the 2517 source to that receiver stays within the EVPN domain. This prevents 2518 problems that might arise if there is a unicast route via L3VPN to S, 2519 but no multicast routers along the routed path. This also prevents 2520 problem that might arise as a result of the fact that the MEGs will 2521 import each others' VPN-IP routes. 2523 In the Section 6.1.2.1.2, we describe the procedures to be used when 2524 the selected route to S is a VPN-IP route. 2526 6.1.2.1.2. Joining a Flow from an MVPN Source 2528 Consider a tenant system, R, on a particular BD, BD-R. Suppose R 2529 wants to receive (S,G) multicast traffic, where source S is not 2530 attached to any PE in the EVPN Tenant Domain, but is attached to an 2531 MVPN PE. 2533 o Suppose R is on a singly homed ethernet segment of BD-R, and that 2534 segment is attached to PE1, where PE1 is a MEG. PE1 learns via 2535 IGMP/MLD listening that R is interested in (S,G). PE1 determines 2536 from its VRF that there is no route to S within the Tenant Domain 2537 (i.e., no EVPN RT-2 route with S's IP address), but that there is 2538 a route to S via L3VPN (i.e., the VRF contains a subnet or host 2539 route to S that was received as a VPN-IP route). PE1 thus 2540 originates (if it hasn't already) an MVPN C-multicast Source Tree 2541 Join(S,G) route. The route is constructed according to normal 2542 MVPN procedures. 2544 The layer 2 multicast state is constructed as specified in 2545 Section 4.1. 2547 In the layer 3 multicast state, the IIF is the appropriate MVPN 2548 tunnel, and the IRB interface to BD-R is added to the OIF list. 2550 When PE1 receives (S,G) traffic from the appropriate MVPN tunnel, 2551 it performs IP processing of the traffic, and then sends the 2552 traffic down its IRB interface to BD-R. Following normal OISM 2553 procedures, the (S,G) traffic will be encapsulated for ethernet 2554 and sent out the AC to which R is attached. 2556 o Suppose R is on a singly homed ethernet segment of BD-R, and that 2557 segment is attached to PE1, where PE1 is an OISM PE but is NOT a 2558 MEG. PE1 learns via IGMP/MLD listening that R is interested in 2559 (S,G). PE1 follows normal OISM procedures, originating an SBD- 2560 SMET route for (S,G); this route will be received by all the MEGs 2561 of the Tenant Domain, including the MEG SBD-DR. The MEG SBD-DR 2562 can determine from PE1's IMET routes whether PE1 is itself a MEG. 2563 If PE1 is not a MEG, the MEG SBD-DR will originate (if it hasn't 2564 already) an MVPN C-multicast Source Tree Join(S,G) route. This 2565 will cause the MEG SBD-DR to receive (S,G) traffic on an MVPN 2566 tunnel. 2568 The layer 2 multicast state is constructed as specified in 2569 Section 4.1. 2571 In the layer 3 multicast state, the IIF is the appropriate MVPN 2572 tunnel, and the IRB interface to the SBD is added to the OIF list. 2574 When the MEG SBD-DR receives (S,G) traffic on an MVPN tunnel, it 2575 performs IP processing of the traffic, and the sends the traffic 2576 down its IRB interface to the SBD. Following normal OISM 2577 procedures, the traffic will be encapsulated for ethernet and 2578 delivered to all PEs in the Tenant Domain that have interest in 2579 (S,G), including PE1. 2581 o If R is on a multi-homed ethernet segment of BD-R, one of the PEs 2582 attached to the segment will be its DF (following normal EVPN 2583 procedures), and the DF will know (via IGMP/MLD listening or the 2584 procedures of [IGMP-Proxy]) that a tenant system reachable via one 2585 of its local ACs to BD-R is interested in (S,G) traffic. The DF 2586 is responsible for originating an SBD-SMET route for (S,G), 2587 following normal OISM procedures. If the DF is a MEG, it MUST 2588 originate the corresponding MVPN C-multicast Source Tree Join(S,G) 2589 route; if the DF is not a MEG, the MEG SBD-DR SBD MUST originate 2590 the C-multicast route when it receives the SMET route. 2592 Optionally, if the non-DF is a MEG, it MAY originate the 2593 corresponding MVPN C-multicast Source Tree Join(S,G) route. This 2594 will cause the traffic to flow to both the DF and the non-DF, but 2595 only the DF will forward the traffic out an AC. This allows for 2596 quicker recovery if the DF's local AC to R fails. 2598 o If R is attached to a non-OISM PE, it will receive the traffic via 2599 an IPMG, as specified in Section 5. 2601 If an EVPN-attached receiver is interested in (*,G) traffic, and if 2602 it is possible for there to be sources of (*,G) traffic that are 2603 attached only to L3VPN nodes, the MEGs will have to know the group- 2604 to-RP mappings. That will enable them to originate MVPN C-multicast 2605 Shared Tree Join(*,G) routes and to send them towards the RP. (Since 2606 we are assuming in this section that there are no tenant multicast 2607 routers attached to the EVPN Tenant Domain, the RP must be attached 2608 via L3VPN. Alternatively, the MEG itself could be configured to 2609 function as an RP for group G.) 2611 The layer 2 multicast states are constructed as specified in 2612 Section 4.1. 2614 In the layer 3 (*,G) multicast state, the IIF is the appropriate MVPN 2615 tunnel. A MEG will add to the (*,G) OIF list its IRB interfaces for 2616 any BDs containing locally attached receivers. If there are 2617 receivers attached to other EVPN PEs, then whenever (S,G) traffic 2618 from an external source matches a (*,G) state, the MEG will create 2619 (S,G) state, with the MVPN tunnel as the IIF, the OIF list copied 2620 from the (*,G) state, and the SBD IRB interface added to the OIF 2621 list. (Please see the discussion in Section 6.1.1 regarding the 2622 inclusion of the SBD IRB interface in a (*,G) state; the SBD IRB 2623 interface is used in the OIF list only for traffic from external 2624 sources.) 2626 Normal MVPN procedures will then result in the MEG getting the (*,G) 2627 traffic from all the multicast sources for G that are attached via 2628 L3VPN. This traffic arrives on MVPN tunnels. When the MEG removes 2629 the traffic from these tunnels, it does the IP processing. If there 2630 are any receivers on a given BD, BD-R, that are attached via local 2631 EVPN ACs, the MEG sends the traffic down its BD-R IRB interface. If 2632 there are any other EVPN PEs that are interested in the (*,G) 2633 traffic, the MEG sends the traffic down the SBD IRB interface. 2634 Normal OISM procedures then distribute the traffic as needed to other 2635 EVPN-PEs. 2637 6.1.2.2. EVPN Sources with MVPN Receivers 2639 6.1.2.2.1. General procedures 2641 Consider the case where an EVPN tenant system S is sending IP 2642 multicast traffic to group G, and there is a receiver R for the (S,G) 2643 traffic that is attached to the L3VPN, but not attached to the EVPN 2644 Tenant Domain. (We assume in this document that the L3VPN/MVPN-only 2645 nodes will not have any special procedures to deal with the case 2646 where a source is inside an EVPN domain.) 2648 In this case, an L3VPN PE through which R can be reached has to send 2649 an MVPN C-multicast Join(S,G) route to one of the MEGs that is 2650 attached to the EVPN Tenant Domain. For this to happen, the L3VPN PE 2651 must have imported a VPN-IP route for S (either a host route or a 2652 subnet route) from a MEG. 2654 If a MEG determines that there is multicast source transmitting on 2655 one of its ACs, the MEG SHOULD originate a VPN-IP host route for that 2656 source. This determination SHOULD be made by examining the IP 2657 multicast traffic that arrives on the ACs. (It MAY be made by 2658 provisioning.) A MEG SHOULD NOT export a VPN-IP host route for any 2659 IP address that is not known to be a multicast source (unless it has 2660 some other reason for exporting such a route). The VPN-IP host route 2661 for a given multicast source MUST be withdrawn if the source goes 2662 silent for a configurable period of time, or if it can be determined 2663 that the source is no longer reachable via a local AC. 2665 A MEG SHOULD also originate a VPN-IP subnet route for each of the BDs 2666 in the Tenant Domain. 2668 VPN-IP routes exported by a MEG must carry any attributes or extended 2669 communities that are required by L3VPN and MVPN. In particular, a 2670 VPN-IP route exported by a MEG must carry a VRF Route Import Extended 2671 Community corresponding to the IP-VRF from which it is imported, and 2672 a Source AS Extended Community. 2674 As a result, if S is attached to a MEG, the L3VPN nodes will direct 2675 their MVPN C-multicast Join routes to that MEG. Normal MVPN 2676 procedures will cause the traffic to be delivered to the L3VPN nodes. 2677 The layer 3 multicast state for (S,G) will have the MVPN tunnel on 2678 its OIF list. The IIF will be the IRB interface leading to the BD 2679 containing S. 2681 If S is not attached to a MEG, the L3VPN nodes will direct their 2682 C-multicast Join routes to whichever MEG appears to be on the best 2683 route to S's subnet. Upon receiving the C-multicast Join, that MEG 2684 will originate an EVPN SMET route for (S,G). As a result, the MEG 2685 will receive the (S,G) traffic at layer 2 via the OISM procedures. 2686 The (S,G) traffic will be sent up the appropriate IRB interface, and 2687 the layer 3 MVPN procedures will ensure that the traffic is delivered 2688 to the L3VPN nodes that have requested it. The layer 3 multicast 2689 state for (S,G) will have the MVPN tunnel in the OIF list, and the 2690 IIF will be one of the following: 2692 o If S belongs to a BD that is attached to the MEG, the IIF will be 2693 the IRB interface to that BD; 2695 o Otherwise the IIF will be the SBD IRB interface. 2697 Note that this works even if S is attached to a non-OISM PE, per the 2698 procedures of Section 5. 2700 6.1.2.2.2. Any-Source Multicast (ASM) Groups 2702 Suppose the MEG SBD-DR learns that one of the PEs in its Tenant 2703 Domain is interested in (*,G), traffic, where G is an Any-Source 2704 Multicast (ASM) group. If there are no tenant multicast routers, the 2705 MEG SBD-DR SHOULD perform the "First Hop Router" (FHR) functionality 2706 for group G on behalf of the Tenant Domain, as described in 2707 [RFC7761]. This means that the MEG SBD-DR must know the identity of 2708 the Rendezvous Point (RP) for each group, must send Register messages 2709 to the Rendezvous Point, etc. 2711 If the MEG SBD-DR is to be the FHR for the Tenant Domain, it must see 2712 all the multicast traffic that is sourced from within the domain and 2713 destined to an ASM group address. The MEG can ensure this by 2714 originating an SBD-SMET route for (*,*). 2716 (As a possible optimization, an SBD-SMET route for (*, "any ASM 2717 group"), or even (*, "any ASM group that might have MVPN sources") 2718 may be defined in a future revision of this draft.) 2720 In some deployment scenarios, it may be preferred that the MEG that 2721 receives the (S,G) traffic over an AC be the one provides the FHR 2722 functionality. This behavior is OPTIONAL. If this option is used, 2723 it MUST be ensured that the MEG DR does not provide the FHR 2724 functionality for (S,G) traffic that is attached to another MEG; FHR 2725 functionality for (S,G) traffic from a particular source S MUST be 2726 provided by only a single router. 2728 Other deployment scenarios are also possible. For example, one might 2729 want to configure the MEGs to themselves be RPs. In this case, the 2730 RPs would have to exchange with each other information about which 2731 sources are active. The method exchanging such information is 2732 outside the scope of this document. 2734 6.1.2.2.3. Source on Multihomed Segment 2736 Suppose S is attached to a segment that is all-active multi-homed to 2737 PEl and PE2. If S is transmitting to two groups, say G1 and G2, it 2738 is possible that PE1 will receive the (S,G1) traffic from S while PE2 2739 receives the (S,G2) traffic from S. 2741 This creates an issue for MVPN/EVPN interworking, because there is no 2742 way to cause L3VPN/MVPN nodes to select PE1 as the ingress PE for 2743 (S,G1) traffic while selecting PE2 as the ingress PE for (S,G2) 2744 traffic. 2746 However, the following procedure ensures that the IP multicast 2747 traffic will still flow, even if the L3VPN/MVPN nodes picks the 2748 "wrong" EVPN-PE as the Upstream PE for (say) the (S,G1) traffic. 2750 Suppose S is on an ethernet segment, belonging to BD1, that is 2751 multi-homed to both PE1 and PE2, where PE1 is a MEG. And suppose 2752 that IP multicast traffic from S to G travels over the AC that 2753 attaches the segment to PE2 . If PE1 receives a C-multicast Source 2754 Tree Join (S,G) route, it MUST originate an SMET route for (S,G). 2755 Normal OISM procedures will then cause PE2 to send the (S,G) traffic 2756 to PE1 on an EVPN IP multicast tunnel. Normal OISM procedures will 2757 also cause PE1 to send the (S,G) traffic up its BD1 IRB interface. 2758 Normal MVPN procedures will then cause PE1 to forward the traffic on 2759 an MVPN tunnel. In this case, the routing is not optimal, but the 2760 traffic does flow correctly. 2762 6.1.2.3. Obtaining Optimal Routing of Traffic Between MVPN and EVPN 2764 The routing of IP multicast traffic between MVPN nodes and EVPN nodes 2765 will be optimal as long as there is a MEG along the optimal route. 2766 There are various deployment strategies that can be used to obtain 2767 optimal routing between MVPN and EVPN. 2769 In one such scenario, a Tenant Domain will have a small number of 2770 strategically placed MEGs. For example, a Data Center may have a 2771 small number of MEGs that connect it to a wide-area network. Then 2772 the optimal route into or out of the Data Center would be through the 2773 MEGs. 2775 In this scenario, the MEGs do not need to originate VPN-IP host 2776 routes for the multicast sources, they only need to originate VPN-IP 2777 subnet routes. The internal structure of the EVPN is completely 2778 hidden from the MVPN node. EVPN actions such as MAC Mobility and 2779 Mass Withdrawal ([RFC7432]) have zero impact on the MVPN control 2780 plane. 2782 While this deployment scenario provides the most optimal routing and 2783 has the least impact on the installed based of MVPN nodes, it does 2784 complicate network planning considerations. 2786 Another way of providing routing that is close to optimal is to turn 2787 each EVPN PE into a MEG. Then routing of MVPN-to-EVPN traffic is 2788 optimal. However, routing of EVPN-to-MVPN traffic is not guaranteed 2789 to be optimal when a source host is on a multi-homed ethernet segment 2790 (as discussed in Section 6.1.2.2.) 2792 The obvious disadvantage of this method is that it requires every 2793 EVPN PE to be a MEG. 2795 The procedures specified in this document allow an operator to add 2796 MEG functionality to any subset of his EVPN OISM PEs. This allows an 2797 operator to make whatever trade-offs he deems appropriate between 2798 optimal routing and MEG deployment. 2800 6.1.2.4. Selecting the MEG SBD-DR 2802 Every PE that is eligible for selection as the MEG SBD-DR originates 2803 an SBD-IMET route. As stated in Section 5, these SBD-IMET routes 2804 carry a Multicast Flags EC with the MEG Flag set. 2806 These SBD-IMET routes SHOULD also carry a DF Election EC. The DF 2807 Election EC and its use is specified in ([DF-Election-Framework]). 2808 When the route is originated, the AC-DF bit in the DF Election EC 2809 SHOULD be set to zero. This bit is not used when selecting a MEG 2810 SBD-DR, i.e., it MUST be ignored by the receiver of an SBD-IMET 2811 route. 2813 In the context of a given Tenant Domain, to select the MEG SBD-DR, 2814 the MEGs of the Tenant Domain perform the following procedure: 2816 o From the set of received SBD-IMET routes for the given tenant 2817 domain, determine he candidate set of PEs that support MEG 2818 functionality for that domain. 2820 o Select a DF Election algorithm as specified in 2821 [DF-Election-Framework]. Some of the possible algorithms can be 2822 found, e.g., in [RFC7432], [DF-Election-Framework], and 2823 [EVPN-DF-WEIGHTED]. 2825 o Apply the DF Election Algorithm (see [DF-Election-Framework]) to 2826 the candidate set of PEs. The "winner" becomes the MEG SBD-DR. 2828 Note that if a given PE supports IPMG (Section 6.1.2) or PEG 2829 (Section 6.1.4) functionality as well as MEG functionality, its 2830 SBD-IMET routes carry only one DF Election EC. 2832 6.1.3. Interworking with 'Global Table Multicast' 2834 If multicast service to the outside sources and/or receivers is 2835 provided via the BGP-based "Global Table Multicast" (GTM) procedures 2836 of [RFC7716], the procedures of Section 6.1.2 can easily be adapted 2837 for EVPN/GTM interworking. The way to adapt the MVPN procedures to 2838 GTM is explained in [RFC7716]. 2840 6.1.4. Interworking with PIM 2842 As we have been discussing, there may be receivers in an EVPN tenant 2843 domain that are interested in multicast flows whose sources are 2844 outside the EVPN Tenant Domain. Or there may be receivers outside an 2845 EVPN Tenant Domain that are interested in multicast flows whose 2846 sources are inside the Tenant Domain. 2848 If the outside sources and/or receivers are part of an MVPN, 2849 interworking procedures are covered in Section 6.1.2. 2851 There are also cases where an external source or receiver are 2852 attached via IP, and the layer 3 multicast routing is done via PIM. 2853 In this case, the interworking between the "PIM domain" and the EVPN 2854 tenant domain is done at L3 Gateways that perform "PIM/EVPN Gateway" 2855 (PEG) functionality. A PEG is very similar to a MEG, except that its 2856 layer 3 multicast routing is done via PIM rather than via BGP. 2858 If external sources or receivers for a given group are attached to a 2859 PEG via a layer 3 interface, that interface should be treated as a 2860 VRF interface attached to the Tenant Domain's L3VPN VRF. The layer 3 2861 multicast routing instance for that Tenant Domain will either run PIM 2862 on the VRF interface or will listen for IGMP/MLD messages on that 2863 interface. If the external receiver is attached elsewhere on an IP 2864 network, the PE has to enable PIM on its interfaces to the backbone 2865 network. In both cases, the PE needs to perform PEG functionality, 2866 and its IMET routes must carry the Muliticast Flags EC with the PEG 2867 flag set. 2869 For each BD on which there is a multicast source or receiver, one of 2870 the PEGs will becomes the PEG DR. DR selection can be done using the 2871 same procedures specified in Section 6.1.2.4, except with "PEG" 2872 substituted for "MEG". 2874 As long as there are no tenant multicast routers within the EVPN 2875 Tenant Domain, the PEGs do not need to run PIM on their IRB 2876 interfaces. 2878 6.1.4.1. Source Inside EVPN Domain 2880 If a PEG receives a PIM Join(S,G) from outside the EVPN tenant 2881 domain, it may find it necessary to create (S,G) state. The PE needs 2882 to determine whether S is within the Tenant Domain. If S is not 2883 within the EVPN Tenant Domain, the PE carries out normal layer 3 2884 multicast routing procedures. If S is within the EVPN tenant domain, 2885 the IIF of the (S,G) state is set as follows: 2887 o if S is on a BD that is attached to the PE, the IIF is the PE's 2888 IRB interface to that BD; 2890 o if S is not on a BD that is attached to the PE, the IIF is the 2891 PE's IRB interface to the SBD. 2893 When the PE creates such an (S,G) state, it MUST originate (if it 2894 hasn't already) an SBD-SMET route for (S,G). This will cause it to 2895 pull the (S,G) traffic via layer 2. When the traffic arrives over an 2896 EVPN tunnel, it gets sent up an IRB interface where the layer 3 2897 multicast routing determines the packet's disposition. The SBD-SMET 2898 route is withdrawn when the (S,G) state no longer exists (unless 2899 there is some other reason for not withdrawing it). 2901 If there are no tenant multicast routers with the EVPN tenant domain, 2902 there cannot be an RP in the Tenant Domain, so a PEG does not have to 2903 handle externally arriving PIM Join(*,G) messages. 2905 The PEG DR for a particular BD MUST act as the a First Hop Router for 2906 that BD. It will examine all (S,G) traffic on the BD, and whenever G 2907 is an ASM group, the PEG DR will send Register messages to the RP for 2908 G. This means that the PEG DR will need to pull all the (S,G) 2909 traffic originating on a given BD, by originating an SMET (*,*) route 2910 for that BD. If a PEG DR is the DR for all the BDS, in SHOULD 2911 originate just an SBD-SMET (*,*) route rather than an SMET (*,*) 2912 route for each BD. 2914 The rules for exporting IP routes to multicast sources are the same 2915 as those specified for MEGs in Section 6.1.2.2, except that the 2916 exported routes will be IP routes rather than VPN-IP routes, and it 2917 is not necessary to attach the VRF Route Import EC or the Source AS 2918 EC. 2920 When a source is on a multi-homed segment, the same issue discussed 2921 in Section 6.1.2.2.3 exists. Suppose S is on an ethernet segment, 2922 belonging to BD1, that is multi-homed to both PE1 and PE2, where PE1 2923 is a PEG. And suppose that IP multicast traffic from S to G travels 2924 over the AC that attaches the segment to PE2. If PE1 receives an 2925 external PIM Join (S,G) route, it MUST originate an SMET route for 2926 (S,G). Normal OISM procedures will cause PE2 to send the (S,G) 2927 traffic to PE1 on an EVPN IP multicast tunnel. Normal OISM 2928 procedures will also cause PE1 to send the (S,G) traffic up its BD1 2929 IRB interface. Normal PIM procedures will then cause PE1 to forward 2930 the traffic along a PIM tree. In this case, the routing is not 2931 optimal, but the traffic does flow correctly. 2933 6.1.4.2. Source Outside EVPN Domain 2935 By means of normal OISM procedures, a PEG learns whether there are 2936 receivers in the Tenant Domain that are interested in receiving (*,G) 2937 or (S,G) traffic. The PEG must determine whether S (or the RP for G) 2938 is outside the EVPN Tenant Domain. If so, and if there is a receiver 2939 on BD1 interested in receiving such traffic, the PEG DR for BD1 is 2940 responsible for originating a PIM Join(S,G) or Join(*,G) control 2941 message. 2943 An alternative would be to allow any PEG that is directly attached to 2944 a receiver to originate the PIM Joins. Then the PEG DR would only 2945 have to originate PIM Joins on behalf of receivers that are not 2946 attached to a PEG. However, if this is done, it is necessary for the 2947 PEGs to run PIM on all their IRB interfaces, so that the PIM Assert 2948 procedures can be used to prevent duplicate delivery to a given BD. 2950 The IIF for the layer 3 (S,G) or (*,G) state is determined by normal 2951 PIM procedures. If a receiver is on BD1, and the PEG DR is attached 2952 to BD1, its IRB interface to BD1 is added to the OIF list. This 2953 ensures that any receivers locally attached to the PEG DR will 2954 receive the traffic. If there are receivers attached to other EVPN 2955 PEs, then whenever (S,G) traffic from an external source matches a 2956 (*,G) state, the PEG will create (S,G) state. The IIF will be set to 2957 whatever external interface the traffic is expected to arrive on 2958 (copied from the (*,G) state), the OIF list is copied from the (*,G) 2959 state, and the SBD IRB interface added to the OIF list. 2961 6.2. Interworking with PIM via an External PIM Router 2963 Section 6.1 describes how to use an OISM PE router as the gateway to 2964 a non-EVPN multicast domain, when the EVPN tenant domain is not being 2965 used as an intermediate transit network for multicast. An 2966 alternative approach is to have one or more external PIM routers 2967 (perhaps operated by a tenant) on one of the BDs of the tenant 2968 domain. We will refer to this BD as the "gateway BD". 2970 In this model: 2972 o The EVPN Tenant Domain is treated as a stub network attached to 2973 the external PIM routers. 2975 o The external PIM routers follow normal PIM procedures, and provide 2976 the FHR and LHR functionality for the entire Tenant Domain. 2978 o The OISM PEs do not run PIM. 2980 o There MUST NOT be more than one gateway BD. 2982 o If an OISM PE not attached to the gateway BD has interest in a 2983 given multicast flow, it conveys that interest, following normal 2984 OISM procedures, by originating an SBD-SMET route for that flow. 2986 o If a PE attached to the gateway BD receives an SBD-SMET, it may 2987 need to generate and transmit a corresponding IGMP/MLD Join out 2988 one or more of its ACs. (Procedures for generating an IGMP/MLD 2989 Join as a result of receiving an SMET route are given in 2990 [IGMP-Proxy].) The PE MUST know which BD is the Gateway BD and 2991 MUST NOT transmit an IGMP/MLD Join to any other BDs. Furthermore, 2992 even if a particular AC is part of that BD, the PE SHOULD NOT 2993 transmit an IGMP/MLD Join on that AC unless that an external PIM 2994 route is attached via that AC. 2996 As a result, IGMP/MLD messages will seen by the external PIM 2997 routers on the gateway BD, and those external PIM routers will 2998 send PIM Join messages externally as required. Traffic of the 2999 given multicast flow will then be received by one of the external 3000 PIM routers, and that traffic will be forwarded by that router to 3001 the gateway BD. 3003 The normal OISM procedures will then cause the given multicast 3004 flow to be tunneled to any PEs of the EVPN Tenant Domain that have 3005 interest in the flow. PEs attached to the gateway BD will see the 3006 flow as originating from the gateway BD, other PEs will see the 3007 flow as originating from the SBD. 3009 o An OISM PE attached to a gateway BD MUST set its layer 2 multicast 3010 state to indicate that each AC to the gateway BD has interest in 3011 all multicast flows. It MUST also originate an SMET route for 3012 (*,*). The procedures for originating SMET routes are discussed 3013 in Section 2.5. 3015 This will cause the OISM PEs attached to the gateway BD to receive 3016 all the IP multicast traffic that is sourced within the EVPN 3017 tenant domain, and to transmit that traffic to the gateway BD, 3018 where the external PIM routers will see it. This enables the 3019 external PIM routers to perform FHR functions on behalf of the 3020 entire Tenant Domain. (Of course, if the gateway BD has a 3021 multi-homed segment, only the PE that is the DF for that segment 3022 will transmit the multicast traffic to the segment.) 3024 7. Using an EVPN Tenant Domain as an Intermediate (Transit) Network for 3025 Multicast traffic 3027 In this section, we consider the scenario where one or more BDs of an 3028 EVPN Tenant Domain are being used to carry IP multicast traffic for 3029 which the source and at least one receiver are not part the tenant 3030 domain. That is, one or more BDs of the Tenant Domain are 3031 intermediate "links" of a larger multicast tree created by PIM. 3033 We define a "tenant multicast router" as a multicast router, running 3034 PIM, that is: 3036 1. attached to one or more BDs of the Tenant Domain, but 3038 2. is not an EVPN PE router. 3040 In order an EVPN Tenant Domain to be used as a transit network for IP 3041 multicast, one or more of its BDs must have tenant multicast routers, 3042 and an OISM PE that attaching to such a BD MUST be provisioned to 3043 enable PIM on its IRB interface to that BD. (This is true even if 3044 none of the tenant routers is on a segment attached to the PE.) 3045 Further, all the OISM PEs (even ones not attached to a BD with tenant 3046 multicast routers) MUST be provisioned to enable PIM on their SBD IRB 3047 interfaces. 3049 If PIM is enabled on a particular BD, the DR Selection procedure of 3050 Section 6.1.2.4 MUST be replaced by the normal PIM DR Election 3051 procedure of [RFC7761]. Note that this may result in one of the 3052 tenant routers being selected as the DR, rather than one of the OISM 3053 PE routers. In this case, First Hop Router and Last Hop Router 3054 functionality will not be performed by any of the EVPN PEs. 3056 A PIM control message on a particular BD is considered to be a 3057 link-local multicast message, and as such is sent transparently from 3058 PE to PE via the BUM tunnel for that BD. This is true whether the 3059 control message was received from an AC, or whether it was received 3060 from the local layer 3 routing instance via an IRB interface. 3062 A PIM Join/Prune message contains three fields that are relevant to 3063 the present discussion: 3065 o Upstream Neighbor 3067 o Group Address (G) 3069 o Source Address (S), omitted in the case of (*,G) Join/Prune 3070 messages. 3072 We will generally speak of a PIM Join as a "Join(S,G)" or a 3073 "Join(*,G)" message, and will use the term "Join(X,G)" to mean 3074 "either Join(S,G) or Join(*,G)". In the context of a Join(X,G), we 3075 will use the term "X" to mean "S in the case of (S,G), or G's RP in 3076 the case of (*,G)". 3078 Suppose BD1 contains two tenant multicast routers, C1 and C2. 3079 Suppose C1 is on a segment attached to PE1, and C2 is on a segment 3080 attached to PE2. When C1 sends a PIM Join(X,G) to BD1, the Upstream 3081 Neighbor field might be set to either PE1, PE2, or C2. C1 chooses 3082 the Upstream Neighbor based on its unicast routing. Typically, it 3083 will choose as the Upstream Neighbor the PIM router on BD1 that is 3084 "closest" (according to the unicast routing) to X. Note that this 3085 will not necessarily be PE1. PE1 may not even be visible to the 3086 unicast routing algorithm used by the tenant routers. Even if it is, 3087 it is unlikely to be the PIM router that is closest to X. So we need 3088 to consider the following two cases: 3090 1. C1 sends a PIM Join(X,G) to BD1, with PE1 as the Upstream 3091 Neighbor. 3093 PE1's PIM routing instance will see the Join arrive on the BD1 3094 IRB interface. If X is not within the Tenant Domain, PE1 3095 handles the Join according to normal PIM procedures. This will 3096 generally result in PE1 selecting an Upstream Neighbor and 3097 sending it a Join(X,G). 3099 If X is within the Tenant Domain, but is attached to some other 3100 PE, PE1 sends (if it hasn't already) an SBD-SMET route for 3101 (X,G). The IIF of the layer 3 (X,G) state will be the SBD IRB 3102 interface, and the OIF list will include the IRB interface to 3103 BD1. 3105 The SBD-SMET route will pull the (X,G) traffic to PE1, and the 3106 (X,G) state will result in the (X,G) traffic being forwarded to 3107 C1. 3109 If X is within the Tenant Domain, but is attached to PE1 itself, 3110 no SBD-SMET route is sent. The IIF of the layer 3 (X,G) state 3111 will be the IRB interface to X's BD, and the OIF list will 3112 include the IRB interface to BD1. 3114 2. C1 sends a PIM Join(X,G) to BD1, with either PE2 or C2 as the 3115 Upstream Neighbor. 3117 PE1's PIM routing instance will see the Join arrive on the BD1 3118 IRB interface. If neither X nor Upstream Neighbor is within the 3119 tenant domain, PE1 handles the Join according to normal PIM 3120 procedures. This will NOT result in PE1 sending a Join(X,G). 3122 If either X or Upstream Neighbor is within the Tenant Domain, 3123 PE1 sends (if it hasn't already) an SBD-SMET route for (X,G). 3124 The IIF of the layer 3 (X,G) state will be the SBD IRB 3125 interface, and the OIF list will include the IRB interface to 3126 BD1. 3128 The SBD-SMET route will pull the (X,G) traffic to PE1, and the 3129 (X,G) state will result in the (X,G) traffic being forwarded to 3130 C1. 3132 8. IANA Considerations 3134 IANA is requested to assign new flags in the "Multicast Flags 3135 Extended Community Flags" registry. These flags are: 3137 o IPMG 3139 o MEG 3141 o PEG 3143 9. Security Considerations 3145 This document uses protocols and procedures defined in the normative 3146 references, and inherits the security considerations of those 3147 references. 3149 This document adds flags or Extended Communities (ECs) to a number of 3150 BGP routes, in order to signal that particular nodes support the 3151 OISM, IPMG, MEG, and/or PEG functionalities that are defined in this 3152 document. Incorrect addition, removal, or modification of those 3153 flags and/or ECs will cause the procedures defined herein to 3154 malfunction, in which case loss or diversion of data traffic is 3155 possible. 3157 10. Acknowledgements 3159 The authors thank Vikram Nagarajan and Princy Elizabeth for their 3160 work on Section 6.2. The authors also benefited tremendously from 3161 discussions with Aldrin Isaac on EVPN multicast optimizations. 3163 11. References 3165 11.1. Normative References 3167 [DF-Election-Framework] 3168 Rabadan, J., Mohanty, S., Sajassi, A., Drake, J., Nagaraj, 3169 K., and S. Sathappan, "Framework for EVPN Designated 3170 Forwarder Election Extensibility", internet-draft draft- 3171 ietf-bess-evpn-df-election-framework-03.txt, May 2018. 3173 [EVPN-AR] Rabadan, J., Ed., "Optimized Ingress Replication solution 3174 for EVPN", internet-draft ietf-bess-evpn-optimized-ir- 3175 03.txt, Febryary 2018. 3177 [EVPN-BUM] 3178 Zhang, Z., Lin, W., Rabadan, J., and K. Patel, "Updates on 3179 EVPN BUM Procedures", internet-draft ietf-bess-evpn-bum- 3180 procedure-updates-03.txt, April 2018. 3182 [EVPN-IRB] 3183 Sajassi, A., Salam, S., Thoria, S., Drake, J., and J. 3184 Rabadan, "Integrated Routing and Bridging in EVPN", 3185 internet-draft draft-ietf-bess-evpn-inter-subnet- 3186 forwarding-04.txt, July 2018. 3188 [EVPN_IP_Prefix] 3189 Rabadan, J., Henderickx, W., Drake, J., Lin, W., and A. 3190 Sajassi, "IP Prefix Advertisement in EVPN", internet- 3191 draft ietf-bess-evpn-prefix-advertisement-11.txt, May 3192 2018. 3194 [IGMP-Proxy] 3195 Sajassi, A., Thoria, S., Patel, K., Yeung, D., Drake, J., 3196 and W. Lin, "IGMP and MLD Proxy for EVPN", internet-draft 3197 draft-ietf-bess-evpn-igmp-mld-proxy-02.txt, June 2018. 3199 [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate 3200 Requirement Levels", BCP 14, RFC 2119, 3201 DOI 10.17487/RFC2119, March 1997, 3202 . 3204 [RFC2236] Fenner, W., "Internet Group Management Protocol, Version 3205 2", RFC 2236, DOI 10.17487/RFC2236, November 1997, 3206 . 3208 [RFC2710] Deering, S., Fenner, W., and B. Haberman, "Multicast 3209 Listener Discovery (MLD) for IPv6", RFC 2710, 3210 DOI 10.17487/RFC2710, October 1999, 3211 . 3213 [RFC3032] Rosen, E., Tappan, D., Fedorkow, G., Rekhter, Y., 3214 Farinacci, D., Li, T., and A. Conta, "MPLS Label Stack 3215 Encoding", RFC 3032, DOI 10.17487/RFC3032, January 2001, 3216 . 3218 [RFC4360] Sangli, S., Tappan, D., and Y. Rekhter, "BGP Extended 3219 Communities Attribute", RFC 4360, DOI 10.17487/RFC4360, 3220 February 2006, . 3222 [RFC6625] Rosen, E., Ed., Rekhter, Y., Ed., Hendrickx, W., and R. 3223 Qiu, "Wildcards in Multicast VPN Auto-Discovery Routes", 3224 RFC 6625, DOI 10.17487/RFC6625, May 2012, 3225 . 3227 [RFC7153] Rosen, E. and Y. Rekhter, "IANA Registries for BGP 3228 Extended Communities", RFC 7153, DOI 10.17487/RFC7153, 3229 March 2014, . 3231 [RFC7432] Sajassi, A., Ed., Aggarwal, R., Bitar, N., Isaac, A., 3232 Uttaro, J., Drake, J., and W. Henderickx, "BGP MPLS-Based 3233 Ethernet VPN", RFC 7432, DOI 10.17487/RFC7432, February 3234 2015, . 3236 [RFC8174] Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC 3237 2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174, 3238 May 2017, . 3240 11.2. Informative References 3242 [EVPN-BIER] 3243 Zhang, Z., Przygienda, T., Sajassi, A., and J. Rabadan, 3244 "EVPN BUM Using BIER", internet-draft ietf-bier-evpn- 3245 01.txt, April 2018. 3247 [EVPN-DF-WEIGHTED] 3248 Rabadan, J., Sathappan, S., Przygienda, T., Lin, W., 3249 Drake, J., Sajassi, A., and S. Mohanty, "Preference-based 3250 EVPN DF Election", internet-draft ietf-bess-evpn-pref-df- 3251 01.txt, April 2018. 3253 [RFC4364] Rosen, E. and Y. Rekhter, "BGP/MPLS IP Virtual Private 3254 Networks (VPNs)", RFC 4364, DOI 10.17487/RFC4364, February 3255 2006, . 3257 [RFC6513] Rosen, E., Ed. and R. Aggarwal, Ed., "Multicast in MPLS/ 3258 BGP IP VPNs", RFC 6513, DOI 10.17487/RFC6513, February 3259 2012, . 3261 [RFC6514] Aggarwal, R., Rosen, E., Morin, T., and Y. Rekhter, "BGP 3262 Encodings and Procedures for Multicast in MPLS/BGP IP 3263 VPNs", RFC 6514, DOI 10.17487/RFC6514, February 2012, 3264 . 3266 [RFC7606] Chen, E., Ed., Scudder, J., Ed., Mohapatra, P., and K. 3267 Patel, "Revised Error Handling for BGP UPDATE Messages", 3268 RFC 7606, DOI 10.17487/RFC7606, August 2015, 3269 . 3271 [RFC7716] Zhang, J., Giuliano, L., Rosen, E., Ed., Subramanian, K., 3272 and D. Pacella, "Global Table Multicast with BGP Multicast 3273 VPN (BGP-MVPN) Procedures", RFC 7716, 3274 DOI 10.17487/RFC7716, December 2015, 3275 . 3277 [RFC7761] Fenner, B., Handley, M., Holbrook, H., Kouvelas, I., 3278 Parekh, R., Zhang, Z., and L. Zheng, "Protocol Independent 3279 Multicast - Sparse Mode (PIM-SM): Protocol Specification 3280 (Revised)", STD 83, RFC 7761, DOI 10.17487/RFC7761, March 3281 2016, . 3283 [RFC8296] Wijnands, IJ., Ed., Rosen, E., Ed., Dolganow, A., 3284 Tantsura, J., Aldrin, S., and I. Meilik, "Encapsulation 3285 for Bit Index Explicit Replication (BIER) in MPLS and Non- 3286 MPLS Networks", RFC 8296, DOI 10.17487/RFC8296, January 3287 2018, . 3289 Appendix A. Integrated Routing and Bridging 3291 This Appendix provides a short tutorial on the interaction of routing 3292 and bridging. First it shows the traditional model, where bridging 3293 and routing are performed in separate boxes. Then it shows the model 3294 specified in [EVPN-IRB], where a single box contains both routing and 3295 bridging functions. The latter model is presupposed in the body of 3296 this document. 3298 Figure 1 shows a "traditional" router that only does routing and has 3299 no L2 bridging capabilities. There are two LANs, LAN1 and LAN2. 3300 LAN1 is realized by switch1, LAN2 by switch2. The router has an 3301 interface, "lan1" that attaches to LAN1 (via switch1) and an 3302 interface "lan2" that attachs to LAN2 (via switch2). Each intreface 3303 is configured, as an IP interface, with an IP address and a subnet 3304 mask. 3306 +-------+ +--------+ +-------+ 3307 | | lan1| |lan2 | | 3308 H1 -----+Switch1+--------+ Router1+--------+Switch2+------H3 3309 | | | | | | 3310 H2 -----| | | | | | 3311 +-------+ +--------+ +-------+ 3312 |_________________| |__________________| 3313 LAN1 LAN2 3315 Figure 1: Conventional Router with LAN Interfaces 3317 IP traffic (unicast or multicast) that remains within a single subnet 3318 never reaches the router. For instance, if H1 emits an ethernet 3319 frame with H2's MAC address in the ethernet destination address 3320 field, the frame will go from H1 to Switch1 to H2, without ever 3321 reaching the router. Since the frame is never seen by a router, the 3322 IP datagram within the frame remains entirely unchanged; e.g., its 3323 TTL is not decremented. The ethernet Source and Destination MAC 3324 addresses are not changed either. 3326 If H1 wants to send a unicast IP datagram to H3, which is on a 3327 different subnet, H1 has to be configured with the IP address of a 3328 "default router". Let's assume that H1 is configured with an IP 3329 address of Router1 as its default router address. H1 compares H3's 3330 IP address with its own IP address and IP subnet mask, and determines 3331 that H3 is on a different subnet. So the packet has to be routed. 3332 H1 uses ARP to map Router1's IP address to a MAC address on LAN1. H1 3333 then encapsulates the datagram in an ethernet frame, using router1's 3334 MAC address as the destination MAC address, and sends the frame to 3335 Router1. 3337 Router1 then receives the frame over its lan1 interface. Router1 3338 sees that the frame is addressed to it, so it removes the ethernet 3339 encapsulation and processes the IP datagram. The datagram is not 3340 addressed to Router1, so it must be forwarded further. Router1 does 3341 a lookup of the datagram's IP destination field, and determines that 3342 the destination (H3) can be reached via Router1's lan2 interface. 3343 Router1 now performs the IP processing of the datagram: it decrements 3344 the IP TTL, adjusts the IP header checksum (if present), may fragment 3345 the packet is necessary, etc. Then the datagram (or its fragments) 3346 are encapsulated in an ethernet header, with Router1's MAC address on 3347 LAN2 as the MAC Source Address, and H3's MAC address on LAN2 (which 3348 Router1 determines via ARP) as the MAC Destination Address. Finally 3349 the packet is sent out the lan2 interface. 3351 If H1 has an IP multicast datagram to send (i.e., an IP datagram 3352 whose Destination Address field is an IP Multicast Address), it 3353 encapsulates it in an ethernet frame whose MAC Destination Address is 3354 computed from the IP Destination Address. 3356 If H2 is a receiver for that multicast address, H2 will receive a 3357 copy of the frame, unchanged, from H1. The MAC Source Address in the 3358 ethernet encapsulation does not change, the IP TTL field does not get 3359 decremented, etc. 3361 If H3 is a receiver for that multicast address, the datagram must be 3362 routed to H3. In order for this to happen, Router1 must be 3363 configured as a multicast router, and it must accept traffic sent to 3364 ethernet multicast addresses. Router1 will receive H1's multicast 3365 frame on its lan1 interface, will remove the ethernet encapsulation, 3366 and will determine how to dispatch the IP datagram based on Router1's 3367 multicast forwarding states. If Router1 knows that there is a 3368 receiver for the multicast datagram on LAN2, makes a copy of the 3369 datagram, decrements the TTL (and performs any other necessary IP 3370 processing), then encapsulates the datagram in ethernet frame for 3371 LAN2. The MAC Source Address for this frame will be Router1's MAC 3372 Source Address on LAN2. The MAC Destination Address is computed from 3373 the IP Destination Address. Finally, the frame is sent out Router1's 3374 LAN2 interface. 3376 Figure 2 shows an Integrated Router/Bridge that supports the routing/ 3377 bridging integration model of [EVPN-IRB]. 3379 +------------------------------------------+ 3380 | Integrated Router/Bridge | 3382 +-------+ +--------+ +-------+ 3383 | | IRB1| L3 |IRB2 | | 3384 H1 -----+ BD1 +--------+Routing +--------+ BD2 +------H3 3385 | | |Instance| | | 3386 H2 -----| | | | | | 3387 +-------+ +--------+ +-------+ 3388 |___________________| |____________________| 3389 LAN1 LAN2 3391 Figure 2: Integrated Router/Bridge 3393 In Figure 2, a single box consists of one or more "L3 Routing 3394 Instances". The routing/forwarding tables of a given routing 3395 instance is known as an IP-VRF ([EVPN-IRB]). In the context of EVPN, 3396 it is convenient to think of each routing instance as representing 3397 the routing of a particular tenant. Each IP-VRF is attached to one 3398 or more interfaces. 3400 When several EVPN PEs have a routing instance of the same tenant 3401 domain, those PEs advertise IP routes to the attached hosts. This is 3402 done as specified in [EVPN-IRB]. 3404 The integrated router/bridge shown in Figure 2 also attaches to a 3405 number of "Broadcast Domains" (BDs). Each BD performs the functions 3406 that are performed by the bridges in Figure 1. To the L3 routing 3407 instance, each BD appears to be a LAN. The interface attaching a 3408 particular BD to a particular IP-VRF is known as an "IRB Interface". 3409 From the perspective of L3 routing, each BD is a subnet. Thus each 3410 IRB interface is configured with a MAC address (which is the router's 3411 MAC address on the corresponding LAN), as well as an IP address and 3412 subnet mask. 3414 The integrated router/bridge shown in Figure 2 may have multiple ACs 3415 to each BD. These ACs are visible only to the bridging function, not 3416 to the routing instance. To the L3 routing instance, there is just 3417 one "interface" to each BD. 3419 If the L3 routing instance represents the IP routing of a particular 3420 tenant, the BDs attached to that routing instance are BDs belonging 3421 to that same tenant. 3423 Bridging and routing now proceed exactly as in the case of Figure 1, 3424 except that BD1 replaces Switch1, BD2 replaces Switch2, interface 3425 IRB1 replaces interface lan1, and interface IRB2 replaces interface 3426 lan2. 3428 It is important to understand that an IRB interface connects an L3 3429 routing instance to a BD, NOT to a "MAC-VRF". (See [RFC7432] for the 3430 definition of "MAC-VRF".) A MAC-VRF may contain several BDs, as long 3431 as no MAC address appears in more than one BD. From the perspective 3432 of the L3 routing instance, each individual BD is an individual IP 3433 subnet; whether each BD has its own MAC-VRF or not is irrelevant to 3434 the L3 routing instance. 3436 Figure 3 illustrates IRB when a pair of BDs (subnets) are attached to 3437 two different PE routers. In this example, each BD has two segments, 3438 and one segment of each BD is attached to one PE router. 3440 +------------------------------------------+ 3441 | Integrated Router/Bridges | 3443 +-------+ +--------+ +-------+ 3444 | | IRB1| |IRB2 | | 3445 H1 -----+ BD1 +--------+ PE1 +--------+ BD2 +------H3 3446 |(Seg-1)| |(L3 Rtg)| |(Seg-1)| 3447 H2 -----| | | | | | 3448 +-------+ +--------+ +-------+ 3449 |___________________| | |____________________| 3450 LAN1 | LAN2 3451 | 3452 | 3453 +-------+ +--------+ +-------+ 3454 | | IRB1| |IRB2 | | 3455 H4 -----+ BD1 +--------+ PE2 +--------+ BD2 +------H5 3456 |(Seg-2)| |(L3 Rtg)| |(Seg-2)| 3457 | | | | | | 3458 +-------+ +--------+ +-------+ 3460 Figure 3: Integrated Router/Bridges with Distributed Subnet 3462 If H1 needs to send an IP packet to H4, it determines from its IP 3463 address and subnet mask that H4 is on the same subnet as H1. 3464 Although H1 and H4 are not attached to the same PE router, EVPN 3465 provides ethernet communication among all hosts that are on the same 3466 BD. H1 thus uses ARP to find H4's MAC address, and sends an ethernet 3467 frame with H4's MAC address in the Destination MAC address field. 3468 The frame is received at PE1, but since the Destination MAC address 3469 is not PE1's MAC address, PE1 assumes that the frame is to remain on 3470 BD1. Therefore the packet inside the frame is NOT decapsulated, and 3471 is NOT send up the IRB interface to PE1's routing instance. Rather, 3472 standard EVPN intra-subnet procedures (as detailed in [RFC7432] are 3473 used to deliver the frame to PE2, which then sends it to H4. 3475 If H1 needs to send an IP packet to H5, it determines from its IP 3476 address and subnet mask that H5 is NOT on the same subnet as H1. 3477 Assuming that H1 has been configured with the IP address of PE1 as 3478 its default router, H1 sends the packet in an ethernet frame with 3479 PE1's MAC address in its Destination MAC Address field. PE1 receives 3480 the frame, and sees that the frame is addressed to it. PE1 thus 3481 sends the frame up its IRB1 interface to the L3 routing instance. 3482 Appropriate IP processing is done (e.g., TTL decrement). The L3 3483 routing instance determines that the "next hop" for H5 is PE2, so the 3484 packet is encapsulated (e.g., in MPLS) and sent across the backbone 3485 to PE2's routing instance. PE2 will see that the packet's 3486 destination, H5, is on BD2 segment-2, and will send the packet down 3487 its IRB2 interface. This causes the IP packet to be encapsulated in 3488 an ethernet frame with PE2's MAC address (on BD2) in the Source 3489 Address field and H5's MAC address in the Destination Address field. 3491 Note that if H1 has an IP packet to send to H3, the forwarding of the 3492 packet is handled entirely within PE1. PE1's routing instance sees 3493 the packet arrive on its IRB1 interface, and then transmits the 3494 packet by sending it down its IRB2 interface. 3496 Often, all the hosts in a particular Tenant Domain will be 3497 provisioned with the same value of the default router IP address. 3498 This IP address can be assigned, as an "anycast address", to all the 3499 EVPN PEs attached to that Tenant Domain. Thus although all hosts are 3500 provisioned with the same "default router address", the actual 3501 default router for a given host will be one of the PEs that is 3502 attached to the same ethernet segment as the host. This provisioning 3503 method ensures that IP packets from a given host are handled by the 3504 closest EVPN PE that supports IRB. 3506 In the topology of Figure 3, one could imagine that H1 is configured 3507 with a default router address that belongs to PE2 but not to PE1. 3508 Inter-subnet routing would still work, but IP packets from H1 to H3 3509 would then follow the non-optimal path H1-->PE1-->PE2-->PE1-->H3. 3510 Sending traffic on this sort of path, where it leaves a router and 3511 then comes back to the same router, is sometimes known as 3512 "hairpinning". Similarly, if PE2 supports IRB but PE1 dos not, the 3513 same non-optimal path from H1 to H3 would have to be followed. To 3514 avoid hairpinning, each EVPN PE needs to support IRB. 3516 It is worth pointing out the way IRB interfaces interact with 3517 multicast traffic. Referring again to Figure 3, suppose PE1 and PE2 3518 are functioning as IP multicast routers. Suppose also that H3 3519 transmits a multicast packet, and both H1 and H4 are interested in 3520 receiving that packet. PE1 will receive the packet from H3 via its 3521 IRB2 interface. The ethernet encapsulation from BD2 is removed, the 3522 IP header processing is done, and the packet is then reencapsulated 3523 for BD1, with PE1's MAC address in the MAC Source Address field. 3524 Then the packet is sent down the IRB1 interface. Layer 2 procedures 3525 (as defined in [RFC7432] would then be used to deliver a copy of the 3526 packet locally to H1, and remotely to H4. 3528 Please be aware that his document modifies the semantics, described 3529 in the previous paragraph, of sending/receiving multicast traffic on 3530 an IRB interface. This is explained in Section 1.5.1 and subsequent 3531 sections. 3533 Authors' Addresses 3535 Wen Lin 3536 Juniper Networks, Inc. 3538 EMail: wlin@juniper.net 3540 Zhaohui Zhang 3541 Juniper Networks, Inc. 3543 EMail: zzhang@juniper.net 3545 John Drake 3546 Juniper Networks, Inc. 3548 EMail: jdrake@juniper.net 3550 Eric C. Rosen (editor) 3551 Juniper Networks, Inc. 3553 EMail: erosen@juniper.net 3555 Jorge Rabadan 3556 Nokia 3558 EMail: jorge.rabadan@nokia.com 3560 Ali Sajassi 3561 Cisco Systems 3563 EMail: sajassi@cisco.com