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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. BGP deployments are 33 configured such that that all BGP speakers within a single AS must be 34 fully meshed so that any external routing information must be re- 35 distributed to all other routers within that AS. This represents a 36 serious scaling problem that has been well documented with several 37 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 today, BGP deployments are configured such 45 that that all BGP speakers within a single AS must be fully meshed 46 and any external routing information must be re-distributed to all 47 other routers within that AS. This "full mesh" requirement clearly 48 does not scale when there are a large number of IBGP speakers as is 49 common in many of todays internet networks. 51 For n BGP speakers within an AS you must maintain n*n-1/2 unique IBGP 52 sessions. With finite resources in both bandwidth and router CPU this 53 clearly does not scale. 55 This scaling problem has been well documented and a number of 56 proposals have been made to alleviate this [2,3]. This document 57 represents another alternative in alleviating the need for a "full 58 mesh" and is known as "Route Reflection". It represents a change in 59 the commonly understood concept of IBGP and the addition of two new 60 optional transitive BGP attributes. 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 Migration 73 It must be possible to migrate 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 compliment 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 89 The basic idea of Route Reflection is very simple. Let us consider 90 the simple example depicted in Figure 1 below. 92 +------ + +-------+ 93 | | IBGP | | 94 | RTR-A |--------| RTR-B | 95 | | | | 96 +-------+ +-------+ 97 \ / 98 IBGP \ ASX / IBGP 99 \ / 100 +-------+ 101 | | 102 | RTR-C | 103 | | 104 +-------+ 106 Figure 1: Full Mesh IBGP 108 In ASX there are three IBGP speakers (routers RTR-A, RTR-B and RTR-C) 109 and each B, C). With the existing BGP model, if RTR-A receives an 110 external route, it must advertise it to both RTR-B and RTR-C. RTR-B 111 and RTR-C (as IBGP speakers) will not re-advertise these IBGP learned 112 routes to other IBGP speakers. 114 If this rule is broken and RTR-C is allowed to reflect IBGP learned 115 routes, then it could re-distribute (or reflect) the IBGP routes 116 learned from RTR-A to RTR-B and vice versa. This would eliminate the 117 need for the IBGP session between RTR-A and RTR-C as shown in Figure 118 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 principle. 138 4. Terminology and Concepts 140 We use the term "Route Reflector" (RR) to represent an IBGP speaker 141 that 142 participates in the reflection. The internal peers of a RR are 143 divided into two groups: 145 1) Client Peers 147 2) Non-Client Peers 149 A RR reflects routes between these groups. A RR along with its 150 client peers form a Cluster. The Non-Client peer must be fully meshed 151 but the Client peers need not be fully meshed. The Client peers 152 should not peer with internal speakers outside of their cluster. 153 Figure 3 depicts a simple example outlining the basic RR components 154 using the terminology noted above. 156 / - - - - - - - - - - - - - -\ 157 | Cluster | 158 +-------+ +-------+ 159 | | | | | | 160 | RTR-A | | RTR-B | 161 | |Client | |Client | | 162 +-------+ +-------+ 163 | \ / | 164 IBGP \ / IBGP 165 | \ / | 166 +-------+ 167 | | | | 168 | RTR-C | 169 | | RR | | 170 +-------+ 171 | / \ | 172 \ - - - - -/- - -\- - - - - - / 173 IBGP / \ IBGP 174 +-------+ +-------+ 175 | RTR-D | IBGP | RTR-E | 176 | Non- |---------| Non- | 177 |Client | |Client | 178 +-------+ +-------+ 180 Figure 3: RR Components 182 5. Operation 184 When a route is received by a RR, it must do the following depending 185 on the type of the peer it is receiving a route from: 187 1) A Route from a Non-Client peer 189 Reflect to all other Clients. 191 2) A Route from a Client peer 193 Reflect to all the Non-Client peers and also to the 194 Client peers (Hence the Client peers are not required 195 to be fully meshed). 197 3) Route from an EBGP peer 199 Send to all the Client and Non-Client Peers. 201 An Autonomous System could have many RRs. A RR treats other RRs just 202 like any other internal BGP speakers. A RR could be configured to 203 have other RRs in a Client group or Non-client group. 205 In a simple configuration the backbone could be divided into many 206 clusters. Each RR would be configured with other RRs as Non-Client 207 peers (thus all the RRs will be fully meshed.). The Clients will be 208 configured to maintain IBGP session only with the RR in their 209 cluster. Due to route reflection, all the IBGP speakers will receive 210 reflected routing information. 212 It is normal in a Autonomous System to have BGP speakers that do not 213 understand the concept of Route-Reflectors (let us call them as 214 conventional BGP speakers). The Route-Reflector Scheme allows such 215 conventional BGP speakers to co-exist. Conventional BGP speakers 216 could be either members of Non-Client group or Client group. This 217 allows for an easy and gradual migration from the current IBGP model 218 to the Route Reflection model. One could start creating clusters by 219 configuring a single router as the designated RR and configuring 220 other RRs and their clients as normal IBGP peers. Additional clusters 221 can be created gradually. 223 6. Redundant RRs 225 Usually a cluster of clients will have a single RR. In that case, the 226 cluster will be identified by the ROUTER_ID of the RR. However, this 227 represents a single point of failure so to make it possible to have 228 multiple RRs in the same cluster, all RRs in the same cluster must be 229 configured with a 4-byte CLUSTER_ID so that an RR can discern routes 230 from other RRs in the same cluster. 232 7. Avoiding Routing Information Loops 234 As IBGP learned routes are reflected, it is possible through mis- 235 configuration to form route redistribution loops. The Route 236 Reflection method defines the following attributes to detect and 237 avoid routing information loops. 239 ORIGINATOR_ID 241 ORIGINATOR_ID is a new optional, non-transitive BGP attribute of Type 242 code 9. This attribute is 4 bytes long and it will be created by a 243 RR. This attribute will carry the ROUTER_ID of the originator of the 244 route in the local AS. A BGP speaker should not create an 245 ORIGINATOR_ID attribute if one already exists If routing information 246 comes back to the originator, it must be ignored. 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. 254 When a RR reflects a route from its Clients to a Non-Client peer, it 255 must append the local CLUSTER_ID to the CLUSTER_LIST. If the 256 CLUSTER_LIST is empty, it must create a new one. Using this attribute 257 an RR can identify if the routing information is looped back to the 258 same cluster due to mis-configuration. If the local CLUSTER_ID is 259 found in the cluster-list, the advertisement will be ignored. 261 8. Implementation and Configuration Considerations 263 Care should be taken to make sure that none of the BGP path 264 attributes defined above can be modified through configuration when 265 exchanging internal routing information between RRs and Clients and 266 Non-Clients. This could result is looping of routes. 268 In some implementations, modification of the BGP path attribute, 269 NEXT_HOP is possible. For example, there could be a need for a RR to 270 modify NEXT_HOP for EBGP learned routes sent to its internal peers. 271 However, this must not be possible for an RR to set on reflected IBGP 272 routes as this breaks the basic principle of Route Reflection and 273 will result in potential black holes. 275 An RR should not modify any AS-PATH attributes (i.e. LOCAL_PREF, MED, 276 DPA)that could change consistent route selection. THis could 277 resulting in potential loops. 279 The BGP protocol provides no way for a Client to identify itself 280 dynamically as a Client to an RR configured BGP speaker and the 281 simplest way to achieve this is by manual configuration. 283 9. Security 285 Security considerations are not discussed in this memo. 287 10. Acknowledgments 289 The authors would like to thank Dennis Ferguson, Enke Chen, Paul 290 Traina and Tony Li for the many discussions resulting in this work. 291 This idea was developed from an earlier discussion between Tony Li 292 and Dimitri Haskin. 294 11. References 296 [1] Rekhter, Y., and Li, T., "A Border Gateway Protocol 4 (BGP-4)", 297 RFC1771, March 1995. 299 [2] Haskin, D., "A BGP/IDRP Route Server alternative to a full mesh 300 routing", RFC1863, October 1995. 302 [3] Traina, P. "Limited Autonomous System Confederations for BGP", 303 INTERNET-DRAFT, , April 1995. 305 12. Author's Addresses 307 Tony Bates 308 MCI 309 2100 Reston Parkway 310 Reston, VA 22091 312 phone: +1 703 715 7521 313 email: Tony.Bates@mci.net 315 Ravishanker Chandrasekeran 316 (Ravi Chandra) 317 cisco Systems 318 170 West Tasman Drive 319 San Jose, CA 95134 321 email: rchandra@cisco.com