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Checking references for intended status: Experimental ---------------------------------------------------------------------------- No issues found here. Summary: 0 errors (**), 0 flaws (~~), 1 warning (==), 1 comment (--). Run idnits with the --verbose option for more detailed information about the items above. -------------------------------------------------------------------------------- 2 Network Working Group A. Przygienda, Ed. 3 Internet-Draft C. Bowers 4 Intended status: Experimental Juniper 5 Expires: 30 May 2022 Y. Lee 6 A. Sharma 7 Comcast 8 R. White 9 Juniper 10 26 November 2021 12 IS-IS Flood Reflection 13 draft-ietf-lsr-isis-flood-reflection-06 15 Abstract 17 This document describes a backwards compatible, optional IS-IS 18 extension that allows the creation of IS-IS flood reflection 19 topologies. Flood reflection allows topologies in which L1 areas 20 provide transit forwarding for L2 using all available L1 nodes 21 internally. It accomplishes this by creating L2 flood reflection 22 adjacencies within each L1 area. Those adjacencies are used to flood 23 L2 LSPDUs, and they are used in the L2 SPF computation. However, 24 they are not used for forwarding within the flood reflection cluster. 25 This arrangement gives the L2 topology significantly better scaling 26 properties. As additional benefit, only those routers directly 27 participating in flood reflection have to support the feature. This 28 allows for the incremental deployment of scalable L1 transit areas in 29 an existing network, without the necessity of upgrading other routers 30 in the network. 32 Requirements Language 34 The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", 35 "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this 36 document are to be interpreted as described in RFC 2119 [RFC2119]. 38 Status of This Memo 40 This Internet-Draft is submitted in full conformance with the 41 provisions of BCP 78 and BCP 79. 43 Internet-Drafts are working documents of the Internet Engineering 44 Task Force (IETF). Note that other groups may also distribute 45 working documents as Internet-Drafts. The list of current Internet- 46 Drafts is at https://datatracker.ietf.org/drafts/current/. 48 Internet-Drafts are draft documents valid for a maximum of six months 49 and may be updated, replaced, or obsoleted by other documents at any 50 time. It is inappropriate to use Internet-Drafts as reference 51 material or to cite them other than as "work in progress." 53 This Internet-Draft will expire on 30 May 2022. 55 Copyright Notice 57 Copyright (c) 2021 IETF Trust and the persons identified as the 58 document authors. All rights reserved. 60 This document is subject to BCP 78 and the IETF Trust's Legal 61 Provisions Relating to IETF Documents (https://trustee.ietf.org/ 62 license-info) in effect on the date of publication of this document. 63 Please review these documents carefully, as they describe your rights 64 and restrictions with respect to this document. Code Components 65 extracted from this document must include Revised BSD License text as 66 described in Section 4.e of the Trust Legal Provisions and are 67 provided without warranty as described in the Revised BSD License. 69 Table of Contents 71 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3 72 2. Glossary . . . . . . . . . . . . . . . . . . . . . . . . . . 8 73 3. Further Details . . . . . . . . . . . . . . . . . . . . . . . 8 74 4. Encodings . . . . . . . . . . . . . . . . . . . . . . . . . . 9 75 4.1. Flood Reflection TLV . . . . . . . . . . . . . . . . . . 9 76 4.2. Flood Reflection Discovery Sub-TLV . . . . . . . . . . . 11 77 4.3. Flood Reflection Discovery Tunnel Type Sub-Sub-TLV . . . 12 78 4.4. Flood Reflection Adjacency Sub-TLV . . . . . . . . . . . 13 79 4.5. Flood Reflection Discovery . . . . . . . . . . . . . . . 14 80 4.6. Flood Reflection Adjacency Formation . . . . . . . . . . 15 81 5. Route Computation . . . . . . . . . . . . . . . . . . . . . . 15 82 5.1. Tunnel Based Deployment . . . . . . . . . . . . . . . . . 16 83 5.2. No Tunnel Deployment . . . . . . . . . . . . . . . . . . 16 84 6. Redistribution of Prefixes . . . . . . . . . . . . . . . . . 16 85 7. Special Considerations . . . . . . . . . . . . . . . . . . . 17 86 8. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 17 87 8.1. New IS-IS TLV Codepoint . . . . . . . . . . . . . . . . . 17 88 8.2. Sub TLVs for TLV 242 . . . . . . . . . . . . . . . . . . 18 89 8.3. Sub-sub TLVs for Flood Reflection Discovery sub-TLV . . . 18 90 8.4. Sub TLVs for TLV 22, 23, 25, 141, 222, and 223 . . . . . 18 91 9. Security Considerations . . . . . . . . . . . . . . . . . . . 19 92 10. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 19 93 11. References . . . . . . . . . . . . . . . . . . . . . . . . . 19 94 11.1. Informative References . . . . . . . . . . . . . . . . . 19 95 11.2. Normative References . . . . . . . . . . . . . . . . . . 19 97 Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 20 99 1. Introduction 101 This section introduces the problem space and outlines the solution. 102 Some of the terms may be unfamiliar to reader without extensive IS-IS 103 background and in such case a glossary is provided in Section 2 and 104 can be referenced. 106 Due to the inherent properties of link-state protocols the number of 107 IS-IS routers within a flooding domain is limited by processing and 108 flooding overhead on each node. While that number can be maximized 109 by well written implementations and techniques such as exponential 110 back-offs, IS-IS will still reach a saturation point where no further 111 routers can be added to a single flooding domain. In some L2 112 backbone deployment scenarios, this limit presents a significant 113 challenge. 115 The traditional approach to increasing the scale of an IS-IS 116 deployement is to break it up into multiple L1 flooding domains and a 117 single L2 backbone. This works well for designs where an L2 backbone 118 connects L1 access topologies, but it is limiting where a large L2 is 119 supposed to span large number of routers. In such scenarios, an 120 alternative approach is to consider multiple L2 flooding domains 121 connected together via L1 flooding domains. In other words, L2 122 flooding domains are connected by "L1/L2 lanes" through the L1 areas 123 to form a single L2 backbone again. Unfortunately, in its simplest 124 implementation, this requires the inclusion of most, or all, of the 125 transit L1 routers as L1/L2 to allow traffic to flow along optimal 126 paths through such transit areas. Consequently, this approach fails 127 to reduce the number of L2 routers involved, so it fails to increase 128 the scalability of the L2 backbone. 130 +----+ +-------+ +-------+ +-------+ +----+ 131 | R1 | | R10 +------------+ R20 +---------------+ R30 | | R4 | 132 | L2 +--+ L1/L2 | | L1 | | L1/L2 +--+ L2 | 133 | | | +--------+ +-+ | +------------+ | | | 134 +----+ ++-+--+-+ | | +---+---+----------+ +-+--+-++ +----+ 135 | | | | | | | | | | | | | 136 | | | | | | | | | +-----------+ | | 137 | | +-------+ | | | | | | | | | | 138 | | | | | | | | | | | +------+ | 139 | +------+ +--------+ | +-------+ | | | 140 | | | | | | | | | | | | | 141 +----+ ++------+---+ | +---+---+---+--+ | +-------+------++ +----+ 142 | R2 | | R11 | | | | | R21 | | | | | R31 | | R5 | 143 | L2 +--+ L1/L2 +------------+ L1 +---------------+ L1/L2 +--+ L2 | 144 | | | | | | | | | | | | | | | | 145 +----+ ++------+---+ | | +---+--++ | +-------+------++ +----+ 146 | | | | | | | | | | | | | 147 | +---------------+ | | | | | | | | 148 | | | | | | | | | | | | | 149 | | +--------------+ | +-----------------+ | 150 | | | | | | | | | | | | | 151 +----+ ++-+--+-+ | | +------+---+---+-----+ | | | ++-----++ +----+ 152 | R3 | | R12 | +----------| R22 | | +----+ R32 | | R6 | 153 | L2 +--+ L1/L2 | +--------| L1 +-------+ | | L1/L2 +--+ L2 | 154 | | | +------------+ |---------------+ | | | 155 +----+ +-------+ +-------+-------------+ +-------+ +----+ 157 Figure 1: Example Topology of L1 with L2 Borders 159 Figure 1 is an example of a network where a topologically rich L1 160 area is used to provide transit between six different L2-only routers 161 (R1-R6). Note that the six L2-only routers do not have connectivity 162 to one another over L2 links. To take advantage of the abundance of 163 paths in the L1 transit area, all the intermediate systems could be 164 placed into both L1 and L2, but this essentially combines the 165 separate L2 flooding domains into a single one, triggering again 166 maximum L2 scale limitation we try to address in first place. 168 A more effective solution would allow to reduce the number of links 169 and routers exposed in L2, while still utilizing the full L1 topology 170 when forwarding through the network. 172 [RFC8099] describes Topology Transparent Zones (TTZ) for OSPF. The 173 TTZ mechanism represents a group of OSPF routers as a full mesh of 174 adjacencies between the routers at the edge of the group. A similar 175 mechanism could be applied to IS-IS as well. However, a full mesh of 176 adjacencies between edge routers (or L1/L2 nodes) significantly 177 limits the scale of the topology. The topology in Figure 1 has 6 L1/ 178 L2 nodes. Figure 2 illustrates a full mesh of L2 adjacencies between 179 the 6 L1/L2 nodes, resulting in (5 * 6)/2 = 15 L2 adjacencies. In a 180 somewhat larger topology containing 20 L1/L2 nodes, the number of L2 181 adjacencies in a full mesh rises to 190. 183 +----+ +-------+ +-------------------------------+-------+ +----+ 184 | R1 | | R10 | | | R30 | | R4 | 185 | L2 +--+ L1/L2 +------------------------------------+ L1/L2 +--+ L2 | 186 | | | | | | | | | 187 +----+ ++-+-+--+-+ | +-+--+---++ +----+ 188 | | | | | | | | 189 | +----------------------------------------------+ | 190 | | | | | | | | 191 | +-----------------------------------+ | | | | 192 | | | | | | | | 193 | +----------------------------------------+ | | 194 | | | | | | | | 195 +----+ ++-----+- | | | | -----+-++ +----+ 196 | R2 | | R11 | | | | | | R31 | | R5 | 197 | L2 +--+ L1/L2 +------------------------------------+ L1/L2 +--+ L2 | 198 | | | | | | | | | | | | 199 +----+ ++------+------------------------------+ | | +----+-++ +----+ 200 | | | | | | | | 201 | | | | | | | | 202 | +-------------------------------------------+ | 203 | | | | | | | | 204 | | | | +----------+ | 205 | | | | | | | | 206 | | | | +-----+ | | 207 | | | | | | | | 208 +----+ ++----+-+-+ | +-+-+--+-++ +----+ 209 | R3 | | R12 | | L2 adjacency | R32 | | R6 | 210 | L2 +--+ L1/L2 +------------------------------------+ L1/L2 +--+ L2 | 211 | | | | | | | | | 212 +----+ +-------+----+ +-------+ +----+ 214 Figure 2: Example topology represented in L2 with a full mesh of 215 L2 adjacencies between L1/L2 nodes 217 BGP, as specified in [RFC4271], faced a similar scaling problem, 218 which has been solved in many networks by deploying BGP route 219 reflectors [RFC4456]. We note that BGP route reflectors do not 220 necessarily have to be in the forwarding path of the traffic. This 221 incongruity of forwarding and control path for BGP route reflectors 222 allows the control plane to scale independently of the forwarding 223 plane. 225 We propose here a similar solution for IS-IS. A simple example of 226 what a flood reflector control plane approach would look like is 227 shown in Figure 3, where router R21 plays the role of a flood 228 reflector. Each L1/L2 ingress/egress router builds a tunnel to the 229 flood reflector, and an L2 adjacency is built over each tunnel. In 230 this solution, we need only 6 L2 adjacencies, instead of the 15 231 needed for a full mesh. In a somewhat larger topology containing 20 232 L1/L2 nodes, this solution requires only 20 L2 adjacencies, instead 233 of the 190 need for a full mesh. Multiple flood reflectors can be 234 used, allowing the network operator to balance between resilience, 235 path utilization, and state in the control plane. The resulting L2 236 adjacency scale is R*n, where R is the number of flood reflectors 237 used and n is the number of L1/L2 nodes. This compares quite 238 favorably with n*(n-1)/2 L2 adjacencies required in a fully meshed L2 239 solution. 241 +----+ +-------+ +-------+ +----+ 242 | R1 | | R10 | | R30 | | R4 | 243 | L2 +--+ L1/L2 +--------------+ +-----------------+ L1/L2 +--+ L2 | 244 | | | | L2 adj | | L2 adj | | | | 245 +----+ +-------+ over | | over +-------+ +----+ 246 tunnel | | tunnel 247 +----+ +-------+ +--+---+--+ +-------+ +----+ 248 | R2 | | R11 | | R21 | | R31 | | R5 | 249 | L2 +--+ L1/L2 +-----------+ L1/L2 +--------------+ L1/L2 +--+ L2 | 250 | | | | L2 adj | flood | L2 adj | | | | 251 +----+ +-------+ over |reflector| over +-------+ +----+ 252 tunnel +--+---+--+ tunnel 253 +----+ +-------+ | | +-------+ +----+ 254 | R3 | | R12 +--------------+ +-----------------+ R32 | | R6 | 255 | L2 +--+ L1/L2 | L2 adj L2 adj | L1/L2 +--+ L2 | 256 | | | | over over | | | | 257 +----+ +-------+ tunnel tunnel +-------+ +----+ 259 Figure 3: Example topology represented in L2 with L2 adjacencies 260 from each L1/ L2 node to a single flood reflector 262 As illustrated in Figure 3, when R21 plays the role of flood 263 reflector, it provides L2 connectivity among all of the previously 264 disconnected L2 islands by reflooding all L2 LSPDUs. At the same 265 time, R20 and R22 in Figure 1 remain L1-only routers. L1-only 266 routers and L1-only links are not visible in L2. In this manner, the 267 flood reflector allows us provide L2 control plane connectivity in a 268 scalable manner. 270 As described so far, the solution illustrated in Figure 3 relies only 271 on currently standardized IS-IS functionality. Without new 272 functionality, however, the data traffic will traverse only R21. 273 This will unnecessarily create a bottleneck at R21 since there is 274 still available capacity in the paths crossing the L1-only routers 275 R20 and R22 in Figure 1. 277 Hence, some new functionality is necessary to allow the L1/L2 edge 278 nodes (R10-12 and R30-32 in Figure 3) to recognize that the L2 279 adjacency to R21 should not be used for forwarding. The L1/L2 edge 280 nodes should forward traffic that would normally be forwarded over 281 the L2 adjacency to R21 over L1 links instead. This would allow the 282 forwarding within the L1 area to use the L1-only nodes and links 283 shown in Figure 1 as well. It allows networks to be built that use 284 the entire forwarding capacity of the L1 areas, while at the same 285 time introducing control plane scaling benefits provided by L2 flood 286 reflectors. 288 This document defines all extensions necessary to support flood 289 reflector deployment: 291 * A 'flood reflector adjacency' for all the adjacencies built for 292 the purpose of reflecting flooding information. This allows these 293 'flood reflectors' to participate in the IS-IS control plane 294 without being used in the forwarding plane. This is a purely 295 local operation on the L1/L2 ingress; it does not require 296 replacing or modifying any routers not involved in the reflection 297 process. Deployment-wise, it is far less tricky to just upgrade 298 the routers involved in flood reflection rather than have a flag 299 day on the whole IS-IS domain. 301 * An (optional) full mesh of tunnels between the L1/L2 routers, 302 ideally load-balancing across all available L1 links. This 303 harnesses all forwarding paths between the L1/L2 edge nodes 304 without injecting unneeded state into the L2 flooding domain or 305 creating 'choke points' at the 'flood reflectors' themselves. The 306 draft is agnostic as to the tunneling technology used but provides 307 enough information for automatic establishment of such tunnels. 308 The discussion of IS-IS adjacency formation and/or liveness 309 discovery on such tunnels is outside the scope of this draft and 310 is largely choice of the underlying implementation. A solution 311 without tunnels is also possible by applying judicious scoping of 312 reachability information between the levels as described in more 313 details later. 315 * Some way to support reflector redundancy, and potentially some way 316 to auto-discover and advertise such adjacencies as flood reflector 317 adjacencies. Such advertisements may allow L2 nodes outside the 318 L1 to perform optimizations in the future based on this 319 information. 321 2. Glossary 323 This section is introduced with the intention of allowing quick 324 reference in the more detailed parts of the document to terms used 326 Flood Reflector: 327 Node configured to connect L2 only to flood reflector clients and 328 reflect (reflood) IS-IS L2 LSPs amongst them. 330 Flood Reflector Client: 331 Node configured to build flood reflector adjacencies and normal L2 332 nodes. 334 Flood Reflector Adjacency: 335 IS-IS L2 adjacency limited by one end being client and the other 336 reflector and agreeing on the same Flood Reflector Cluster ID. 338 Flood Reflector Cluster: 339 Collection of clients and flood reflectors configured with the 340 same cluster identifier. Cluster ID value of 0 SHOULD NOT be used 341 since it may be used in the future for special purposes. 343 Tunnel Deployment: 344 Deployment where flood reflector clients build a full mesh of 345 tunnels in L1 to "shortcut" forwarding of L2 traffic through the 346 cluster. 348 No Tunnel Deployment: 349 Deployment where flood reflector clients redistribute L2 350 reachability into L1 to allow forwarding through the cluster 351 without underlying tunnels. 353 3. Further Details 355 Several considerations should be noted in relation to such a flood 356 reflection mechanism. 358 First, this allows multi-area IS-IS deployments to scale without any 359 major modifications in the IS-IS implementation on most of the nodes 360 deployed in the network. Unmodified (traditional) L2 routers will 361 compute reachability across the transit L1 area using the flood 362 reflector adjacencies. 364 Second, the flood reflectors are not required to participate in 365 forwarding traffic through the L1 transit area. These flood 366 reflectors can be hosted on virtual devices outside the forwarding 367 topology. 369 Third, astute readers will realize that flooding reflection may cause 370 the use of suboptimal paths. This is similar to the BGP route 371 reflection suboptimal routing problem described in 372 [ID.draft-ietf-idr-bgp-optimal-route-reflection-28]. The L2 373 computation determines the egress L1/L2 and with that can create 374 illusions of ECMP where there is none. And in certain scenarios lead 375 to an L1/L2 egress which is not globally optimal. This represents a 376 straightforward instance of the trade-off between the amount of 377 control plane state and the optimal use of paths through the network 378 often encountered when aggregating routing information. 380 One possible solution to this problem is to expose additional 381 topology information into the L2 flooding domains. In the example 382 network given, links from router 01 to router 02 can be exposed into 383 L2 even when 01 and 02 are participating in flood reflection. This 384 information would allow the L2 nodes to build 'shortcuts' when the L2 385 flood reflected part of the topology looks more expensive to cross 386 distance wise. 388 Another possible variation is for an implementation to approximate 389 with the tunnel cost the cost of the underlying topology. 391 Redundancy can be achieved by building multiple flood reflectors in a 392 L1 area. Multiple flood reflectors do not need any synchronization 393 mechanisms amongst themselves, except standard IS-IS flooding and 394 database maintenance procedures. 396 4. Encodings 398 4.1. Flood Reflection TLV 400 The Flood Reflection TLV is a new top-level TLV that MAY appear in L2 401 IIHs. The Flood Reflection TLV indicates the flood reflector cluster 402 (based on Flood Reflection Cluster ID) that a given router is 403 configured to participate in. It also indicates whether the router 404 is configured to play the role of either flood reflector or flood 405 reflector client. The Flood Reflection Cluster ID and flood 406 reflector roles advertised in the IIHs are used to ensure that flood 407 reflector adjacencies are only formed between a flood reflector and 408 flood reflector client, and that the Flood Reflection Cluster IDs 409 match. The Flood Reflection TLV has the following format: 411 0 1 2 3 412 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 413 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 414 | Type | Length |C| RESERVED | 415 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 416 | Flood Reflection Cluster ID | 417 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 418 | Sub-TLVs ... 419 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 421 Type: TBD 423 Length: The length, in octets, of the following fields. 425 C (Client): This bit is set to indicate that the router acts as a 426 flood reflector client. When this bit is NOT set, the router acts 427 as a flood reflector. On a given router, the same value of the 428 C-bit MUST be advertised across all interfaces advertising the 429 Flood Reflection TLV in IIHs. 431 RESERVED: This field is reserved for future use. It MUST be set to 432 0 when sent and MUST be ignored when received. 434 Flood Reflection Cluster ID: Flood Reflection Cluster Identifier. 435 These same 32-bit value MUST be assigned to all of the flood 436 reflectors and flood reflector clients in the same L1 area. The 437 value MUST be unique across different L1 areas within the IGP 438 domain. In case of violation of those rules multiple L1 areas may 439 become a single cluster or a single area may split in flood 440 reflection sense and several mechanisms such as auto-discovery of 441 tunnels may not work correctly. On a given router, the same value 442 of the Flood Reflection Cluster ID MUST be advertised across all 443 interfaces advertising the Flood Reflection TLV in IIHs. When a 444 router discovers that a node is using multiple Cluster IDs based 445 on its advertised TLVs and IIHs, the node MAY adequately log such 446 violations subject to rate limiting. This implies that a flood 447 reflector MUST NOT participate in more than a single L1 area. In 448 case of Cluster ID value of 0, the TLV containing it MUST be 449 ignored. 451 Sub-TLVs: Optional sub-TLVs. For future extensibility, the format 452 of the Flood Reflection TLV allows for the possibility of 453 including optional sub-TLVs. No sub-TLVs of the Flood Reflection 454 TLV are defined in this document. 456 The Flood Reflection TLV SHOULD NOT appear more than once in an IIH. 457 A router receiving multiple Flood Reflection TLVs in the same IIH 458 MUST use the values in the first TLV and it SHOULD adequately log 459 such violations subject to rate limiting. 461 4.2. Flood Reflection Discovery Sub-TLV 463 Flood Reflection Discovery sub-TLV is advertised as a sub-TLV of the 464 IS-IS Router Capability TLV-242, defined in [RFC7981]. The Flood 465 Reflection Discovery sub-TLV is advertised in L1 and L2 LSPs with 466 area flooding scope in order to enable the auto-discovery of flood 467 reflection capabilities. The Flood Reflection Discovery sub-TLV has 468 the following format: 470 0 1 2 3 471 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 472 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 473 | Type | Length |C| Reserved | 474 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 475 | Flood Reflection Cluster ID | 476 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 478 Type: TBD 480 Length: The length, in octets, of the following fields. 482 C (Client): This bit is set to indicate that the router acts as a 483 flood reflector client. When this bit is NOT set, the router acts 484 as a flood reflector. 486 RESERVED: This field is reserved for future use. It MUST be set to 487 0 when sent and MUST be ignored when received. 489 Flood Reflection Cluster ID: The Flood Reflection Cluster Identifier 490 is the same as that defined in the Flood Reflection TLV and obeys 491 the same rules. 493 The Flood Reflection Discovery sub-TLV SHOULD NOT appear more than 494 once in TLV 242. A router receiving multiple Flood Reflection 495 Discovery sub-TLVs in TLV 242 MUST use the values in the first sub- 496 TLV and it SHOULD adequately log such violations subject to rate 497 limiting. 499 4.3. Flood Reflection Discovery Tunnel Type Sub-Sub-TLV 501 Flood Reflection Discovery Tunnel Type sub-sub-TLV is advertised 502 optionally as a sub-sub-TLV of the Flood Reflection Discovery Sub- 503 TLV, defined in Section 4.2. It allows the automatic creation of L2 504 tunnels to be used as flood reflector adjacencies and L1 shortcut 505 tunnels. The Flood Reflection Tunnel Type sub-sub-TLV has the 506 following format: 508 0 1 2 3 509 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 510 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+---------------+ 511 | Type | Length | Reserved |F| 512 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 513 | Tunnel Encapsulation Attribute | 514 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 516 Type: TBD 518 Length: The length, in octets, of zero or more of the following 519 fields. 521 Reserved: SHOULD be 0 on transmission and ignored on reception. 523 F Flag: When set indicates flood reflection tunnel endpoint, when 524 clear, indicates possible L1 shortcut tunnel endpoint. 526 Tunnel Encapsulation Attribute: Carries encapsulation type and 527 further attributes necessary for tunnel establishment as defined 528 in [RFC9012]. Protocol type sub-TLV as defined in [RFC9012] MAY 529 be included but MUST when F flag is set include according type 530 that allows carrying of encapsulated IS-IS frames. Such tunnel 531 type MUST provide according mechanisms to carry up to 532 `originatingL2LSPBufferSize` sized IS-IS frames across. 534 A flood reflector receiving multiple Flood Reflection Discovery 535 Tunnel Type sub-sub-TLVs in Flood Reflection Discovery sub-TLV with F 536 flag set SHOULD use one or more of the specified tunnel endpoints to 537 automatically establish one or more tunnels that will serve as flood 538 reflection adjacency(-ies). 540 A flood reflection client receiving multiple Flood Reflection 541 Discovery Tunnel Type sub-sub-TLVs in Flood Reflection Discovery sub- 542 TLV with F flag clear from other leaves MAY use one or more of the 543 specified tunnel endpoints to automatically establish one or more 544 tunnels that will serve as L1 tunnel shortcuts. 546 Optional address validation procedures as defined in [RFC9012] MUST 547 be disregarded. 549 4.4. Flood Reflection Adjacency Sub-TLV 551 The Flood Reflection Adjacency sub-TLV is advertised as a sub-TLV of 552 TLVs 22, 23, 25, 141, 222, and 223. Its presence indicates that a 553 given adjacency is a flood reflector adjacency. It is included in L2 554 area scope flooded LSPs. Flood Reflection Adjacency sub-TLV has the 555 following format: 557 0 1 2 3 558 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 559 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 560 | Type | Length |C| Reserved | 561 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 562 | Flood Reflection Cluster ID | 563 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 565 Type: TBD 567 Length: The length, in octets, of the following fields. 569 C (Client): This bit is set to indicate that the router advertising 570 this adjacency is a flood reflector client. When this bit is NOT 571 set, the router advertising this adjacency is a flood reflector. 573 RESERVED: This field is reserved for future use. It MUST be set to 574 0 when sent and MUST be ignored when received. 576 Flood Reflection Cluster ID: The Flood Reflection Cluster Identifier 577 is the same as that defined in the Flood Reflection TLV and obeys 578 the same rules. 580 The Flood Reflection Adjacency sub-TLV SHOULD NOT appear more than 581 once in a given TLV. A router receiving multiple Flood Reflection 582 Adjacency sub-TLVs in a TLV MUST use the values in the first sub-TLV 583 and it SHOULD adequately log such violations subject to rate 584 limiting. 586 4.5. Flood Reflection Discovery 588 A router participating in flood reflection as client or reflector 589 MUST be configured as an L1/L2 router. It SHOULD originate the Flood 590 Reflection Discovery sub-TLV with area flooding scope in L1 and L2. 591 Normally, all routers on the edge of the L1 area (those having 592 traditional L2 adjacencies) will advertise themselves as route 593 reflector clients. Therefore, a flood reflector client will have 594 both traditional L2 adjacencies and flood reflector L2 adjacencies. 596 A router acting as a flood reflector MUST NOT have any traditional L2 597 adjacencies. It will be an L1/L2 router only by virtue of having 598 flood reflector L2 adjacencies. A router desiring to act as a flood 599 reflector SHOULD advertise itself as such using the Flood Reflection 600 Discovery sub-TLV in L1 and L2. 602 A given flood reflector or flood reflector client can only 603 participate in a single cluster, as determined by the value of its 604 Flood Reflection Cluster ID and should disregard other routers' TLVs 605 for flood reflection purposes if the cluster ID is not matching. 607 Upon reception of Flood Reflection Discovery sub-TLVs, a router 608 acting as flood reflector client SHOULD initiate a tunnel towards 609 each flood reflector with which it shares an Flood Reflection Cluster 610 ID using one or more of the tunnel encapsulations provided with F 611 flag being set. The L2 adjacencies formed over such tunnels MUST be 612 marked as flood reflector adjacencies. If the client or reflector 613 has a direct L2 adjacency with the according remote side it SHOULD 614 use it instead of instantiating a new tunnel. 616 In absence of auto-discovery an implementation MAY use statically 617 configured tunnels to create flood reflection adjacencies. 619 The IS-IS metrics for all flood reflection adjacencies in a cluster 620 SHOULD be uniform. 622 Upon reception of Flood Reflection Discover TLVs, a router acting as 623 a flood reflector client MAY initiate tunnels with L1-only 624 adjacencies towards any of the other flood reflector clients with 625 lower router IDs in its cluster using encapsulations with F flag 626 clear. These tunnels MAY be used for forwarding to improve the load- 627 balancing characteristics of the L1 area. If the clients have a 628 direct L2 adjacency they SHOULD use it instead of instantiating a new 629 tunnel. 631 4.6. Flood Reflection Adjacency Formation 633 In order to simplify both implementations and network deployments, 634 this draft does not allow the formation of complex hierarchies of 635 flood reflectors and clients or allow multiple clusters in a single 636 L1 area. Consequently, all flood reflectors and flood reflector 637 clients in the same L1 area MUST share the same Flood Reflector 638 Cluster ID. Deployment of multiple cluster IDs in the same L1 area 639 are outside the scope of this document. 641 A flood reflector MUST only form flood reflection adjacencies with 642 flood reflector clients with matching Cluster ID. A flood reflector 643 MUST NOT form any traditional L2 adjacencies. 645 Flood reflector clients MUST only form flood reflection adjacencies 646 with flood reflectors with matching Cluster ID. 648 Flood reflector clients MAY form traditional L2 adjacencies with 649 flood reflector clients or nodes not participating in flood 650 reflection. When two clients form traditional L2 adjacency Cluster 651 ID is disregarded. 653 The Flood Reflector Cluster ID and flood reflector roles advertised 654 in the Flood Reflection TLVs in IIHs are used to ensure that flood 655 reflection adjacencies that are established meet the above criteria. 657 On change in either flood reflection role or cluster ID on IIH on the 658 local or remote side the adjacency has to be reset and re-established 659 if possible. 661 Once a flood reflection adjacency is established, the flood reflector 662 and the flood reflector client MUST advertise the adjacency by 663 including the Flood Reflection Adjacency Sub-TLV in the Extended IS 664 reachability TLV or MT-ISN TLV. 666 5. Route Computation 668 To ensure loop-free routing, the route reflection client MUST follow 669 the normal L2 computation to determine L2 routes. This is because 670 nodes outside the L1 area will generally not be aware that flood 671 reflection is being performed. The flood reflection clients need to 672 produce the same result for the L2 route computation as a router not 673 participating in flood reflection. 675 5.1. Tunnel Based Deployment 677 In tunnel based option the reflection client, after L2 and L1 678 computation, MUST examine all L2 routes and replace all flood 679 reflector adjacencies with the correct underlying tunnel next-hop to 680 the egress. 682 5.2. No Tunnel Deployment 684 In case of deployment without underlying tunnels, the necessary L2 685 routes are distributed into the area, normally as L2->L1 routes. Due 686 to the rules in Section 4.6 the computation in the resulting topology 687 is relatively simple, the L2 SPF from a flood reflector client is 688 guaranteed to reach within a hop the Flood Reflector and in the 689 following hop the L2 egress to which it has a forwarding tunnel 690 again. All the flood reflector tunnel nexthops in the according L2 691 route can hence be removed and if the L2 route has no other ECMP L2 692 nexthops, the L2 route MUST be suppressed in the RIB by some means to 693 allow the less preferred L2->L1 route to be used to forward traffic 694 towards the advertising egress. 696 In the particular case the client has L2 routes which are not route 697 reflected, those will be naturally preferred (such routes normally 698 "hot-potato" route of the L1 area). However in the case the L2 route 699 through the flood reflector egress is "shorter" than such present non 700 flood reflected L2 routes, the node SHOULD ensure that such routes 701 are suppressed so the L2->L1 towards the egress still takes 702 preference. Observe that operationally this can be resolved in a 703 relatively simple way by configuring flood reflector adjacencies to 704 have a high metric, i.e. the flood reflector topology becomes "last 705 resort" and the leaves will try to "hot-potato" out the area as fast 706 as possible which is normally the desirable behavior. 708 In deployment scenarios where tunnels are not used, all L1/L2 edge 709 nodes MUST be ultimately flood reflector clients except during during 710 transition phase. 712 6. Redistribution of Prefixes 714 When L2 prefixes need to be redistributed into L1 by the route 715 reflector clients a client that does not have any L2 flood reflector 716 adjacencies MUST NOT redistribute those routes into the area in case 717 of application of Section 5.2. The L2 prefixes advertisements 718 redistributed into L1 with flood reflectors SHOULD be normally 719 limited to L2 intra-area routes (as defined in [RFC7775]), if the 720 information exists to distinguish them from other other L2 prefix 721 advertisements. 723 On the other hand, in topologies that make use of flood reflection to 724 hide the structure of L1 areas while still providing transit 725 forwarding across them using tunnels, we generally do not need to 726 redistribute L1 prefixes advertisements into L2. 728 7. Special Considerations 730 In pathological cases setting the overload bit in L1 (but not in L2) 731 can partition L1 forwarding, while allowing L2 reachability through 732 flood reflector adjacencies to exist. In such a case a node cannot 733 replace a route through a flood reflector adjacency with a L1 734 shortcut and the client can use the L2 tunnel to the flood reflector 735 for forwarding while it MUST initiate an alarm and declare 736 misconfiguration. 738 A flood reflector with directly L2 attached prefixes should advertise 739 those in L1 as well since based on preference of L1 routes the 740 clients will not try to use the L2 flood reflector adjacency to route 741 the packet towards them. A very, very corner case is when the flood 742 reflector is reachable via L2 flood reflector adjacency (due to 743 underlying L1 partition) only in which case the client can use the L2 744 tunnel to the flood reflector for forwarding towards those prefixes 745 while it MUST initiate an alarm and declare misconfiguration. 747 A flood reflector SHOULD NOT set the attached bit on its LSPs. 749 Instead of modifying the computation procedures one could imagine a 750 flood reflector solution where the Flood Reflector would re-advertise 751 the L2 prefixes with a 'third-party' next-hop but that would have 752 less desirable convergence properties than the solution proposed and 753 force a fork-lift of all L2 routers to make sure they disregard such 754 prefixes unless in the same L1 domain as the Flood Reflector. 756 Depending on pseudo-node choice in case of a broadcast domain with 757 multiple flood reflectors attached this can lead to a partitioned LAN 758 and hence a router discovering such a condition MUST initiate an 759 alarm and declare misconfiguration. 761 8. IANA Considerations 763 This document requests allocation for the following IS-IS TLVs and 764 Sub-TLVs. 766 8.1. New IS-IS TLV Codepoint 768 This document requests the following IS-IS TLV: 770 Value Name IIH LSP SNP Purge 771 ----- --------------------------------- --- --- --- ----- 772 TBD1 Flood Reflection y n n n 774 Suggested value for TBD1 is 161. 776 8.2. Sub TLVs for TLV 242 778 This document request the following registration in the "sub-TLVs for 779 TLV 242" registry. 781 Type Description 782 ---- ----------- 783 TBD2 Flood Reflection Discovery 785 Suggested value for TBD2 is 161. 787 8.3. Sub-sub TLVs for Flood Reflection Discovery sub-TLV 789 This document request the following registration in the "sub-sub-TLVs 790 for Flood Reflection Discovery sub-TLV" registry. 792 Type Description 793 ---- ----------- 794 TBD3 Flood Reflection Discovery Tunnel Encapsulation Attribute 796 Suggested value for TBD3 is 161. 798 8.4. Sub TLVs for TLV 22, 23, 25, 141, 222, and 223 800 This document requests the following registration in the "sub-TLVs 801 for TLV 22, 23, 25, 141, 222, and 223" registry. 803 Type Description 22 23 25 141 222 223 804 ---- -------------------------------- --- --- --- --- --- --- 805 TBD4 Flood Reflector Adjacency y y n y y y 807 Suggested value for TBD4 is 161. 809 9. Security Considerations 811 This document introduces tunnels carrying IS-IS control traffic via 812 tunnels. In case of statically configured tunnels a deployment 813 SHOULD provide enough security protection to prevent malicious 814 attackers from using the tunnel endpoints. For information used to 815 form dynamically discovered tunnels, it SHOULD be protected by the 816 the deployed IS-IS security mechanism preventing malicious nodes from 817 spoofing rogue information on behalf of other members. 819 10. Acknowledgements 821 The authors thank Shraddha Hegde, Peter Psenak, Acee Lindem and Les 822 Ginsberg for their thorough review and detailed discussions. Thanks 823 are also extended to Michael Richardson for an excellent routing 824 directorate review. 826 11. References 828 11.1. Informative References 830 [ID.draft-ietf-idr-bgp-optimal-route-reflection-28] 831 Raszuk et al., R., "BGP Optimal Route Reflection", July 832 2019, . 835 [RFC4271] Rekhter, Y., Ed., Li, T., Ed., and S. Hares, Ed., "A 836 Border Gateway Protocol 4 (BGP-4)", RFC 4271, 837 DOI 10.17487/RFC4271, January 2006, 838 . 840 [RFC4456] Bates, T., Chen, E., and R. Chandra, "BGP Route 841 Reflection: An Alternative to Full Mesh Internal BGP 842 (IBGP)", RFC 4456, DOI 10.17487/RFC4456, April 2006, 843 . 845 [RFC8099] Chen, H., Li, R., Retana, A., Yang, Y., and Z. Liu, "OSPF 846 Topology-Transparent Zone", RFC 8099, 847 DOI 10.17487/RFC8099, February 2017, 848 . 850 11.2. Normative References 852 [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate 853 Requirement Levels", BCP 14, RFC 2119, 854 DOI 10.17487/RFC2119, March 1997, 855 . 857 [RFC7775] Ginsberg, L., Litkowski, S., and S. Previdi, "IS-IS Route 858 Preference for Extended IP and IPv6 Reachability", 859 RFC 7775, DOI 10.17487/RFC7775, February 2016, 860 . 862 [RFC7981] Ginsberg, L., Previdi, S., and M. Chen, "IS-IS Extensions 863 for Advertising Router Information", RFC 7981, 864 DOI 10.17487/RFC7981, October 2016, 865 . 867 [RFC9012] Patel, K., Van de Velde, G., Sangli, S., and J. Scudder, 868 "The BGP Tunnel Encapsulation Attribute", RFC 9012, 869 DOI 10.17487/RFC9012, April 2021, 870 . 872 Authors' Addresses 874 Tony Przygienda (editor) 875 Juniper 876 1137 Innovation Way 877 Sunnyvale, CA 878 United States of America 880 Email: prz@juniper.net 882 Chris Bowers 883 Juniper 884 1137 Innovation Way 885 Sunnyvale, CA 886 United States of America 888 Email: cbowers@juniper.net 890 Yiu Lee 891 Comcast 892 1800 Bishops Gate Blvd 893 Mount Laurel, NJ 08054 894 United States of America 896 Email: Yiu_Lee@comcast.com 897 Alankar Sharma 898 Comcast 899 1800 Bishops Gate Blvd 900 Mount Laurel, NJ 08054 901 United States of America 903 Email: Alankar_Sharma@comcast.com 905 Russ White 906 Juniper 907 1137 Innovation Way 908 Sunnyvale, CA 909 United States of America 911 Email: russw@juniper.net