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Run idnits with the --verbose option for more detailed information about the items above. -------------------------------------------------------------------------------- 1 L2VPN Working Group B. Kothari 2 Internet Draft K. Kompella 3 Intended status: Standards Track Juniper Networks 4 Expires: January 2011 W. Henderickx 5 F. Balus 6 Alcatel-Lucent 7 J. Uttaro 8 AT&T 9 July 12, 2010 11 BGP based Multi-homing in Virtual Private LAN Service 12 draft-ietf-l2vpn-vpls-multihoming-01 14 Status of this Memo 16 This Internet-Draft is submitted to IETF in full conformance with the 17 provisions of BCP 78 and BCP 79. This document may contain material 18 from IETF Documents or IETF Contributions published or made publicly 19 available before November 10, 2008. The person(s) controlling the 20 copyright in some of this material may not have granted the IETF 21 Trust the right to allow modifications of such material outside the 22 IETF Standards Process. 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Code Components extracted from this document must 57 include Simplified BSD License text as described in Section 4.e of 58 the Trust Legal Provisions and are provided without warranty as 59 described in the Simplified BSD License. 61 Abstract 63 Virtual Private LAN Service (VPLS) is a Layer 2 Virtual Private 64 Network (VPN) that gives its customers the appearance that their 65 sites are connected via a Local Area Network (LAN). It is often 66 required for the Service Provider (SP) to give the customer redundant 67 connectivity to some sites, often called "multi-homing". This memo 68 shows how BGP-based multi-homing can be offered in the context of LDP 69 and BGP VPLS solutions. 71 Table of Contents 73 1. Introduction...................................................3 74 1.1. General Terminology.......................................4 75 1.2. Conventions used in this document.........................4 76 2. Background.....................................................4 77 2.1. Scenarios.................................................4 78 2.2. VPLS Multi-homing Considerations..........................5 79 3. Multi-homing Operation.........................................6 80 3.1. Provisioning Model........................................6 81 3.2. Multi-homing NLRI.........................................6 82 3.3. Designated Forwarder Election.............................7 83 3.3.1. Attributes...........................................7 84 3.3.2. Variables Used.......................................8 85 3.3.2.1. RD..............................................8 86 3.3.2.2. MH-ID...........................................8 87 3.3.2.3. VBO.............................................8 88 3.3.2.4. DOM.............................................8 89 3.3.2.5. ACS.............................................8 90 3.3.2.6. PREF............................................8 91 3.3.2.7. PE-ID...........................................9 93 3.3.3. Election Procedures..................................9 94 3.3.3.1. Bucketization for BGP DF Election..............10 95 3.3.3.2. Bucketization for VPLS DF Election.............10 96 3.3.3.3. Tie-breaking Rules.............................10 97 3.4. DF Election on PEs.......................................11 98 4. Multi-AS VPLS.................................................12 99 4.1. Route Origin Extended Community..........................12 100 4.2. VPLS Preference..........................................12 101 4.3. Use of BGP-MH attributes in Inter-AS Methods.............13 102 4.3.1. Inter-AS Method (b): EBGP Redistribution of VPLS 103 Information between ASBRs..................................14 104 4.3.2. Inter-AS Method (c): Multi-Hop EBGP Redistribution of 105 VPLS Information between ASes..............................15 106 5. MAC Flush Operations..........................................15 107 5.1. MAC List Flush...........................................16 108 5.2. Implicit MAC Flush.......................................16 109 6. Backwards Compatibility.......................................16 110 6.1. BGP based VPLS...........................................17 111 6.2. LDP VPLS with BGP Auto-discovery.........................17 112 7. Security Considerations.......................................17 113 8. IANA Considerations...........................................17 114 9. References....................................................17 115 9.1. Normative References.....................................17 116 9.2. Informative References...................................18 117 10. Acknowledgments..............................................18 119 1. Introduction 121 Virtual Private LAN Service (VPLS) is a Layer 2 Virtual Private 122 Network (VPN) that gives its customers the appearance that their 123 sites are connected via a Local Area Network (LAN). It is often 124 required for a Service Provider (SP) to give the customer redundant 125 connectivity to one or more sites, often called "multi-homing". 126 [RFC4761] explains how VPLS can be offered using BGP for auto- 127 discovery and signaling; section 3.5 of that document describes how 128 multi-homing can be achieved in this context. [I-D.ietf-l2vpn- 129 signaling] explains how VPLS can be offered using BGP for auto- 130 discovery (BGP-AD) and [RFC4762] explains how VPLS can be offered 131 using LDP for signaling. This document provides a BGP-based multi- 132 homing solution applicable to both BGP and LDP VPLS technologies. 133 Note that BGP MH can be used for LDP VPLS without the use of the BGP- 134 AD solution. 136 Section 2 lays out some of the scenarios for multi-homing, other ways 137 that this can be achieved, and some of the expectations of BGP-based 138 multi-homing. Section 3 defines the components of BGP-based multi- 139 homing, and the procedures required to achieve this. Section 7 may 140 someday discuss security considerations. 142 1.1. General Terminology 144 Some general terminology is defined here; most is from [RFC4761], 145 [RFC4762] or [RFC4364]. Terminology specific to this memo is 146 introduced as needed in later sections. 148 A "Customer Edge" (CE) device, typically located on customer 149 premises, connects to a "Provider Edge" (PE) device, which is owned 150 and operated by the SP. A "Provider" (P) device is also owned and 151 operated by the SP, but has no direct customer connections. A "VPLS 152 Edge" (VE) device is a PE that offers VPLS services. 154 A VPLS domain represents a bridging domain per customer. A Route 155 Target community as described in [RFC4360] is typically used to 156 identify all the PE routers participating in a particular VPLS 157 domain. A VPLS site is a grouping of ports on a PE that belong to the 158 same VPLS domain. A Multi-homed (MH) site is uniquely identified by a 159 MH site ID (MH-ID). Sites are referred to as local or remote 160 depending on whether they are configured on the PE router in context 161 or on one of the remote PE routers (network peers). 163 1.2. Conventions used in this document 165 The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", 166 "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this 167 document are to be interpreted as described in [RFC2119]. 169 2. Background 171 This section describes various scenarios where multi-homing may be 172 required, and the implications thereof. It also describes some of the 173 singular properties of VPLS multi-homing, and what that means from 174 both an operational point of view and an implementation point of 175 view. There are other approaches for providing multi-homing such as 176 Spanning Tree Protocol, and this document specifies use of BGP for 177 multi-homing. Comprehensive comparison among the approaches is 178 outside the scope of this document. 180 2.1. Scenarios 182 The most basic scenario is shown in Figure 1. 184 CE1 is a VPLS CE that is dual-homed to both PE1 and PE2 for redundant 185 connectivity. 187 ............... 188 . . ___ CE2 189 ___ PE1 . / 190 / : PE3 191 __/ : Service : 192 CE1 __ : Provider PE4 193 \ : : \___ CE3 194 \___ PE2 . 195 . . 196 ............... 198 Figure 1 Scenario 1 200 CE1 is a VPLS CE that is dual-homed to both PE1 and PE2 for redundant 201 connectivity. However, CE4, which is also in the same VPLS domain, is 202 single-homed to just PE1. 204 CE4 ------- ............... 205 \ . . ___ CE2 206 ___ PE1 . / 207 / : PE3 208 __/ : Service : 209 CE1 __ : Provider PE4 210 \ : : \___ CE3 211 \___ PE2 . 212 . . 213 ............... 215 Figure 2 Scenario 2 217 2.2. VPLS Multi-homing Considerations 219 The first (perhaps obvious) fact about a multi-homed VPLS CE, such as 220 CE1 in Figure 1 is that if CE1 is an Ethernet switch or bridge, a 221 loop has been created in the customer VPLS. This is a dangerous 222 situation for an Ethernet network, and the loop must be broken. Even 223 if CE1 is a router, it will get duplicates every time a packet is 224 flooded, which is clearly undesirable. 226 The next is that (unlike the case of IP-based multi-homing) only one 227 of PE1 and PE2 can be actively sending traffic, either towards CE1 or 228 into the SP cloud. That is to say, load balancing techniques will not 229 work. All other PEs MUST choose the same designated forwarder for a 230 multi-homed site. Call the PE that is chosen to send traffic to/from 231 CE1 the "designated forwarder". 233 In Figure 2, CE1 and CE4 must be dealt with independently, since CE1 234 is dual-homed, but CE4 is not. 236 3. Multi-homing Operation 238 This section describes procedures for electing a designated forwarder 239 among the set of PEs that are multi-homed to a customer site. The 240 procedures described in this section are applicable to BGP based 241 VPLS, LDP based VPLS with BGP-AD or a VPLS that contains a mix of 242 both BGP and LDP signaled PWs. 244 3.1. Provisioning Model 246 Figure 1 shows a customer site, CE1, multi-homed to two VPLS PEs, PE1 247 and PE2. In order for all VPLS PEs within the same VPLS domain to 248 elect one of the multi-homed PEs as the designated forwarder, an 249 indicator that the PEs are multi-homed to the same customer site is 250 required. This is achieved by assigning the same multi-homed site ID 251 (MH-ID) on PE1 and PE2 for CE1. When remote VPLS PEs receive NLRI 252 advertisement from PE1 and PE2 for CE1, the two NLRI advertisements 253 for CE1 are identified as candidates for designated forwarder 254 selection due to the same MH-ID. Thus, same MH-ID SHOULD be assigned 255 on all VPLS PEs that are multi-homed to the same customer site. Note 256 that a MH-ID=0 is invalid and a PE should discard such an 257 advertisement. 259 3.2. Multi-homing NLRI 261 Section 3.2.2 in [RFC4761] describes the encoding of the BGP VPLS 262 NLRI. This NLRI contains fields VE-ID, VE block offset, VE block size 263 and label base. For multi-homing operation, the same NLRI is used for 264 identifying the multi-homed customers sites. The VE-ID field in the 265 NLRI is set to MH-ID; the VE block offset, VE block size and label 266 base are set to zero. Thus, the NLRI contains 2 octets indicating the 267 length, 8 octets for Route Distinguisher, 2 octets for MH-ID and 7 268 octets with value zero. 270 Figure 2 shows two customer sites, CE1 and CE4, connected to PE1 with 271 CE1 multi-homed to PE1 and PE2. CE4 does not require special 272 addressing, being associated with the base VPLS instance identified 273 by the VSI-ID for LDP VPLS and VE-ID for BGP VPLS. However, CE1 which 274 is multi-homed to PE1 and PE2 requires configuration of MH-ID and 275 both PE1 and PE2 MUST be provisioned with the same MH-ID for CE1. It 276 is valid to have non-zero VE block offset, VE block size and label 277 base in the VPLS NLRI for a multi-homed site. However, multi-homing 278 operations in such a case are outside the scope of this document. 280 3.3. Designated Forwarder Election 282 BGP-based multi-homing for VPLS relies on BGP DF election and VPLS DF 283 election. The net result of doing both BGP and VPLS DF election is 284 that of electing a single designated forwarder (DF) among the set of 285 PEs to which a customer site is multi-homed. All the PEs that are 286 elected as non-designated forwarders MUST keep their attachment 287 circuit to the multi-homed CE in blocked status (no forwarding). 289 These election algorithms operate on VPLS advertisements, which 290 include both the NLRI and attached BGP attributes. In order to 291 simplify the explanation of these algorithms, we will use a number of 292 variables derived from fields in the VPLS advertisement. These 293 variables are: RD, MH-ID, VBO, DOM, ACS, PREF and PE-ID. The notation 294 ADV -> means that from a 295 received VPLS advertisement ADV, the respective variables were 296 derived. The following sections describe two attributes needed for DF 297 election, then describe the variables and how they are derived from 298 fields in VPLS advertisement ADV, and finally describe how DF 299 election is done. 301 3.3.1. Attributes 303 The procedures below refer to two attributes: the Route Origin 304 community (see Section 4.1) and the L2-info community (see Section 305 4.2.1). These attributes are required for inter-AS operation; for 306 generality, the procedures below show how they are to be used. The 307 procedures also say how to handle the case that either or both are 308 not present. 310 3.3.2. Variables Used 312 3.3.2.1. RD 314 RD is simply set to the Route Distinguisher field in the NLRI part of 315 ADV. 317 3.3.2.2. MH-ID 319 MH-ID is simply set to the VE-ID field in the NLRI part of ADV. 321 3.3.2.3. VBO 323 VBO is simply set to the VE Block Offset field in the NLRI part of 324 ADV. This field will typically be zero. 326 3.3.2.4. DOM 328 This variable, indicating the VPLS domain to which ADV belongs, is 329 derived by applying BGP policy to the Route Target extended 330 communities in ADV. The details of how this is done are outside the 331 scope of this document. 333 3.3.2.5. ACS 335 ACS is the status of the attachment circuits for a given site of a 336 VPLS. ACS = 1 if all attachment circuits for the site are down, and 0 337 otherwise. 339 For BGP-based Multi-homing, ADV MUST contain an L2-info extended 340 community; within this community are control flags. One of these 341 flags is the 'D' bit, described in [I-D.kothari-l2vpn-auto-site-id]. 342 ACS is set to the value of the 'D' bit in ADV. 344 3.3.2.6. PREF 346 PREF is derived from the Local Preference (LP) attribute in ADV as 347 well as the VPLS Preference field (VP) in the L2-info extended 348 community. If the Local Preference attribute is missing, LP is set to 349 0; if the L2-info community is missing, VP is set to 0. The following 350 table shows how PREF is computed from LP and VP. 352 +---------+---------------+----------+------------------------------+ 353 | VP | LP Value | PREF | Comment | 354 | Value | | Value | | 355 +---------+---------------+----------+------------------------------+ 356 | 0 | 0 | 0 | malformed advertisement, | 357 | | | | unless ACS=1 | 358 | | | | | 359 | 0 | 1 to (2^16-1) | LP | backwards compatibility | 360 | | | | | 361 | 0 | 2^16 to | (2^16-1) | backwards compatibility | 362 | | (2^32-1) | | | 363 | | | | | 364 | >0 | LP same as VP | VP | Implementation supports VP | 365 | | | | | 366 | >0 | LP != VP | 0 | malformed advertisement | 367 +---------+---------------+----------+------------------------------+ 368 Figure 3 PREF table 370 3.3.2.7. PE-ID 372 If ADV contains a Route Origin (RO) community (see Section 4.1) with 373 type 0x01, then PE-ID is set to the Global Administrator sub-field of 374 the RO. Otherwise, if ADV has an ORIGINATOR_ID attribute, then PE-ID 375 is set to the ORIGINATOR_ID. Otherwise, PE-ID is set to the BGP 376 Identifier. 378 3.3.3. Election Procedures 380 The election procedures described in this section apply equally to 381 BGP VPLS and LDP VPLS. 383 Election occurs in two stages. The first stage divides all received 384 VPLS advertisements into buckets of relevant and comparable 385 advertisements. Distinction MUST NOT be made on whether the NLRI is a 386 multi-homing NLRI or not. In this stage, advertisements may be 387 discarded as not being relevant to DF election. The second stage 388 picks a single "winner" from each bucket by repeatedly applying a 389 tie-breaking algorithm on a pair of advertisements from that bucket. 390 The tie-breaking rules are such that the order in which 391 advertisements are picked from the bucket does not affect the final 392 result. Note that this is a conceptual description of the process; an 393 implementation MAY choose to realize this differently as long as the 394 semantics are preserved. 396 Note: these procedures supersede the tie breaking rules described in 397 (Section 9.1.2.2) [RFC4271] 399 3.3.3.1. Bucketization for BGP DF Election 401 An advertisement 403 ADV -> 405 is discarded if DOM is not of interest to the BGP speaker. Otherwise, 406 ADV is put into the bucket for . In other words, 407 the information in BGP DF election consists of 408 and only advertisements with exact same 409 are candidates for DF election. 411 3.3.3.2. Bucketization for VPLS DF Election 413 An advertisement 415 ADV -> 417 is discarded if DOM is not of interest to the VPLS PE. Otherwise, ADV 418 is put into the bucket for . In other words, all 419 advertisements for a particular VPLS domain that have the same MH-ID 420 are candidates for VPLS DF election. 422 3.3.3.3. Tie-breaking Rules 424 This section describes the tie-breaking rules for both BGP and VPLS 425 DF election. Tie-breaking rules for BGP DF election are applied to 426 candidate advertisements by any BGP speaker. Since RD must be same 427 for advertisements to be candidates for BGP DF election, use of 428 unique RDs will result in no candidate advertisements for BGP tie- 429 breaking rules and thus, a BGP speaker in such a case will simply not 430 do BGP DF election. Tie-breaking rules for VPLS DF election are 431 applied to candidate advertisements by all VPLS PEs and the actions 432 taken by VPLS PEs based on the VPLS DF election result are described 433 in Section 3.4. 435 Given two advertisements ADV1 and ADV2 from a given bucket, first 436 compute the variables needed for DF election: 438 ADV1 -> 440 ADV2 -> 442 Note that MH-ID1 = MH-ID2 and DOM1 = DOM2, since ADV1 and ADV2 came 443 from the same bucket. If this is for BGP DF election, RD1 = RD2 and 444 VBO1 = VBO2 as well. Then the following tie-breaking rules MUST be 445 applied in the given order. 447 1. if (ACS1 != 1) AND (ACS2 == 1) ADV1 wins; stop 449 if (ACS1 == 1) AND (ACS2 != 1) ADV2 wins; stop 451 else continue 453 2. if (PREF1 > PREF2) ADV1 wins; stop; 455 else if (PREF1 < PREF2) ADV2 wins; stop; 457 else continue 459 3. if (PE-ID1 < PE-ID2) ADV1 wins; stop; 461 else if (PE-ID1 > PE-ID2) ADV2 wins; stop; 463 else ADV1 and ADV2 are from the same VPLS PE 465 For BGP DF election, if there is no winner and ADV1 and ADV2 are from 466 the same PE, BGP DF election should simply consider this as an 467 update. 469 For VPLS DF election, if there is no winner and ADV1 and ADV2 are 470 from the same PE, a VPLS PE MUST retain both ADV1 and ADV2. 472 3.4. DF Election on PEs 474 DF election algorithm MUST be run by all multi-homed VPLS PEs. In 475 addition, all other PEs SHOULD also run the DF election algorithm. As 476 a result of the DF election, multi-homed PEs that loose the DF 477 election for a MH-ID MUST put the ACs associated with the MH-ID in 478 non-forwarding state. 480 DF election result on the egress PEs can be used in traffic 481 forwarding decision. Figure 2 shows two customer sites, CE1 and CE4, 482 connected to PE1 with CE1 multi-homed to PE1 and PE2. If PE1 is the 483 designated forwarder for CE1, based on the DF election result, PE3 484 can chose to not send unknown unicast and multicast traffic to PE2 as 485 PE2 is not the designated forwarder for any customer site and it has 486 no other single homed sites connected to it. 488 4. Multi-AS VPLS 490 This section describes multi-homing in an inter-AS context. 492 4.1. Route Origin Extended Community 494 Due to lack of information about the PEs that originate the VPLS 495 NLRIs in inter-AS operations, Route Origin Extended Community 496 [RFC4360] is used to carry the source PE's IP address. 498 To use Route Origin Extended Community for carrying the originator 499 VPLS PE's loopback address, the type field of the community MUST be 500 set to 0x01 and the Global Administrator sub-field MUST be set to the 501 PE's loopback IP address. 503 4.2. VPLS Preference 505 When multiple PEs are assigned the same site ID for multi-homing, it 506 is often desired to be able to control the selection of a particular 507 PE as the designated forwarder. Section 3.5 in [RFC4761] describes 508 the use of BGP Local Preference in path selection to choose a 509 particular NLRI, where Local Preference indicates the degree of 510 preference for a particular VE. The use of Local Preference is 511 inadequate when VPLS PEs are spread across multiple ASes as Local 512 Preference is not carried across AS boundary. A new field, VPLS 513 preference (VP), is introduced in this document that can be used to 514 accomplish this. VPLS preference indicates a degree of preference for 515 a particular customer site. VPLS preference is not mandatory for 516 intra-AS operation; the algorithm explained in Section 3.3 will work 517 with or without the presence of VPLS preference. 519 Section 3.2.4 in [RFC4761] describes the Layer2 Info Extended 520 Community that carries control information about the pseudowires. The 521 last two octets that were reserved now carries VPLS preference as 522 shown in Figure 3. 524 +------------------------------------+ 525 | Extended community type (2 octets) | 526 +------------------------------------+ 527 | Encaps Type (1 octet) | 528 +------------------------------------+ 529 | Control Flags (1 octet) | 530 +------------------------------------+ 531 | Layer-2 MTU (2 octet) | 532 +------------------------------------+ 533 | VPLS Preference (2 octets) | 534 +------------------------------------+ 536 Figure 4 Layer 2 Info Extended Community 538 A VPLS preference is a 2-octets unsigned integer. A value of zero 539 indicates absence of a VP and is not a valid preference value. This 540 interpretation is required for backwards compatibility. 541 Implementations using Layer2 Info Extended Community as described in 542 (Section 3.2.4) [RFC4761] MUST set the last two octets as zero since 543 it was a reserved field. 545 For backwards compatibility, if VP is used, then BGP Local Preference 546 MUST be set to the value of VP. Note that a Local Preference value of 547 zero for a MH-Site is not valid unless 'D' bit in the control flags 548 is set (see [I-D.kothari-l2vpn-auto-site-id]). In addition, Local 549 Preference value greater than or equal to 2^16 for VPLS 550 advertisements is not valid. 552 4.3. Use of BGP-MH attributes in Inter-AS Methods 554 Section 3.4 in [RFC4761] and section 4 in [I-D.ietf-l2vpn-signaling] 555 describe three methods (a, b and c) to connect sites in a VPLS to PEs 556 that are across multiple AS. Since VPLS advertisements in method (a) 557 do not cross AS boundaries, multi-homing operations for method (a) 558 remain exactly the same as they are within as AS. However, for method 559 (b) and (c), VPLS advertisements do cross AS boundary. This section 560 describes the VPLS operations for method (b) and method (c). Consider 561 Figure 5 for inter-AS VPLS with multi-homed customer sites. 563 4.3.1. Inter-AS Method (b): EBGP Redistribution of VPLS Information 564 between ASBRs 566 AS1 AS2 567 ........ ........ 568 CE2 _______ . . . . 569 ___ PE1 . . PE3 --- CE3 570 / : . . : 571 __/ : : : : 572 CE1 __ : ASBR1 --- ASBR2 : 573 \ : : : : 574 \___ PE2 . . PE4 ---- CE4 575 . . . . 576 ........ ........ 578 Figure 5 Inter-AS VPLS 580 A customer has four sites, CE1, CE2, CE3 and CE4. CE1 is multi-homed 581 to PE1 and PE2 in AS1. CE2 is single-homed to PE1. CE3 and CE4 are 582 also single homed to PE3 and PE4 respectively in AS2. Assume that in 583 addition to the base LDP/BGP VPLS addressing (VSI-IDs/VE-IDs), MH ID 584 1 is assigned for CE1. After running DF election algorithm, all four 585 VPLS PEs must elect the same designated forwarder for CE1 site. Since 586 BGP Local Preference is not carried across AS boundary, VPLS 587 preference as described in Section 4.2 MUST be used for carrying site 588 preference in inter-AS VPLS operations. 590 For Inter-AS method (b) ASBR1 will send a VPLS NLRI received from PE1 591 to ASBR2 with itself as the BGP nexthop. ASBR2 will send the received 592 NLRI from ASBR1 to PE3 and PE4 with itself as the BGP nexthop. Since 593 VPLS PEs use BGP Local Preference in DF election, for backwards 594 compatibility, ASBR2 MUST set the Local Preference value in the VPLS 595 advertisements it sends to PE3 and PE4 to the VPLS preference value 596 contained in the VPLS advertisement it receives from ASBR1. ASBR1 597 MUST do the same for the NLRIs it sends to PE1 and PE2. If ASBR1 598 receives a VPLS advertisement without a valid VPLS preference from a 599 PE within its AS, then ASBR1 MUST set the VPLS preference in the 600 advertisements to the Local Preference value before sending it to 601 ASBR2. Similarly, ASBR2 must do the same for advertisements without 602 VPLS Preference it receives from PEs within its AS. Thus, in method 603 (b), ASBRs MUST update the VPLS and Local Preference based on the 604 advertisements they receive either from an ASBR or a PE within their 605 AS. 607 In Figure 5, PE1 will send the VPLS advertisements, including the 608 ones for MH site CE1, with Route Origin Extended Community containing 609 its loopback address. PE2 will do the same. Even though PE3 receives 610 the VPLS advertisements from the same BGP nexthop, ASBR2, the source 611 PE address contained in the Route Origin Extended Community is 612 different for the VPLS advertisements received from PE1 and PE2, and 613 thus, PE3 can apply correctly the DF Election algorithm as the 614 resulting PE-IDs are different. 616 4.3.2. Inter-AS Method (c): Multi-Hop EBGP Redistribution of VPLS 617 Information between ASes 619 In this method, there is a multi-hop E-BGP peering between the PEs or 620 Route Reflectors in AS1 and the PEs or Route Reflectors in AS2. There 621 is no VPLS state in either control or data plane on the ASBRs. 623 The multi-homing operations on the PEs in this method are exactly the 624 same as they are in intra-AS scenario. However, since Local 625 Preference is not carried across AS boundary, the translation of LP 626 to VP and vice versa MUST be done by RR, if RR is used to reflect 627 VPLS advertisements to other ASes. This is exactly the same as what a 628 ASBR does in case of method (b). A RR must set the VP to the LP value 629 in an advertisement before sending it to other ASes and must set the 630 LP to the VP value in an advertisement that it receives from other 631 ASes before sending to the PEs within the AS. 633 5. MAC Flush Operations 635 In a service provider VPLS network, customer MAC learning is confined 636 to PE devices and any intermediate nodes, such as a Route Reflector, 637 do not have any state for MAC addresses. 639 Topology changes either in the service provider's network or in 640 customer's network can result in the movement of MAC addresses from 641 one PE device to another. Such events can result into traffic being 642 dropped due to stale state of MAC addresses on the PE devices. Age 643 out timers that clear the stale state will resume the traffic 644 forwarding, but age out timers are typically in minutes, and 645 convergence of the order of minutes can severely impact customer's 646 service. To handle such events and expedite convergence of traffic, 647 flushing of affected MAC addresses is highly desirable. 649 This section describes the scenarios where VPLS flush is desirable 650 and the specific VPLS Flush TLVs that provide capability to flush the 651 affected MAC addresses on the PE devices. All operations described in 652 this section are in context of a particular VPLS domain and not 653 across multiple VPLS domains. Mechanisms for MAC flush are described 654 in [I-D.kothari-l2vpn-vpls-flush] for BGP based VPLS and in [RFC4762] 655 for LDP based VPLS. 657 5.1. MAC List Flush 659 If multiple customer sites are connected to the same PE, PE1 as shown 660 in Figure 2, and redundancy per site is desired when multi-homing 661 procedures described in this document are in effect, then it is 662 desirable to flush just the relevant MAC addresses from a particular 663 site when the site connectivity is lost. 665 To flush particular set of MAC addresses, a PE SHOULD originate a 666 flush message with MAC list that contains a list of MAC addresses 667 that needs to be flushed. In Figure 2, if connectivity between CE1 668 and PE1 goes down and if PE1 was the designated forwarder for CE1, 669 PE1 SHOULD send a list of MAC addresses that belong to CE1 to all its 670 BGP peers. 672 It is RECOMMENDED that in case of excessive link flap of customer 673 attachment circuit in a short duration, a PE should have a means to 674 throttle advertisements of flush messages so that excessive flooding 675 of such advertisements do not occur. 677 5.2. Implicit MAC Flush 679 When connectivity to a customer site is lost, remote PEs learn that a 680 particular site is no longer reachable. The local PE either withdraws 681 the VPLS NLRI that it previously advertised for the site or it sends 682 a BGP update message for the site's VPLS NLRI with the 'D' bit set. 684 If a remote PE detects that a multi-homed PE has transitioned from 685 being a DF to a non-DF, then the remote PE can choose to flush all 686 MAC addresses that it learned from the multi-homed PE transitioning 687 from DF to non-DF. Alternatively the remote PE may chose to react 688 when detecting the non-DF to DF transition for a multi-homed PE by 689 flushing in the related VPLS context all the MACs learned with the 690 exception of the MACs associated with the new DF PE. 692 6. Backwards Compatibility 694 No forwarding loops are formed when PEs or Route Reflectors that do 695 not support procedures defined in this section co exist in the 696 network with PEs or Route Reflectors that do support. 698 6.1. BGP based VPLS 700 As explained in this section, multi-homed PEs to the same customer 701 site MUST assign the same MH-ID and related NLRI SHOULD contain the 702 block offset, block size and label base as zero. Remote PEs that lack 703 support of multi-homing operations specified in this document will 704 fail to create any PWs for the multi-homed MH-IDs due to the label 705 value of zero and thus, the multi-homing NLRI should have no impact 706 on the operation of Remote PEs that lack support of multi-homing 707 operations specified in this document. 709 6.2. LDP VPLS with BGP Auto-discovery 711 The BGP-AD NLRI has a prefix length of 12 containing only a 8 bytes 712 RD and a 4 bytes VSI-ID. If a LDP VPLS PEs running BGP AD lacks 713 support of multi-homing operations specified in this document, it 714 SHOULD ignore a MH NLRI with the length field of 17. As a result it 715 will not ask LDP to create any PWs for the multi-homed Site-ID and 716 thus, the multi-homing NLRI should have no impact on LDP VPLS 717 operation. 719 7. Security Considerations 721 No new security issues are introduced beyond those that are described 722 in [RFC4761] and [RFC4762]. 724 8. IANA Considerations 726 At this time, this memo includes no request to IANA. 728 9. References 730 9.1. Normative References 732 [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate 733 Requirement Levels", BCP 14, RFC 2119, March 1997. 735 [RFC4761] Kompella, K. and Y. Rekhter, "Virtual Private LAN Service 736 (VPLS) Using BGP for Auto-Discovery and Signaling", RFC 737 4761, January 2007. 739 [RFC4447] Martini, L., Rosen, E., El-Aawar, N., Smith, T., and G. 740 Heron, "Pseudowire Setup and Maintenance Using the Label 741 Distribution Protocol (LDP)", RFC 4447, April 2006. 743 [RFC4446] Martini, L., "IANA Allocations for Pseudowire Edge to Edge 744 Emulation (PWE3)", BCP 116, RFC 4446, April 2006. 746 [I-D.ietf-l2vpn-signaling] Rosen, E., "Provisioning, Autodiscovery, 747 and Signaling in L2VPNs", draft-ietf-l2vpn-signaling-08 748 (work in progress), May 2006. 750 [I-D.kothari-l2vpn-vpls-flush] Kothari, B. and R. Fernando, "VPLS 751 Flush in BGP-based Virtual Private LAN Service", draft- 752 kothari-l2vpn-vpls-flush-00 (work in progress), October 753 2008. 755 [I-D.kothari-l2vpn-auto-site-id] Kothari, B., Kompella, K., and T. 756 IV, "Automatic Generation of Site IDs for Virtual Private 757 LAN Service", draft-kothari-l2vpn-auto-site-id-01 (work in 758 progress), October 2008. 760 [I-D.ietf-pwe3-redundancy-bit] Muley, P., Bocci, M., and L. Martini, 761 "Preferential Forwarding Status bit definition", draft- 762 ietf-pwe3-redundancy-bit-01 (work in progress), September 763 2008. 765 9.2. Informative References 767 [RFC4360] Sangli, S., Tappan, D., and Y. Rekhter, "BGP Extended 768 Communities Attribute", RFC 4360, February 2006. 770 [RFC4364] Rosen, E. and Y. Rekhter, "BGP/MPLS IP Virtual Private 771 Networks (VPNs)", RFC 4364, February 2006. 773 [RFC4456] Bates, T., Chen, E., and R. Chandra, "BGP Route Reflection: 774 An Alternative to Full Mesh Internal BGP (IBGP)", RFC 4456, 775 April 2006. 777 [RFC4762] Lasserre, M. and V. Kompella, "Virtual Private LAN Service 778 (VPLS) Using Label Distribution Protocol (LDP) Signaling", 779 RFC 4762, January 2007. 781 [RFC4271] Rekhter, Y., Li, T., and S. Hares, "A Border Gateway 782 Protocol 4 (BGP-4)", RFC 4271, January 2006. 784 10. Acknowledgments 786 The authors would like to thank Yakov Rekhter, Nischal Sheth, Mitali 787 Singh, Deven Raut and Nehal Bhau for their insightful comments and 788 probing questions. 790 Authors' Addresses 792 Bhupesh Kothari 793 Juniper Networks 794 1194 N. Mathilda Ave. 795 Sunnyvale, CA 94089 US 797 Email: bhupesh@juniper.net 799 Kireeti Kompella 800 Juniper Networks 801 1194 N. Mathilda Ave. 802 Sunnyvale, CA 94089 US 804 Email: kireeti@juniper.net 806 Wim Henderickx 807 Alcatel-Lucent 808 Copernicuslaan 50 809 2018 Antwerp, Belgium 811 Email: wim.henderickx@alcatel-lucent.be 813 Florin Balus 814 Alcatel-Lucent 815 701 E. Middlefield Road 816 Mountain View, CA, USA 94043 818 Email: florin.balus@alcatel-lucent.com 820 James Uttaro 821 AT&T 822 200 S. Laurel Avenue 823 Middletown, NJ 07748 824 USA 826 Email: uttaro@att.com