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Run idnits with the --verbose option for more detailed information about the items above. -------------------------------------------------------------------------------- 2 INTERNET-DRAFT Tony Bates 3 Ravi Chandra 4 Enke Chen 5 Cisco Systems 6 November 1998 8 BGP Route Reflection 9 An alternative to full mesh IBGP 10 12 Status of this Memo 14 This document is an Internet Draft. Internet Drafts are working 15 documents of the Internet Engineering Task Force (IETF), its Areas, 16 and its Working Groups. Note that other groups may also distribute 17 working documents as Internet Drafts. 19 Internet Drafts are draft documents valid for a maximum of six 20 months. Internet Drafts may be updated, replaced, or obsoleted by 21 other documents at any time. It is not appropriate to use Internet 22 Drafts as reference material or to cite them other than as a "working 23 draft" or "work in progress". 25 Please check the I-D abstract listing contained in each Internet 26 Draft directory to learn the current status of this or any other 27 Internet Draft. 29 Abstract 31 The Border Gateway Protocol [1] is an inter-autonomous system routing 32 protocol designed for TCP/IP internets. Currently in the Internet BGP 33 deployments are configured such that that all BGP speakers within a 34 single AS must be fully meshed so that any external routing 35 information must be re-distributed to all other routers within that 36 AS. This represents a serious scaling problem that has been well 37 documented with several alternatives proposed [2,3]. 39 This document describes the use and design of a method known as 40 'Route Reflection' to alleviate the the need for 'full mesh' IBGP. 42 1. Introduction 44 Currently in the Internet, BGP deployments are configured such that 45 that all BGP speakers within a single AS must be fully meshed and any 46 external routing information must be re-distributed to all other 47 routers within that AS. For n BGP speakers within an AS that 48 requires to maintain n*(n-1)/2 unique IBGP sessions. This "full 49 mesh" requirement clearly does not scale when there are a large 50 number of IBGP speakers each exchanging a large volume of routing 51 information, as is common in many of todays internet networks. 53 This scaling problem has been well documented and a number of 54 proposals have been made to alleviate this [2,3]. This document 55 represents another alternative in alleviating the need for a "full 56 mesh" and is known as "Route Reflection". This approach allows a BGP 57 speaker (known as "Route Reflector") to advertise IBGP learned routes 58 to certain IBGP peers. It represents a change in the commonly 59 understood concept of IBGP, and the addition of two new optional 60 transitive BGP attributes to prevent loops in routing updates. 62 2. Design Criteria 64 Route Reflection was designed to satisfy the following criteria. 66 o Simplicity 68 Any alternative must be both simple to configure as well 69 as understand. 71 o Easy Transition 73 It must be possible to transition from a full mesh 74 configuration without the need to change either topology 75 or AS. This is an unfortunate management overhead of the 76 technique proposed in [3]. 78 o Compatibility 80 It must be possible for non compliant IBGP peers 81 to continue be part of the original AS or domain 82 without any loss of BGP routing information. 84 These criteria were motivated by operational experiences of a very 85 large and topology rich network with many external connections. 87 3. Route Reflection 88 The basic idea of Route Reflection is very simple. Let us consider 89 the simple example depicted in Figure 1 below. 91 +------ + +-------+ 92 | | IBGP | | 93 | RTR-A |--------| RTR-B | 94 | | | | 95 +-------+ +-------+ 96 \ / 97 IBGP \ ASX / IBGP 98 \ / 99 +-------+ 100 | | 101 | RTR-C | 102 | | 103 +-------+ 105 Figure 1: Full Mesh IBGP 107 In ASX there are three IBGP speakers (routers RTR-A, RTR-B and RTR- 108 C). With the existing BGP model, if RTR-A receives an external route 109 and it is selected as the best path it must advertise the external 110 route to both RTR-B and RTR-C. RTR-B and RTR-C (as IBGP speakers) 111 will not re-advertise these IBGP learned routes to other IBGP 112 speakers. 114 If this rule is relaxed and RTR-C is allowed to advertise IBGP 115 learned routes to IBGP peers, then it could re-advertise (or reflect) 116 the IBGP routes learned from RTR-A to RTR-B and vice versa. This 117 would eliminate the need for the IBGP session between RTR-A and RTR-B 118 as shown in Figure 2 below. 120 +------ + +-------+ 121 | | | | 122 | RTR-A | | RTR-B | 123 | | | | 124 +-------+ +-------+ 125 \ / 126 IBGP \ ASX / IBGP 127 \ / 128 +-------+ 129 | | 130 | RTR-C | 131 | | 132 +-------+ 134 Figure 2: Route Reflection IBGP 136 The Route Reflection scheme is based upon this basic principle. 138 4. Terminology and Concepts 140 We use the term "Route Reflection" to describe the operation of a BGP 141 speaker advertising an IBGP learned route to another IBGP peer. Such 142 a BGP speaker is said to be a "Route Reflector" (RR), and such a 143 route is said to be a reflected route. 145 The internal peers of a RR are divided into two groups: 147 1) Client Peers 149 2) Non-Client Peers 151 A RR reflects routes between these groups, and may reflect routes 152 among client peers. A RR along with its client peers form a Cluster. 153 The Non-Client peer must be fully meshed but the Client peers need 154 not be fully meshed. Figure 3 depicts a simple example outlining the 155 basic RR components using the terminology noted above. 157 / - - - - - - - - - - - - - - 158 | Cluster | 159 +-------+ +-------+ 160 | | | | | | 161 | RTR-A | | RTR-B | 162 | |Client | |Client | | 163 +-------+ +-------+ 164 | \ / | 165 IBGP \ / IBGP 166 | \ / | 167 +-------+ 168 | | | | 169 | RTR-C | 170 | | RR | | 171 +-------+ 172 | / \ | 173 - - - - - /- - -\- - - - - - / 174 IBGP / \ IBGP 175 +-------+ +-------+ 176 | RTR-D | IBGP | RTR-E | 177 | Non- |---------| Non- | 178 |Client | |Client | 179 +-------+ +-------+ 181 Figure 3: RR Components 183 5. Operation 185 When a RR receives a route from an IBGP peer, it selects the best 186 path based on its path selection rule. After the best path is 187 selected, it must do the following depending on the type of the peer 188 it is receiving the best path from: 190 1) A Route from a Non-Client IBGP peer 192 Reflect to all the Clients. 194 2) A Route from a Client peer 196 Reflect to all the Non-Client peers and also to the 197 Client peers. (Hence the Client peers are not required 198 to be fully meshed.) 200 An Autonomous System could have many RRs. A RR treats other RRs just 201 like any other internal BGP speakers. A RR could be configured to 202 have other RRs in a Client group or Non-client group. 204 In a simple configuration the backbone could be divided into many 205 clusters. Each RR would be configured with other RRs as Non-Client 206 peers (thus all the RRs will be fully meshed.). The Clients will be 207 configured to maintain IBGP session only with the RR in their 208 cluster. Due to route reflection, all the IBGP speakers will receive 209 reflected routing information. 211 It is possible in a Autonomous System to have BGP speakers that do 212 not understand the concept of Route-Reflectors (let us call them 213 conventional BGP speakers). The Route-Reflector Scheme allows such 214 conventional BGP speakers to co-exist. Conventional BGP speakers 215 could be either members of a Non-Client group or a Client group. This 216 allows for an easy and gradual migration from the current IBGP model 217 to the Route Reflection model. One could start creating clusters by 218 configuring a single router as the designated RR and configuring 219 other RRs and their clients as normal IBGP peers. Additional clusters 220 can be created gradually. 222 6. Redundant RRs 224 Usually a cluster of clients will have a single RR. In that case, the 225 cluster will be identified by the ROUTER_ID of the RR. However, this 226 represents a single point of failure so to make it possible to have 227 multiple RRs in the same cluster, all RRs in the same cluster can be 228 configured with a 4-byte CLUSTER_ID so that an RR can discard routes 229 from other RRs in the same cluster. 231 7. Avoiding Routing Information Loops 233 When a route is reflected, it is possible through mis-configuration 234 to form route re-distribution loops. The Route Reflection method 235 defines the following attributes to detect and avoid routing 236 information loops: 238 ORIGINATOR_ID 240 ORIGINATOR_ID is a new optional, non-transitive BGP attribute of Type 241 code 9. This attribute is 4 bytes long and it will be created by a RR 242 in reflecting a route. This attribute will carry the ROUTER_ID of 243 the originator of the route in the local AS. A BGP speaker should not 244 create an ORIGINATOR_ID attribute if one already exists. A router 245 should ignore a route received with its ROUTER_ID as the 246 ORIGINATOR_ID. 248 CLUSTER_LIST 250 Cluster-list is a new optional, non-transitive BGP attribute of Type 251 code 10. It is a sequence of CLUSTER_ID values representing the 252 reflection path that the route has passed. It is encoded as follows: 254 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 255 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 256 | Attr. Flags |Attr. Type Code| Length | value ... 257 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 259 Where Length is the number of octets. 261 When a RR reflects a route, it must append the local CLUSTER_ID to 262 the CLUSTER_LIST. If the CLUSTER_LIST is empty, it must create a new 263 one. Using this attribute an RR can identify if the routing 264 information is looped back to the same cluster due to mis- 265 configuration. If the local CLUSTER_ID is found in the cluster-list, 266 the advertisement received will be ignored. 268 8. Implementation Considerations 270 Care should be taken to make sure that none of the BGP path 271 attributes defined above can be modified through configuration when 272 exchanging internal routing information between RRs and Clients and 273 Non-Clients. Their modification could potential result in routing 274 loops. 276 In addition, when a RR reflects a route, it should not modify the 277 following path attributes: NEXT_HOP, AS_PATH, LOCAL_PREF, and MED. 278 Their modification could potential result in routing loops. 280 9. Configuration and Deployment Considerations 282 The BGP protocol provides no way for a Client to identify itself 283 dynamically as a Client of an RR. The simplest way to achieve this 284 is by manual configuration. 286 One of the key component of the route reflection approach in 287 addressing the scaling issue is that the RR summarizes routing 288 information and only reflects its best path. 290 Both MEDs and IGP metrics may impact the BGP route selection. 291 Because MEDs are not always comparable and the IGP metric may differ 292 for each router, with certain route reflection topologies the route 293 reflection approach may not yield the same route selection result as 294 that of the full IBGP mesh approach. A way to make route selection 295 the same as it would be with the full IBGP mesh approach is to make 296 sure that route reflectors are never forced to perform the BGP route 297 selection based on IGP metrics which are significantly different from 298 the IGP metrics of their clients, or based on incomparable MEDs. The 299 former can be achieved by configuring the intra-cluster IGP metrics 300 to be better than the inter-cluster IGP metrics, and maintaining full 301 mesh within the cluster. The latter can be achieved by: 303 o setting the local preference of a route at the border router 304 to reflect the MED values. 306 o or by making sure the AS-path lengths from different ASs are 307 different when the AS-path length is used as a route 308 selection criteria. 310 o or by configuring community based policies using which the 311 reflector can decide on the best route. 313 One could argue though that the latter requirement is overly 314 restrictive, and perhaps impractical in some cases. One could 315 further argue that as long as there are no routing loops, there are 316 no compelling reasons to force route selection with route reflectors 317 to be the same as it would be with the full IBGP mesh approach. 319 To prevent routing loops and maintain consistent routing view, it is 320 essential that the network topology be carefully considered in 321 designing a route reflection topology. In general, the route 322 reflection topology should congruent with the network topology when 323 there exist multiple paths for a prefix. One commonly used approach 324 is the POP-based reflection, in which each POP maintains its own 325 route reflectors serving clients in the POP, and all route reflectors 326 are fully meshed. In addition, clients of the reflectors in each POP 327 are often fully meshed for the purpose of optimal intra-POP routing, 328 and the intra-POP IGP metrics are configured to be better than the 329 inter-POP IGP metrics. 331 10. Security 333 Security considerations are not discussed in this memo. 335 11. Acknowledgments 337 The authors would like to thank Dennis Ferguson, John Scudder, Paul 338 Traina and Tony Li for the many discussions resulting in this work. 339 This idea was developed from an earlier discussion between Tony Li 340 and Dimitri Haskin. 342 In addition, the authors would like to acknowledge valuable review 343 and suggestions from Yakov Rekhter on this document, and helpful 344 comments from Tony Li, Rohit Dube, and John Scudder on Section 9. 346 12. References 348 [1] Rekhter, Y., and Li, T., "A Border Gateway Protocol 4 (BGP-4)", 349 RFC1771, March 1995. 351 [2] Haskin, D., "A BGP/IDRP Route Server alternative to a full mesh 352 routing", RFC1863, October 1995. 354 [3] Traina, P. "Limited Autonomous System Confederations for BGP", 355 RFC1965, June 1996. 357 13. Author's Addresses 359 Tony Bates 360 Cisco Systems 361 170 West Tasman Drive 363 email: tbates@cisco.com 365 Ravishanker Chandrasekeran 366 (Ravi Chandra) 367 Cisco Systems 368 170 West Tasman Drive 369 San Jose, CA 95134 371 email: rchandra@cisco.com 373 Enke Chen 374 Cisco Systems 375 170 West Tasman Drive 376 San Jose, CA 95134 378 email: enkechen@cisco.com