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Checking references for intended status: Informational ---------------------------------------------------------------------------- == Unused Reference: 'RFC8518' is defined on line 352, but no explicit reference was found in the text Summary: 0 errors (**), 0 flaws (~~), 3 warnings (==), 1 comment (--). Run idnits with the --verbose option for more detailed information about the items above. -------------------------------------------------------------------------------- 2 LSR Working Group U. Chunduri 3 Internet-Draft Futurewei USA 4 Intended status: Informational J. Tantsura 5 Expires: November 18, 2020 Apstra, Inc. 6 S. Hegde 7 Juniper Networks 8 May 17, 2020 10 IS-IS Multi Topology Deployment Considerations 11 draft-chunduri-lsr-isis-mt-deployment-cons-03 13 Abstract 15 This document analyzes IS-IS Multi Topology (MT) applicability in 16 various IS-IS deployments. This document explores the nuances around 17 the terminology and usage of various IS-IS address families, 18 topologies with different considerations, for choosing the right 19 combination for a specific deployment scenario. 21 This document also discusses various ways one can deploy IPv6 only 22 IS-IS topology. 24 Requirements Language 26 The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", 27 "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this 28 document are to be interpreted as described in RFC2119 [RFC2119], 29 RFC8174 [RFC8174] when, and only when they appear in all capitals, as 30 shown here. 32 Status of This Memo 34 This Internet-Draft is submitted in full conformance with the 35 provisions of BCP 78 and BCP 79. 37 Internet-Drafts are working documents of the Internet Engineering 38 Task Force (IETF). Note that other groups may also distribute 39 working documents as Internet-Drafts. The list of current Internet- 40 Drafts is at https://datatracker.ietf.org/drafts/current/. 42 Internet-Drafts are draft documents valid for a maximum of six months 43 and may be updated, replaced, or obsoleted by other documents at any 44 time. It is inappropriate to use Internet-Drafts as reference 45 material or to cite them other than as "work in progress." 47 This Internet-Draft will expire on November 18, 2020. 49 Copyright Notice 51 Copyright (c) 2020 IETF Trust and the persons identified as the 52 document authors. All rights reserved. 54 This document is subject to BCP 78 and the IETF Trust's Legal 55 Provisions Relating to IETF Documents 56 (https://trustee.ietf.org/license-info) in effect on the date of 57 publication of this document. Please review these documents 58 carefully, as they describe your rights and restrictions with respect 59 to this document. Code Components extracted from this document must 60 include Simplified BSD License text as described in Section 4.e of 61 the Trust Legal Provisions and are provided without warranty as 62 described in the Simplified BSD License. 64 Table of Contents 66 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2 67 2. Need for MT in IS-IS networks . . . . . . . . . . . . . . . . 3 68 3. Acronyms . . . . . . . . . . . . . . . . . . . . . . . . . . 3 69 4. Topologies and Address Families . . . . . . . . . . . . . . . 4 70 4.1. Single Topology Mode and Multiple Address Families . . . 4 71 4.2. Multiple Topology Mode and Multiple Address Families . . 5 72 4.2.1. Transition Mode . . . . . . . . . . . . . . . . . . . 6 73 4.3. IPv6 Only Topology . . . . . . . . . . . . . . . . . . . 6 74 5. IS-IS MT and LFA . . . . . . . . . . . . . . . . . . . . . . 7 75 6. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 7 76 7. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 7 77 8. Security Considerations . . . . . . . . . . . . . . . . . . . 7 78 9. References . . . . . . . . . . . . . . . . . . . . . . . . . 7 79 9.1. Normative References . . . . . . . . . . . . . . . . . . 7 80 9.2. Informative References . . . . . . . . . . . . . . . . . 8 81 Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 8 83 1. Introduction 85 IS-IS originally developed for OSI [ISO.10589.1992] and extensions 86 have been made available to support IPv4 [RFC1195]. A method for 87 exchanging IPv6 routing information using the IS-IS routing protocol 88 is specified in [RFC5308]. How to run a set of independent IP 89 topologies with topology specific adjacencies, within a single IS-IS 90 domain has been defined in IS-IS MT [RFC5120]. 92 There are number of networks, including mobile backhaul networks 93 seeking to use IPv6 only solutions. It is possible to conceive, 94 various parts of the backhaul networks use IPv4 and appropriate 95 migration strategy needed before eventually moving towards IPv6 only 96 network. While any IGP can be used in these networks, this document 97 covers only IS-IS protocol aspects. 99 Various layer-3 DC fabric routing options (refs: openfabric, spine- 100 leaf, controller-based) by changing or optimizing some aspects w.r.t 101 adjacency formation, flooding optimizations, or/and mechanisms to 102 automatically compute the location of the node in the fat tree 103 topology are proposed recently and this document brings some of the 104 multi topology deployment aspects relevant to these networks. Please 105 note, part of the discussion around IS-IS MT is not specific to DC or 106 CLOS fabrics and generally applicable to any IS-IS deployment but 107 discussed here because of multiple proposals to use various forms of 108 IS-IS in this context. 110 2. Need for MT in IS-IS networks 112 For mobile transport backhaul networks seeking only IPv6 network or 113 transitioning from parts of the network with only IPv4, IS-IS MT is 114 needed. For layer-3 DC fabric underlay, which provide reachability, 115 only one address family (either IPv4 or IPv6) SHOULD be sufficient. 116 However if either only IPv6 address family is needed in the underlay 117 or deploying both IPv4 and IPv6 address families are desired 118 discussion in Section 4 is relevant. 120 It is an unlikely requirement, where DC fabric to be partitioned 121 logically to have different topologies in the underlay but this can 122 happen in various scenarios as listed in Section 4.1. If one does 123 the same to meet a particular requirement, it introduces a 124 manageability complexity of these logical topologies. IS-IS MT 125 [RFC5120] also designed to address the above need and discussion in 126 Section 4.2 is relevant. It is worth noting, majority of the IS-IS 127 deployments use MT primarily to have a separate logical topology for 128 IPv6 address family. 130 3. Acronyms 132 IIH : IS-IS Hello Protocol Data Unit 134 LSP : Link State PDU 136 MT : Multi Topology 138 SPF : Shortest Path First 140 4. Topologies and Address Families 142 Terminology around IS-IS topologies and address families is somewhat 143 confusing at best. Just to give an example, MT ID #2 defined in 144 [RFC5120] says, it is "Reserved for IPv6 routing topology". While 145 multiple MT ID's can be deployed in a network with IPv6 topologies, 146 MT ID #2, perhaps referring to a first such topology with IPv6 only 147 address family. This section details various topology and address 148 family options possible with currently available IS-IS specifications 149 with respective defined TLVs. 151 4.1. Single Topology Mode and Multiple Address Families 153 IS-IS with IPv4 address family and with wide-metrics [RFC5305] is 154 widely deployed, with TLV 22 defined for IS Reachability and TLV 135 155 for IP (IPv4) reachability information . This is essentially a single 156 topology for the entire IS-IS area/domain with a single address 157 family (IPv4 unicast). 159 IS-IS can also be enabled with IPv6 unicast address family in a 160 single topology mode along with IPv4 unicast address family. Here 161 IPv6 uses the same underlying topology that is used for IPv4 and this 162 can be done as specified in IS-IS IPv6 [RFC5308] which introduces TLV 163 236, an IPv6 reachability TLV. It is important to note same IS-IS 164 adjacency is used for both address families and with a single SPF 165 (decision process) both IPv4 and IPv6 reachability would be computed. 167 However, for the above to work effectively, both IPv4 and IPv6 168 address families MUST share a common network topology. That is to 169 use IS-IS for IPv4 and IPv6 routing, any interface configured for 170 IPv4 IS-IS MUST also be configured for IPv6 IS-IS, and vice versa. 171 All routers within an IS-IS area (Level 1 routing) or domain (Level 2 172 routing) MUST also support the same set of address families: IPv4 173 only, IPv6 only, or both IPv4 and IPv6. Any discrepancy in the 174 configuration w.r.t above can cause routing black holes and one such 175 scenario is discussed below. 177 | / \| 178 ...Rx Ry... 179 | \ / 180 | \ / | 181 | \ / | 182 | /\ | 183 | / \ | 184 | / \| 185 ... R1 R2... 186 | \ / | 188 Figure 1: IS-IS with multiple address families 190 As shown, in the above diagram all routers in the network enabled 191 with both IPv4 and IPv6 unicast address families at the IS level and 192 single topology would be built. However, at a link level all but 193 except one link, say if IPv6 is not configured on the link between 194 the routers Rx and R2; due to a single IS-IS topology, the shortest 195 path between Rx and R2 is the direct link and since IPv6 is not 196 enabled on that link, Rx and R2 cannot exchange IPv6 data traffic 197 even though there's an alternate path between them in the topology 198 through Rx, R1, Ry and R2. 200 Hence to summarize the restrictions: all routers in the topology MUST 201 support only IPv4, only IPv6 or both IPv4 and IPv6 address families 202 on all links and node. In other words, network MUST be congruent. 203 While this model is to simpler to operate, might not be flexible 204 enough for some IS-IS deployments. Some examples where congruency is 205 not possible as follows: 207 a. When IPv6 is getting introduced in the network legacy nodes that 208 are IPv6 incapable. 210 b. Implementation issues causing IPv6 to be disabled on some nodes. 212 c. Hardware scale limitations causing IPv6 to be disabled on some 213 low-end nodes. 215 4.2. Multiple Topology Mode and Multiple Address Families 217 Multi-topology IS-IS uses multiple SPFs to compute routes and removes 218 the restriction that all interfaces MUST support all configured 219 address families and that all routers in an IS-IS area or domain MUST 220 support the same set of address families. This introduces the 221 concept of topology specific adjacency with MT IS Reachability TLV 222 222 and MT capable IPv4 Reachability with TLV 235 and MT capable IPv6 223 Reachability with TLV 237. 225 When MT IS-IS is enabled with IPv4 and IPv6 address families, the 226 routers build two topologies, one for each address family (IPv4 and 227 IPv6) and can find the optimum path for each address family even when 228 some links in the network support only one of them. IS-IS MT 229 [RFC5120] defines MT ID #0 for backward compatibility, as the 230 "standard" topology and this essentially operate as IS-IS single 231 topology mode as specified in Section 4.1 and supports both IPv4 and 232 IPv6 address families. MT ID #2 [RFC5120] is defined for IPv6 233 address family in MT mode. 235 4.2.1. Transition Mode 237 Most of the vendors supported MT transition feature (though some 238 vendors disabled to avoid confusion around this) in the IS-IS 239 networks to facilitate MT deployments without disrupting the single 240 topology mode. The MT transition mode allows a network operating in 241 single topology IS-IS IPv6 [RFC5308] to continue to work while 242 upgrading routers to include MT IS-IS IPv6 support i.e., MT ID #2 243 with [RFC5120] . While in transition mode, both types of TLVs 244 (single-topology with TLVs 22/236 and MT with TLVs 222/237) are sent 245 in LSPs for all configured IPv6 addresses, nodes can continue to 246 process these and operate in single topology mode though being in MT 247 mode ("standard" IS-IS topology with MT ID #0). After all routers in 248 the area or domain have been upgraded to support MT IPv6 transition 249 mode can be removed from the configuration. Once all routers in the 250 area or domain are operating in MT IPv6 mode, the topological 251 restrictions of single-topology mode can be made no longer in effect. 253 When transition mode is enabled, the router advertises both MT TLVs 254 and the old style IS-IS IPv6 TLVs but the topological restrictions of 255 the single topology mode discussed above are in effect. However, 256 there were instances while this mode is enabled and expectations for 257 different result in the actual deployments. 259 4.3. IPv6 Only Topology 261 Though it is theoretically possible to build IPv6 only underlay (with 262 TLV 236 for IPv6 reachability prefixes) in single topology mode as 263 discussed in Section 4.1, lot of legacy implementations require IPv4 264 address families too be configured in single topology mode (ingrained 265 code structures for IPv4 address family). IPv6 only DC underlay 266 network can be built with multi topology adjacencies (TLV 222) and 267 reachability prefixes (TLV 237) with MT ID #2 as discussed above in 268 Section 4.2. With this, any other address family can be introduced 269 including "standard" topology MT ID #0 (Single topology mode with 270 both address families) and there are no restrictions on which address 271 family has to enable on which link as specified in Section 4.1. 273 5. IS-IS MT and LFA 275 IP Fast Reroute (FRR) or Loop Free Alternative (LFA) computation in 276 MT mode are described in detail in Section 5.2 of [RFC5120]. 278 6. Acknowledgements 280 Thanks to Acee Lindem, Chris Hopps, Michael Abramson and Les Ginsberg 281 for various inputs on this work. 283 7. IANA Considerations 285 This document has no actions for IANA. 287 8. Security Considerations 289 Security concerns for IS-IS are addressed in [RFC5304] and [RFC5310]. 290 Further security analysis for IS-IS protocol is done in [RFC7645]. 292 This document does not introduce any change in any of the IS-IS 293 protocol or IS-IS protocol extensions. This document also does not 294 introduce any new security issues other than as noted in the 295 referenced IS-IS protocol extensions. 297 9. References 299 9.1. Normative References 301 [ISO.10589.1992] 302 International Organization for Standardization, 303 "Intermediate system to intermediate system intra-domain- 304 routing routine information exchange protocol for use in 305 conjunction with the protocol for providing the 306 connectionless-mode Network Service (ISO 8473)", 307 ISO Standard 10589, 1992. 309 [RFC1195] Callon, R., "Use of OSI IS-IS for routing in TCP/IP and 310 dual environments", RFC 1195, DOI 10.17487/RFC1195, 311 December 1990, . 313 [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate 314 Requirement Levels", BCP 14, RFC 2119, 315 DOI 10.17487/RFC2119, March 1997, 316 . 318 [RFC8174] Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC 319 2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174, 320 May 2017, . 322 9.2. Informative References 324 [RFC5120] Przygienda, T., Shen, N., and N. Sheth, "M-ISIS: Multi 325 Topology (MT) Routing in Intermediate System to 326 Intermediate Systems (IS-ISs)", RFC 5120, 327 DOI 10.17487/RFC5120, February 2008, 328 . 330 [RFC5304] Li, T. and R. Atkinson, "IS-IS Cryptographic 331 Authentication", RFC 5304, DOI 10.17487/RFC5304, October 332 2008, . 334 [RFC5305] Li, T. and H. Smit, "IS-IS Extensions for Traffic 335 Engineering", RFC 5305, DOI 10.17487/RFC5305, October 336 2008, . 338 [RFC5308] Hopps, C., "Routing IPv6 with IS-IS", RFC 5308, 339 DOI 10.17487/RFC5308, October 2008, 340 . 342 [RFC5310] Bhatia, M., Manral, V., Li, T., Atkinson, R., White, R., 343 and M. Fanto, "IS-IS Generic Cryptographic 344 Authentication", RFC 5310, DOI 10.17487/RFC5310, February 345 2009, . 347 [RFC7645] Chunduri, U., Tian, A., and W. Lu, "The Keying and 348 Authentication for Routing Protocol (KARP) IS-IS Security 349 Analysis", RFC 7645, DOI 10.17487/RFC7645, September 2015, 350 . 352 [RFC8518] Sarkar, P., Ed., Chunduri, U., Ed., Hegde, S., Tantsura, 353 J., and H. Gredler, "Selection of Loop-Free Alternates for 354 Multi-Homed Prefixes", RFC 8518, DOI 10.17487/RFC8518, 355 March 2019, . 357 Authors' Addresses 359 Uma Chunduri 360 Futurewei USA 361 2330 Central Expressway 362 Santa Clara, CA 95050 363 USA 365 Email: umac.ietf@gmail.com 366 Jeff Tantsura 367 Apstra, Inc. 369 Email: jefftant.ietf@gmail.com 371 Shraddha Hegde 372 Juniper Networks 373 Elnath-Exora Business Park Survey 374 Bangalore, Karnataka 560103 375 USA 377 Email: shraddha@juniper.net