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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(See RFCs 3967 and 4897 for information about using normative references to lower-maturity documents in RFCs) == Outdated reference: A later version (-15) exists of draft-ietf-idr-add-paths-10 Summary: 0 errors (**), 0 flaws (~~), 3 warnings (==), 1 comment (--). Run idnits with the --verbose option for more detailed information about the items above. -------------------------------------------------------------------------------- 2 Network Working Group D. Walton 3 Internet-Draft Cumulus Networks 4 Intended status: Standards Track A. Retana 5 Expires: April 9, 2016 E. Chen 6 Cisco Systems, Inc. 7 J. Scudder 8 Juniper Networks 9 October 7, 2015 11 BGP Persistent Route Oscillation Solutions 12 draft-ietf-idr-route-oscillation-stop-01 14 Abstract 16 In this document we present two sets of paths for an address prefix 17 that can be advertised by a BGP route reflector or confederation ASBR 18 to eliminate the MED-induced route oscillations in a network. The 19 first set involves all the available paths, and would achieve the 20 same routing consistency as the full IBGP mesh. The second set, 21 which is a subset of the first one, involves the neighbor-AS based 22 Group Best Paths, and would be sufficient to eliminate the MED- 23 induced route oscillations (subject to certain commonly adopted 24 topological constrains). 26 Status of This Memo 28 This Internet-Draft is submitted in full conformance with the 29 provisions of BCP 78 and BCP 79. 31 Internet-Drafts are working documents of the Internet Engineering 32 Task Force (IETF). Note that other groups may also distribute 33 working documents as Internet-Drafts. The list of current Internet- 34 Drafts is at http://datatracker.ietf.org/drafts/current/. 36 Internet-Drafts are draft documents valid for a maximum of six months 37 and may be updated, replaced, or obsoleted by other documents at any 38 time. It is inappropriate to use Internet-Drafts as reference 39 material or to cite them other than as "work in progress." 41 This Internet-Draft will expire on April 9, 2016. 43 Copyright Notice 45 Copyright (c) 2015 IETF Trust and the persons identified as the 46 document authors. All rights reserved. 48 This document is subject to BCP 78 and the IETF Trust's Legal 49 Provisions Relating to IETF Documents 50 (http://trustee.ietf.org/license-info) in effect on the date of 51 publication of this document. Please review these documents 52 carefully, as they describe your rights and restrictions with respect 53 to this document. Code Components extracted from this document must 54 include Simplified BSD License text as described in Section 4.e of 55 the Trust Legal Provisions and are provided without warranty as 56 described in the Simplified BSD License. 58 Table of Contents 60 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2 61 2. Requirements Language . . . . . . . . . . . . . . . . . . . . 3 62 3. Advertise the Available Paths . . . . . . . . . . . . . . . . 3 63 4. Advertise the Group Best Paths . . . . . . . . . . . . . . . 4 64 5. Route Reflection and Confederation . . . . . . . . . . . . . 4 65 5.1. Route Reflection . . . . . . . . . . . . . . . . . . . . 5 66 5.2. Confederation . . . . . . . . . . . . . . . . . . . . . . 5 67 6. Deployment Considerations . . . . . . . . . . . . . . . . . . 5 68 7. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 6 69 8. Security Considerations . . . . . . . . . . . . . . . . . . . 6 70 9. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 7 71 10. References . . . . . . . . . . . . . . . . . . . . . . . . . 7 72 10.1. Normative References . . . . . . . . . . . . . . . . . . 7 73 10.2. Informative References . . . . . . . . . . . . . . . . . 7 74 Appendix A. Why the Group Best Paths Are Adequate? . . . . . . . 7 75 Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 8 77 1. Introduction 79 As documented in [RFC3345], the routing information reduction by BGP 80 Route Reflection [RFC4456] or BGP Confederation [RFC5065] can result 81 in persistent IBGP route oscillations with certain routing setup and 82 network topologies. Except for a couple artificially engineered 83 network topologies, the MED attribute [RFC4271] has played a pivotal 84 role in virtually all of the known persistent IBGP route 85 oscillations. For the sake of brevity, we use the term "MED-induced 86 route oscillation" hereafter to refer to a persistent IBGP route 87 oscillation in which the MED plays a role. 89 In order to eliminate the MED-induced route oscillations and to 90 achieve consistent routing in a network, clearly a route reflector or 91 a confederation ASBR needs to advertise more than just the best path 92 for an address prefix. Our goal is to identify the "right" set of 93 paths for an address prefix that needs to be advertised by a route 94 reflector or a confederation ASBR. 96 In this document we present two sets of paths for an address prefix 97 that can be advertised by a BGP route reflector or confederation ASBR 98 to eliminate the MED-induced route oscillations in a network. The 99 first set involves all the available paths, and would achieve the 100 same routing consistency as the full IBGP mesh. The second set, 101 which is a subset of the first one, involves the neighbor-AS based 102 Group Best Paths, and would be sufficient to eliminate the MED- 103 induced route oscillations (subject to certain commonly adopted 104 topological constrains). 106 These paths can be advertised using the mechanism described in ADD- 107 PATH [I-D.ietf-idr-add-paths] for advertising multiple paths. No 108 other assumptions in functionality beyond the base BGP specification 109 [RFC4271] is assumed. 111 2. Requirements Language 113 The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", 114 "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this 115 document are to be interpreted as described in [RFC2119]. 117 3. Advertise the Available Paths 119 Observe that in a network that maintains a full IBGP mesh all the BGP 120 speakers have consistent and equivalent routing information. Such a 121 network is thus free of the MED-induced route oscillations and other 122 routing inconsistencies such as forwarding loops. 124 Therefore one approach is to allow a route reflector or a 125 confederation ASBR to advertise all the available paths for an 126 address prefix. Clearly this approach would yield the same amount of 127 routing information and achieve the same routing consistency as the 128 full IBGP mesh in a network. 130 This approach can be implemented using the mechanism described in 131 ADD-PATH [I-D.ietf-idr-add-paths] for advertising multiple paths for 132 certain prefixes. 134 For the sake of scalability the advertisement of multiple paths 135 should be limited to those prefixes which are affected by MED-induced 136 route oscillation in a network carrying a large number of alternate 137 paths. A detailed description of how these oscillations can occur 138 can be found in [RFC3345]; the description of how a node would 139 locally detect such condition is outside the scope of this document. 141 4. Advertise the Group Best Paths 143 The term neighbor-AS for a route refers to the neighboring AS from 144 which the route was received. The calculation of the neighbor-AS is 145 specified in Sect. 9.1.2.2 of [RFC4271], and Section 7.2 of 146 [RFC5065]. By definition the MED is comparable only among routes 147 with the same neighbor-AS. Thus the route selection procedures 148 specified in [RFC4271] would conceptually involve two steps: first 149 organize the paths for an address prefix into groups according to 150 their respective neighbor-AS's, and calculate the most preferred one 151 (termed "Group Best Path") for each of the groups; Then calculate the 152 overall best path among all the Group Best Paths. 154 As a generally recommended ([RFC4456], [RFC5065]) and widely adopted 155 practice, a route reflection cluster or a confederation sub-AS should 156 be designed such that the IGP metrics for links within a cluster (or 157 confederation sub-AS) are much smaller than the IGP metrics for the 158 links between the clusters (or confederation sub-AS). This practice 159 helps achieve consistent routing within a route reflection cluster or 160 a confederation sub-AS. 162 When the aforementioned practice for devising a route reflection 163 cluster or confederation sub-AS is followed in a network, we claim 164 that the advertisement of all the Group Best Paths by a route 165 reflector or a confederation ASBR is sufficient to eliminate the MED- 166 induced route oscillations in the network. This claim is validated 167 in Appendix A. 169 Note that a Group Best Path for an address prefix can be identified 170 by the combination of the address prefix and the neighbor-AS. Thus 171 this approach can be implemented using the mechanism described in 172 ADD-PATH [I-D.ietf-idr-add-paths] for advertising multiple paths, and 173 in this case the neighbor-AS of a path may be used as the path 174 identifier of the path. 176 It should be noted that the approach of advertising the Group Best 177 Paths requires certain topological constrains to be satisfied in 178 order to eliminate the MED-induced route oscillation. Specific 179 topological considerations are described in [RFC3345]. 181 5. Route Reflection and Confederation 183 To allow a route reflector or a confederation ASBR to advertise 184 either the Available Paths or Group Best Paths using the mechanism 185 described in ADD-PATH [I-D.ietf-idr-add-paths], the following 186 revisions are proposed for BGP route reflection and BGP 187 Confederation. 189 5.1. Route Reflection 191 Depending on the configuration, for a particular a route 192 reflector SHOULD include the with the "Send/Receive" 193 field set to 2 or 3 in the ADD-PATH Capability 194 [I-D.ietf-idr-add-paths] advertised to an IBGP peer. When the ADD- 195 PATH Capability is also received from the IBGP peer with the "Send/ 196 Receive" field set to 1 or 3 for the same , then the 197 following procedures shall be followed: 199 If the peer is a route reflection client, the route reflector SHOULD 200 advertise to the peer the Group Best Paths (or the Available Paths) 201 received from its non-client IBGP peers. Depending on the 202 configuration, the route reflector MAY also advertise to the peer the 203 Group Best Paths (or the Available Paths) received from its clients. 205 If the peer is a non-client, the route reflector SHOULD advertise to 206 the peer the Group Best Paths (or the Available Paths) received from 207 its clients. 209 5.2. Confederation 211 Depending on the configuration, for a particular a 212 confederation ASBR SHOULD include the with the "Send/ 213 Receive" field set to 2 or 3 in the ADD-PATH Capability 214 [I-D.ietf-idr-add-paths] advertised to an IBGP peer, and to a 215 confederation external peer. When the ADD-PATH Capability is also 216 received from the IBGP peer or the confederation external peer with 217 the "Send/Receive" field set to 1 or 3 for the same , then 218 the following procedures shall be followed: 220 If the peer is internal, the confederation ASBR SHOULD advertise to 221 the peer the Group Best Paths (or the Available Paths) received from 222 its confederation external peers. 224 If the peer is confederation external, the confederation ASBR SHOULD 225 advertise to the peer the Group Best Paths (or the Available Paths) 226 received from its IBGP peers. 228 6. Deployment Considerations 230 Some route oscillations, once detected, can be eliminated by simple 231 configuration workarounds. As carrying additional paths impacts the 232 memory usage and routing convergence in a network, it is recommended 233 that the impact be evaluated and the approach of using a 234 configuration workaround be considered in deciding whether to deploy 235 the proposed mechanism in a network. In addition, the advertisement 236 of multiple paths should be limited to those prefixes which are 237 affected by MED-induced route oscillation. 239 While the route reflectors or confederation ASBRs in a network need 240 to advertise the Group Best Paths or Available Paths, the vast 241 majority of the BGP speakers in the network only need to receive the 242 Group Best Paths or Available Paths, which would involve only minor 243 software changes. 245 It should be emphasized that in order to eliminate the MED-induced 246 route oscillations in a network using the approach of advertising the 247 Group Best Paths, the recommended practice for devising a route 248 reflection cluster or confederation sub-AS with respect to the IGP 249 metrics ([RFC4456], [RFC5065]) should be followed. 251 It is expected that the approach of advertising the Group Best Paths 252 would be adequate to achieve consistent routing for the vast majority 253 of the networks. For a network that has large number of alternate 254 paths, the approach should be a good choice as the number of paths 255 advertised by a reflector or a confederation ASBR is bounded by the 256 number of the neighbor-AS's for a particular address prefix. The 257 additional states for an address prefix would also be per neighbor-AS 258 based rather than per path based. The number of the neighbor-AS's 259 for a particular address prefix is typically small because of the 260 limited number of upstream providers for a customer and the nature of 261 advertising only customer routes at the inter-exchange points. 263 The approach of advertising the Group Best Paths, however, may still 264 be inadequate for certain networks to avoid other routing 265 inconsistencies such as forwarding loops. The required topological 266 constrains could also be operationally challenging. In these cases 267 the approach of advertising the Available Paths may be used, but 268 should be limited to those prefixes which are affected by MED-induced 269 route oscillation in a network carrying a large number of alternate 270 paths. Note that the number of paths that need to be maintained and 271 advertised can be greatly reduced by accepting MEDs from other 272 peering networks. 274 7. IANA Considerations 276 This memo includes no request to IANA. 278 8. Security Considerations 280 This extension to BGP does not change the underlying security issues 281 inherent in the existing BGP [RFC4271]. 283 9. Acknowledgements 285 We would like to thank David Cook and Naiming Shen for their 286 contributions to the design and development of the solutions. 288 10. References 290 10.1. Normative References 292 [I-D.ietf-idr-add-paths] 293 Walton, D., Retana, A., Chen, E., and J. Scudder, 294 "Advertisement of Multiple Paths in BGP", draft-ietf-idr- 295 add-paths-10 (work in progress), October 2014. 297 [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate 298 Requirement Levels", BCP 14, RFC 2119, 299 DOI 10.17487/RFC2119, March 1997, 300 . 302 [RFC4271] Rekhter, Y., Ed., Li, T., Ed., and S. Hares, Ed., "A 303 Border Gateway Protocol 4 (BGP-4)", RFC 4271, 304 DOI 10.17487/RFC4271, January 2006, 305 . 307 [RFC4456] Bates, T., Chen, E., and R. Chandra, "BGP Route 308 Reflection: An Alternative to Full Mesh Internal BGP 309 (IBGP)", RFC 4456, DOI 10.17487/RFC4456, April 2006, 310 . 312 [RFC5065] Traina, P., McPherson, D., and J. Scudder, "Autonomous 313 System Confederations for BGP", RFC 5065, 314 DOI 10.17487/RFC5065, August 2007, 315 . 317 10.2. Informative References 319 [RFC3345] McPherson, D., Gill, V., Walton, D., and A. Retana, 320 "Border Gateway Protocol (BGP) Persistent Route 321 Oscillation Condition", RFC 3345, DOI 10.17487/RFC3345, 322 August 2002, . 324 Appendix A. Why the Group Best Paths Are Adequate? 326 It is assumed that the following common practice is followed. A 327 route reflection cluster or a confederation sub-AS should be designed 328 such that the IGP metrics for links within a cluster (or 329 confederation sub-AS) are much smaller than the IGP metrics for the 330 links between the clusters (or confederation sub-AS). This practice 331 helps achieve consistent routing within a route reflection cluster or 332 a confederation sub-AS. 334 Observe that in a network that maintains full IBGP mesh only the 335 paths that survive the (Local_Pref, AS-PATH Length, Origin, MED) 336 comparisons [RFC4271] would contribute to the route selection in the 337 network. 339 Consider a route reflection cluster that sources one or more paths 340 that would survive the (Local_Pref, AS-PATH Length, Origin, MED) 341 comparisons among all the paths in the network. One of these 342 surviving paths would be selected as the Group Best Path by the route 343 reflector in the cluster. Due to the constrain on the IGP metrics as 344 described previously, this path would remain as the Group Best Path 345 and would be advertised to all other clusters even after a path is 346 received from another cluster. 348 On the other hand, when no path in a route reflection cluster would 349 survive the (Local_Pref, AS-PATH Length, Origin, MED) comparisons 350 among all the paths in the network, the Group Best Path (when exists) 351 for a route reflector would be from another cluster. Clearly the 352 advertise of the Group Best Path by the route reflector to the 353 clients only depends on the paths received from other clusters. 355 Therefore there is no MED-induced route oscillation in the network as 356 the advertisement of a Group Best Path to a peer does not depend on 357 the paths received from that peer. 359 The claim for the confederation can be validated similarly. 361 Authors' Addresses 363 Daniel Walton 364 Cumulus Networks 365 140C S. Whisman Rd. 366 Mountain View, CA 94041 367 USA 369 Email: dwalton@cumulusnetworks.com 371 Alvaro Retana 372 Cisco Systems, Inc. 373 7025 Kit Creek Rd. 374 Research Triangle Park, NC 27709 375 USA 377 Email: aretana@cisco.com 378 Enke Chen 379 Cisco Systems, Inc. 380 170 W. Tasman Dr. 381 San Jose, CA 95134 382 USA 384 Email: enkechen@cisco.com 386 John Scudder 387 Juniper Networks 388 1194 N. Mathilda Ave 389 Sunnyvale, CA 94089 390 USA 392 Email: jgs@juniper.net