idnits 2.17.1 draft-ietf-trill-multilevel-single-nickname-08.txt: Checking boilerplate required by RFC 5378 and the IETF Trust (see https://trustee.ietf.org/license-info): ---------------------------------------------------------------------------- No issues found here. Checking nits according to https://www.ietf.org/id-info/1id-guidelines.txt: ---------------------------------------------------------------------------- No issues found here. Checking nits according to https://www.ietf.org/id-info/checklist : ---------------------------------------------------------------------------- No issues found here. Miscellaneous warnings: ---------------------------------------------------------------------------- == The copyright year in the IETF Trust and authors Copyright Line does not match the current year -- The document date (March 8, 2019) is 1848 days in the past. Is this intentional? Checking references for intended status: Proposed Standard ---------------------------------------------------------------------------- (See RFCs 3967 and 4897 for information about using normative references to lower-maturity documents in RFCs) -- Looks like a reference, but probably isn't: '256' on line 449 -- Looks like a reference, but probably isn't: '257' on line 450 Summary: 0 errors (**), 0 flaws (~~), 1 warning (==), 3 comments (--). Run idnits with the --verbose option for more detailed information about the items above. -------------------------------------------------------------------------------- 2 INTERNET-DRAFT M. Zhang 3 Intended Status: Proposed Standard D. Eastlake 4 Huawei 5 R. Perlman 6 EMC 7 M. Cullen 8 Painless Security 9 H. Zhai 10 JIT 11 Expires: September 9, 2019 March 8, 2019 13 Transparent Interconnection of Lots of Links (TRILL) 14 Single Area Border RBridge Nickname for Multilevel 15 draft-ietf-trill-multilevel-single-nickname-08.txt 17 Abstract 19 A major issue in multilevel TRILL is how to manage RBridge nicknames. 20 In this document, the area border RBridge uses a single nickname in 21 both Level 1 and Level 2. RBridges in Level 2 must obtain unique 22 nicknames but RBridges in different Level 1 areas may have the same 23 nicknames. 25 Status of This Memo 27 This Internet-Draft is submitted to IETF in full conformance with the 28 provisions of BCP 78 and BCP 79. 30 Internet-Drafts are working documents of the Internet Engineering 31 Task Force (IETF), its areas, and its working groups. Note that other 32 groups may also distribute working documents as Internet-Drafts. 34 Internet-Drafts are draft documents valid for a maximum of six months 35 and may be updated, replaced, or obsoleted by other documents at any 36 time. It is inappropriate to use Internet-Drafts as reference 37 material or to cite them other than as "work in progress." 39 The list of current Internet-Drafts can be accessed at 40 http://www.ietf.org/1id-abstracts.html 42 The list of Internet-Draft Shadow Directories can be accessed at 43 http://www.ietf.org/shadow.html 45 Copyright and License Notice 47 Copyright (c) 2019 IETF Trust and the persons identified as the 48 document authors. All rights reserved. 50 This document is subject to BCP 78 and the IETF Trust's Legal 51 Provisions Relating to IETF Documents 52 (http://trustee.ietf.org/license-info) in effect on the date of 53 publication of this document. Please review these documents 54 carefully, as they describe your rights and restrictions with respect 55 to this document. Code Components extracted from this document must 56 include Simplified BSD License text as described in Section 4.e of 57 the Trust Legal Provisions and are provided without warranty as 58 described in the Simplified BSD License. 60 Table of Contents 62 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 2 63 2. Acronyms and Terminology . . . . . . . . . . . . . . . . . . . 3 64 3. Nickname Handling on Border RBridges . . . . . . . . . . . . . 3 65 3.1. Actions on Unicast Packets . . . . . . . . . . . . . . . . 4 66 3.2. Actions on Multi-Destination Packets . . . . . . . . . . . 5 67 4. Per-flow Load Balancing . . . . . . . . . . . . . . . . . . . . 6 68 4.1. Ingress Nickname Replacement . . . . . . . . . . . . . . . 7 69 4.2. Egress Nickname Replacement . . . . . . . . . . . . . . . . 7 70 5. Protocol Extensions for Discovery . . . . . . . . . . . . . . . 7 71 5.1. Discovery of Border RBridges in L1 . . . . . . . . . . . . 7 72 5.2. Discovery of Border RBridge Sets in L2 . . . . . . . . . . 8 73 6. One Border RBridge Connects Multiple Areas . . . . . . . . . . 8 74 7. E-L1FS/E-L2FS Backwards Compatibility . . . . . . . . . . . . . 9 75 8. Security Considerations . . . . . . . . . . . . . . . . . . . . 9 76 9. IANA Considerations . . . . . . . . . . . . . . . . . . . . . . 10 77 10. References . . . . . . . . . . . . . . . . . . . . . . . . . . 10 78 10.1. Normative References . . . . . . . . . . . . . . . . . . . 10 79 10.2. Informative References . . . . . . . . . . . . . . . . . . 11 80 Appendix A. Clarifications . . . . . . . . . . . . . . . . . . . . 11 81 A.1. Level Transition . . . . . . . . . . . . . . . . . . . . . 11 82 Author's Addresses . . . . . . . . . . . . . . . . . . . . . . . . 13 84 1. Introduction 86 TRILL (Transparent Interconnection of Lots of Links [RFC6325] 87 [RFC7780]) multilevel techniques are designed to improve TRILL 88 scalability issues. As described in [RFC8243], there have been two 89 proposed approaches. One approach, which is referred to as the 90 "unique nickname" approach, gives unique nicknames to all the TRILL 91 switches in the multilevel campus, either by having the Level- 92 1/Level-2 border TRILL switches advertise which nicknames are not 93 available for assignment in the area, or by partitioning the 16-bit 94 nickname into an "area" field and a "nickname inside the area" field. 95 The other approach, which is referred to in [RFC8243] as the 96 "aggregated nickname" approach, involves assigning nicknames to the 97 areas, and allowing nicknames to be reused in different areas, by 98 having the border TRILL switches rewrite the nickname fields when 99 entering or leaving an area. 101 The approach specified in this document is somewhat similar to the 102 "aggregated nickname" approach in [RFC8243] but with very important 103 difference. In this document, the nickname of an area border RBridge 104 is used in both Level 1 (L1) and Level 2 (L2). No additional 105 nicknames are assigned to the L1 areas. Instead, each L1 area is 106 denoted by the set of all nicknames of those border RBridges of the 107 area. For this approach, nicknames in L2 MUST be unique but nicknames 108 inside an L1 areas MAY be reused in other L1 areas that also use this 109 approach. The use of the approach specified in this document in one 110 L1 area does not prohibit the use of other approaches in other L1 111 areas in the same TRILL campus. 113 2. Acronyms and Terminology 115 Data Label: VLAN or FGL Fine-Grained Label (FGL) 117 DBRB: Designated Border RBridge. 119 IS-IS: Intermediate System to Intermediate System [IS-IS] 121 The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", 122 "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and 123 "OPTIONAL" in this document are to be interpreted as described in BCP 124 14 [RFC2119] [RFC8174] when, and only when, they appear in all 125 capitals, as shown here. 127 Familiarity with [RFC6325] is assumed in this document. 129 3. Nickname Handling on Border RBridges 131 This section provides an illustrative example and description of the 132 border learning border RBridge nicknames. 134 Area {2,20} level 2 Area {3,30} 135 +-------------------+ +-----------------+ +--------------+ 136 | | | | | | 137 | S--RB27---Rx--Rz----RB2---Rb---Rc--Rd---Re--RB3---Rk--RB44---D | 138 | 27 | | | | 44 | 139 | ----RB20--- ----RB30--- | 140 +-------------------+ +-----------------+ +--------------+ 142 Figure 1: An Example Topology for TRILL Multilevel 144 In Figure 1, RB2, RB20, RB3 and RB30 are area border TRILL switches 145 (RBridges). Their nicknames are 2, 20, 3 and 30 respectively and are 146 used as TRILL switch identifiers in their areas [RFC6325]. Area 147 border RBridges use the set of border nicknames to denote the L1 area 148 that they are attached to. For example, RB2 and RB20 use nicknames 149 {2,20} to denote the L1 area on the left. 151 A source S is attached to RB27 and a destination D is attached to 152 RB44. RB27 has a nickname, say 27, and RB44 has a nickname, say 44 153 (and in fact, they could even have the same nickname, since the TRILL 154 switch nickname will not be visible outside these Level 1 areas). 156 3.1. Actions on Unicast Packets 158 Let's say that S transmits a frame to destination D and let's say 159 that D's location is learned by the relevant TRILL switches already. 160 These relevant switches have learned the following: 162 1) RB27 has learned that D is connected to nickname 3. 163 2) RB3 has learned that D is attached to nickname 44. 165 The following sequence of events will occur: 167 - S transmits an Ethernet frame with source MAC = S and destination 168 MAC = D. 170 - RB27 encapsulates with a TRILL header with ingress RBridge = 27, 171 and egress RBridge = 3 producing a TRILL Data packet. 173 - RB2 and RB20 have announced in the Level 1 IS-IS instance in area 174 {2,20}, that they are attached to all those area nicknames, 175 including {3,30}. Therefore, IS-IS routes the packet to RB2 (or 176 RB20, if RB20 on the least-cost route from RB27 to RB3). 178 - RB2, when transitioning the packet from Level 1 to Level 2, 179 replaces the ingress TRILL switch nickname with its own nickname, 180 so replaces 27 with 2. Within Level 2, the ingress RBridge field 181 in the TRILL header will therefore be 2, and the egress RBridge 182 field will be 3. (The egress nickname MAY be replaced with an area 183 nickname selected from {3,30}. See Section 4 for the detail of the 184 selection method. Here, suppose nickname 3 is used.) Also RB2 185 learns that S is attached to nickname 27 in area {2,20} to 186 accommodate return traffic. RB2 SHOULD synchronize with RB20 using 187 ESADI protocol [RFC7357] that MAC = S is attached to nickname 27. 189 - The packet is forwarded through Level 2, to RB3, which has 190 advertised, in Level 2, its L2 nickname as 3. 192 - RB3, when forwarding into area {3,30}, replaces the egress 193 nickname in the TRILL header with RB44's nickname (44). (The 194 ingress nickname MAY be replaced with an area nickname selected 195 from {2,20}. See Section 4 for the detail of the selection method. 196 Here, suppose nickname 2 is selected.) So, within the destination 197 area, the ingress nickname will be 2 and the egress nickname will 198 be 44. 200 - RB44, when decapsulating, learns that S is attached to nickname 2, 201 which is one of the area nicknames of the ingress. 203 3.2. Actions on Multi-Destination Packets 205 Distribution trees for flooding of multi-destination packets are 206 calculated separately within each L1 area and in L2. When a multi- 207 destination packet arrives at the border, it needs to be transitioned 208 either from L1 to L2, or from L2 to L1. All border RBridges are 209 eligible for Level transition. However, for each multi-destination 210 packet, only one of them acts as the Designated Border RBridge (DBRB) 211 to do the transition while other non-DBRBs MUST drop the received 212 copies. All border RBridges of an area MUST agree on a pseudorandom 213 algorithm and locally determine the DBRB as they do in the "Per-flow 214 Load Balancing" section. It's also possible to implement a certain 215 election protocol to elect the DBRB. However, such kind of 216 implementations are out the scope of this document. By default, the 217 border RBridge with the smallest nickname, considered as an unsigned 218 integer, is elected DBRB. 220 As per [RFC6325], multi-destination packets can be classified into 221 three types: unicast packet with unknown destination MAC address 222 (unknown-unicast packet), multicast packet and broadcast packet. Now 223 suppose that D's location has not been learned by RB27 or the frame 224 received by RB27 is recognized as broadcast or multicast. What will 225 happen, as it would in TRILL today, is that RB27 will forward the 226 packet as multi-destination, setting its M bit to 1 and choosing an 227 L1 tree, flooding the packet on the distribution tree, subject to 228 possible pruning. 230 When the copies of the multi-destination packet arrive at area border 231 RBridges, non-DBRBs MUST drop the packet while the DBRB, say RB2, 232 needs to do the Level transition for the multi-destination packet. 233 For a unknown-unicast packet, if the DBRB has learnt the destination 234 MAC address, it SHOULD convert the packet to unicast and set its M 235 bit to 0. Otherwise, the multi-destination packet will continue to be 236 flooded as multicast packet on the distribution tree. The DBRB 237 chooses the new distribution tree by replacing the egress nickname 238 with the new root RBridge nickname. The following sequence of events 239 will occur: 241 - RB2, when transitioning the packet from Level 1 to Level 2, 242 replaces the ingress TRILL switch nickname with its own nickname, 243 so replaces 27 with 2. RB2 also needs to replace the egress 244 RBridge nickname with an L2 tree root RBridge nickname, say 2. In 245 order to accommodate return traffic, RB2 records that S is 246 attached to nickname 27 and SHOULD use ESADI protocol to 247 synchronize this attachment information with other border RBridges 248 (say RB20) in the area. 250 - RB20, will receive the packet flooded on the L2 tree by RB2. It is 251 important that RB20 does not transition this packet back to L1 as 252 it does for a multicast packet normally received from another 253 remote L1 area. RB20 should examine the ingress nickname of this 254 packet. If this nickname is found to be a border RBridge nickname 255 of the area {2,20}, RB2 must not forwarded the packet into this 256 area. 258 - The packet is flooded on the Level 2 tree to reach both RB3 and 259 RB30. Suppose RB3 is the selected DBRB. The non-DBRB RB30 will 260 drop the packet. 262 - RB3, when forwarding into area {3,30}, replaces the egress 263 nickname in the TRILL header with the root RBridge nickname, say 264 3, of the distribution tree of L1 area {3,30}. (Here, the ingress 265 nickname MAY be replaced with a different area nickname selected 266 from {2,20}, the set of border RBridges to the ingress area, as 267 specified in Section 4.) Now suppose that RB27 has learned the 268 location of D (attached to nickname 3), but RB3 does not know 269 where D is. In that case, RB3 must turn the packet into a multi- 270 destination packet and floods it on the distribution tree of L1 271 area {3,30}. 273 - RB30, will receive the packet flooded on the L1 tree by RB3. It is 274 important that RB30 does not transition this packet back to L2. 275 RB30 should also examine the ingress nickname of this packet. If 276 this nickname is found to be an L2 border RBridge nickname, RB30 277 must not transition the packet back to L2. 279 - The multicast listener RB44, when decapsulating the received 280 packet, learns that S is attached to nickname 2, which is one of 281 the area nicknames of the ingress. 283 4. Per-flow Load Balancing 285 Area border RBridges perform ingress/egress nickname replacement when 286 they transition TRILL data packets between Level 1 and Level 2. This 287 nickname replacement enables the per-flow load balance which is 288 specified as follows. 290 4.1. Ingress Nickname Replacement 292 When a TRILL data packet from other areas arrives at an area border 293 RBridge, this RBridge MAY select one area nickname of the ingress 294 area to replace the ingress nickname of the packet so that the 295 returning TRILL data packet can be forwarded to this selected 296 nickname. The selection is simply based on a pseudorandom algorithm 297 as defined in Section 5.3 of [RFC7357]. With the random ingress 298 nickname replacement, the border RBridge actually achieves a per-flow 299 load balance for returning traffic. 301 All area border RBridges in an L1 area MUST agree on the same 302 pseudorandom algorithm. The source MAC address, ingress area 303 nicknames, egress area nicknames and the Data Label of the received 304 TRILL data packet are candidate factors of the input of this 305 pseudorandom algorithm. Note that the value of the destination MAC 306 address SHOULD be excluded from the input of this pseudorandom 307 algorithm, otherwise the egress RBridge will see one source MAC 308 address flip flopping among multiple ingress RBridges. 310 4.2. Egress Nickname Replacement 312 When a TRILL data packet originated from an L1 area arrives at an 313 area border RBridge, that RBridge MAY select one area nickname of the 314 egress area to replace the egress nickname of the packet. By default, 315 it SHOULD choose the egress area border RBridge with the least cost 316 route to reach. The pseudorandom algorithm as defined in Section 5.3 317 of [RFC7357] may be used as an alternative. In that case, however, 318 the ingress area border RBridge may take the non-least-cost Level 2 319 route to forward the TRILL data packet to the egress area border 320 RBridge. 322 5. Protocol Extensions for Discovery 324 5.1. Discovery of Border RBridges in L1 326 The following Level 1 Border RBridge APPsub-TLV will be included in 327 an E-L1FS FS-LSP fragment zero [RFC7780] as an APPsub-TLV of the 328 TRILL GENINFO-TLV. Through listening for this APPsub-TLV, an area 329 border RBridge discovers all other area border RBridges in this area. 331 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 332 | Type = L1-BORDER-RBRIDGE | (2 bytes) 333 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 334 | Length | (2 bytes) 335 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 336 | Sender Nickname | (2 bytes) 337 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 338 o Type: Level 1 Border RBridge (TRILL APPsub-TLV type tbd1) 340 o Length: 2 342 o Sender Nickname: The nickname the originating IS will use as the 343 L1 Border RBridge nickname. This field is useful because the 344 originating IS might own multiple nicknames. 346 5.2. Discovery of Border RBridge Sets in L2 348 The following APPsub-TLV will be included in an E-L2FS FS-LSP 349 fragment zero [RFC7780] as an APPsub-TLV of the TRILL GENINFO-TLV. 350 Through listening to this APPsub-TLV in L2, an area border RBridge 351 discovers all groups of L1 border RBridges and each such group 352 identifies an area. 354 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 355 | Type = L1-BORDER-RB-GROUP | (2 bytes) 356 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 357 | Length | (2 bytes) 358 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 359 | L1 Border RBridge Nickname 1 | (2 bytes) 360 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 361 | ... | 362 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 363 | L1 Border RBridge Nickname k | (2 bytes) 364 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 366 o Type: Level 1 Border RBridge Group (TRILL APPsub-TLV type tbd2) 368 o Length: 2 * k. If length is not a multiple of 2, the APPsub-TLV is 369 corrupt and MUST be ignored. 371 o L1 Border RBridge Nickname: The nickname that an area border 372 RBridge uses as the L1 Border RBridge nickname. The L1-BORDER-RB- 373 GROUP TLV generated by an area border RBridge MUST include all L1 374 Border RBridge nicknames of the area. It's RECOMMENDED that these 375 k nicknames are ordered in ascending order according to the 2- 376 octet nickname considered as an unsigned integer. 378 When an L1 area is partitioned [RFC8243], border RBridges will re- 379 discover each other in both L1 and L2 through exchanging LSPs. In L2, 380 the set of border RBridge nicknames for this splitting area will 381 change. Border RBridges that detect such a change MUST flush the 382 reach-ability information associated to any RBridge nickname from 383 this changing set. 385 6. One Border RBridge Connects Multiple Areas 386 It's possible that one border RBridge (say RB1) connects multiple L1 387 areas. RB1 SHOULD use a single area nickname for all these areas. 389 Nicknames used within one of these areas can be reused within other 390 areas. It's important that packets destined to those duplicated 391 nicknames are sent to the right area. Since these areas are connected 392 to form a layer 2 network, duplicated {MAC, Data Label} across these 393 areas ought not occur. Now suppose a TRILL data packet arrives at the 394 area border nickname of RB1. For a unicast packet, RB1 can lookup the 395 {MAC, Data Label} entry in its MAC table to identify the right 396 destination area (i.e., the outgoing interface) and the egress 397 RBridge's nickname. For a multicast packet: suppose RB1 is not the 398 DBRB, RB1 will not transition the packet; otherwise, RB1 is the DBRB, 400 - if this packet originated from an area out of the connected areas, 401 RB1 replicates this packet and floods it on the proper Level 1 402 trees of all the areas in which it acts as the DBRB. 404 - if the packet originated from one of the connected areas, RB1 405 replicates the packet it receives from the Level 1 tree and floods 406 it on other proper Level 1 trees of all the areas in which it acts 407 as the DBRB except the originating area (i.e., the area connected 408 to the incoming interface). RB1 might also receive the replication 409 of the packet from the Level 2 tree. This replication MUST be 410 dropped by RB1. It recognizes such packets by their ingress 411 nickname being the nickname of one of the border RBridges of an L1 412 area to which the receiving border RBridge is attached. 414 7. E-L1FS/E-L2FS Backwards Compatibility 416 All Level 2 RBridges MUST support E-L2FS [RFC7356] [RFC7780]. The 417 Extended TLVs defined in Section 5 are to be used in Extended Level 418 1/2 Flooding Scope (E-L1FS/E-L2FS) PDUs. Area border RBridges MUST 419 support both E-L1FS and E-L2FS. RBridges that do not support both 420 E-L1FS or E-L2FS cannot serve as area border RBridges but they can 421 appear in an L1 area acting as non-area-border RBridges. 423 8. Security Considerations 425 For general TRILL Security Considerations, see [RFC6325]. 427 The newly defined TRILL APPsub-TLVs in Section 5 are transported in 428 IS-IS PDUs whose authenticity can be enforced using regular IS-IS 429 security mechanism [IS-IS] [RFC5310]. This document raises no new 430 security issues for IS-IS. 432 Using a variation of aggregated nicknames, and the resulting possible 433 duplication of nicknames between areas, increases the possibility of 434 a TRILL Data packet being delivered to the wrong egress RBridge if 435 areas are unexpectedly merged. However, in many cases the data would 436 be discarded at that egress because it would not match a known end 437 station data label/MAC address. 439 9. IANA Considerations 441 IANA is requested to allocate two new types under the TRILL GENINFO 442 TLV [RFC7357] from the range allocated by standards action for the 443 TRILL APPsub-TLVs defined in Section 5. The following entries are 444 added to the "TRILL APPsub-TLV Types under IS-IS TLV 251 Application 445 Identifier 1" Registry on the TRILL Parameters IANA web page. 447 Type Name Reference 448 --------- ---- --------- 449 tbd1[256] L1-BORDER-RBRIDGE [This document] 450 tbd2[257] L1-BORDER-RB-GROUP [This document] 452 10. References 454 10.1. Normative References 456 [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate 457 Requirement Levels", BCP 14, RFC 2119, DOI 458 10.17487/RFC2119, March 1997, . 461 [RFC6325] Perlman, R., Eastlake 3rd, D., Dutt, D., Gai, S., and A. 462 Ghanwani, "Routing Bridges (RBridges): Base Protocol 463 Specification", RFC 6325, DOI 10.17487/RFC6325, July 2011, 464 . 466 [RFC7356] Ginsberg, L., Previdi, S., and Y. Yang, "IS-IS Flooding 467 Scope Link State PDUs (LSPs)", RFC 7356, DOI 468 10.17487/RFC7356, September 2014, . 471 [RFC7357] Zhai, H., Hu, F., Perlman, R., Eastlake 3rd, D., and O. 472 Stokes, "Transparent Interconnection of Lots of Links 473 (TRILL): End Station Address Distribution Information 474 (ESADI) Protocol", RFC 7357, DOI 10.17487/RFC7357, 475 September 2014, . 477 [RFC7780] Eastlake 3rd, D., Zhang, M., Perlman, R., Banerjee, A., 478 Ghanwani, A., and S. Gupta, "Transparent Interconnection of 479 Lots of Links (TRILL): Clarifications, Corrections, and 480 Updates", RFC 7780, DOI 10.17487/RFC7780, February 2016, 481 . 483 [RFC8174] Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC 2119 484 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174, May 485 2017, . 487 10.2. Informative References 489 [IS-IS] International Organization for Standardization, ISO/IEC 490 10589:2002, "Information technology -- Telecommunications 491 and information exchange between systems -- Intermediate 492 System to Intermediate System intra-domain routeing 493 information exchange protocol for use in conjunction with 494 the protocol for providing the connectionless-mode network 495 service", ISO 8473, Second Edition, November 2002. 497 [RFC5310] Bhatia, M., Manral, V., Li, T., Atkinson, R., White, R., 498 and M. Fanto, "IS-IS Generic Cryptographic Authentication", 499 RFC 5310, DOI 10.17487/RFC5310, February 2009, 500 . 502 [RFC8243] Perlman, R., Eastlake 3rd, D., Zhang, M., Ghanwani, A., 503 and H. Zhai, "Alternatives for Multilevel Transparent 504 Interconnection of Lots of Links (TRILL)", RFC 8243, DOI 505 10.17487/RFC8243, September 2017, . 508 Appendix A. Clarifications 510 A.1. Level Transition 512 It's possible that an L1 RBridge is only reachable from a non-DBRB 513 RBridge. If this non-DBRB RBridge refrains from Level transition, the 514 question is, how can a multicast packet reach this L1 RBridge? The 515 answer is, it will be reached after the DBRB performs the Level 516 transition and floods the packet using an L1 distribution tree. 518 Take the following figure as an example. RB77 is reachable from the 519 border RBridge RB30 while RB3 is the DBRB. RB3 transitions the 520 multicast packet into L1 and floods the packet on the distribution 521 tree rooted from RB3. This packet will finally flooded to RB77 via 522 RB30. 524 Area{3,30} 525 +--------------+ (root) RB3 o 526 | | \ 527 -RB3 | | o RB30 528 | | | / 529 -RB30-RB77 | RB77 o 530 +--------------+ 531 Example Topology L1 Tree 533 In the above example, the multicast packet is forwarded along a non- 534 optimal path. A possible improvement is to have RB3 configured not to 535 belong to this area. In this way, RB30 will surely act as the DBRB to 536 do the Level transition. 538 Author's Addresses 540 Mingui Zhang 541 Huawei Technologies 542 No. 156 Beiqing Rd. Haidian District 543 Beijing 100095 544 China 546 Email: zhangmingui@huawei.com 548 Donald E. Eastlake, 3rd 549 Huawei Technologies 550 1424 Pro Shop Court 551 Davenport, FL 33896 552 United States 554 Phone: +1-508-333-2270 555 Email: d3e3e3@gmail.com 557 Radia Perlman 558 EMC 559 2010 256th Avenue NE, #200 560 Bellevue, WA 98007 561 United States 563 Email: radia@alum.mit.edu 565 Margaret Cullen 566 Painless Security 567 356 Abbott Street 568 North Andover, MA 01845 569 United States 571 Phone: +1-781-405-7464 572 Email: margaret@painless-security.com 573 URI: http://www.painless-security.com 575 Hongjun Zhai 576 Jinling Institute of Technology 577 99 Hongjing Avenue, Jiangning District 578 Nanjing, Jiangsu 211169 579 China 581 Email: honjun.zhai@tom.com