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Checking references for intended status: Proposed Standard ---------------------------------------------------------------------------- (See RFCs 3967 and 4897 for information about using normative references to lower-maturity documents in RFCs) ** Obsolete normative reference: RFC 1771 (ref. '1') (Obsoleted by RFC 4271) ** Obsolete normative reference: RFC 1863 (ref. '2') (Obsoleted by RFC 4223) -- Possible downref: Non-RFC (?) normative reference: ref. '3' Summary: 11 errors (**), 0 flaws (~~), 2 warnings (==), 3 comments (--). Run idnits with the --verbose option for more detailed information about the items above. -------------------------------------------------------------------------------- 1 INTERNET-DRAFT Tony Bates 2 MCI 3 Ravi Chandra 4 cisco Systems 5 March 1996 7 BGP Route Reflection 8 An alternative to full mesh IBGP 9 11 Status of this Memo 13 This document is an Internet Draft. Internet Drafts are working 14 documents of the Internet Engineering Task Force (IETF), its Areas, 15 and its Working Groups. Note that other groups may also distribute 16 working documents as Internet Drafts. 18 Internet Drafts are draft documents valid for a maximum of six 19 months. Internet Drafts may be updated, replaced, or obsoleted by 20 other documents at any time. It is not appropriate to use Internet 21 Drafts as reference material or to cite them other than as a "working 22 draft" or "work in progress". 24 Please check the I-D abstract listing contained in each Internet 25 Draft directory to learn the current status of this or any other 26 Internet Draft. 28 Abstract 30 The Border Gateway Protocol [1] is an inter-autonomous system routing 31 protocol designed for TCP/IP internets. BGP deployments are 32 configured such that that all BGP speakers within a single AS must be 33 fully meshed so that any external routing information must be re- 34 distributed to all other routers within that AS. This represents a 35 serious scaling problem that has been well documented with several 36 alternatives proposed [2,3]. 38 This document describes the use and design of a method known as 39 "Route Reflection" to alleviate the the need for "full mesh" IBGP. 41 1. Introduction 43 Currently in the Internet today, BGP deployments are configured such 44 that that all BGP speakers within a single AS must be fully meshed 45 and any external routing information must be re-distributed to all 46 other routers within that AS. This "full mesh" requirement clearly 47 does not scale when there are a large number of IBGP speakers as is 48 common in many of todays internet networks. 50 For n BGP speakers within an AS you must maintain n*(n-1)/2 unique 51 IBGP sessions. With finite resources in both bandwidth and router CPU 52 this clearly does not scale. 54 This scaling problem has been well documented and a number of 55 proposals have been made to alleviate this [2,3]. This document 56 represents another alternative in alleviating the need for a "full 57 mesh" and is known as "Route Reflection". It represents a change in 58 the commonly understood concept of IBGP and the addition of two new 59 optional transitive BGP attributes. 61 2. Design Criteria 63 Route Reflection was designed to satisfy the following criteria. 65 o Simplicity 67 Any alternative must be both simple to configure as well 68 as understand. 70 o Easy Migration 72 It must be possible to migrate from a full mesh 73 configuration without the need to change either topology 74 or AS. This is an unfortunate management overhead of the 75 technique proposed in [3]. 77 o Compatibility 79 It must be possible for non compliant IBGP peers 80 to continue be part of the original AS or domain 81 without any loss of BGP routing information. 83 These criteria were motivated by operational experiences of a very 84 large and topology rich network with many external connections. 86 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 reflect IBGP learned 115 routes, then it could re-advertise (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 basic principle. 138 4. Terminology and Concepts 140 We use the term "Route Reflector" (RR) to represent an IBGP speaker 141 that participates in the reflection. The internal peers of a RR are 142 divided into two groups: 144 1) Client Peers 146 2) Non-Client Peers 148 A RR reflects routes between these groups. A RR along with its 149 client peers form a Cluster. The Non-Client peer must be fully meshed 150 but the Client peers need not be fully meshed. The Client peers 151 should not peer with internal speakers outside of their cluster. 152 Figure 3 depicts a simple example outlining the basic RR components 153 using the terminology noted above. 155 / - - - - - - - - - - - - - -\ 156 | Cluster | 157 +-------+ +-------+ 158 | | | | | | 159 | RTR-A | | RTR-B | 160 | |Client | |Client | | 161 +-------+ +-------+ 162 | \ / | 163 IBGP \ / IBGP 164 | \ / | 165 +-------+ 166 | | | | 167 | RTR-C | 168 | | RR | | 169 +-------+ 170 | / \ | 171 \ - - - - -/- - -\- - - - - - / 172 IBGP / \ IBGP 173 +-------+ +-------+ 174 | RTR-D | IBGP | RTR-E | 175 | Non- |---------| Non- | 176 |Client | |Client | 177 +-------+ +-------+ 179 Figure 3: RR Components 181 5. Operation 183 When a route is received by a RR, it selects the best path based on 184 its path selection rule. After the best path is selected, it must do 185 the following depending on the type of the peer it is receiving the 186 best path from: 188 1) A Route from a Non-Client peer 190 Reflect to all other Clients. 192 2) A Route from a Client peer 194 Reflect to all the Non-Client peers and also to the 195 Client peers (Hence the Client peers are not required 196 to be fully meshed). 198 3) Route from an EBGP peer 200 Send to all the Client and Non-Client Peers. 202 An Autonomous System could have many RRs. A RR treats other RRs just 203 like any other internal BGP speakers. A RR could be configured to 204 have other RRs in a Client group or Non-client group. 206 In a simple configuration the backbone could be divided into many 207 clusters. Each RR would be configured with other RRs as Non-Client 208 peers (thus all the RRs will be fully meshed.). The Clients will be 209 configured to maintain IBGP session only with the RR in their 210 cluster. Due to route reflection, all the IBGP speakers will receive 211 reflected routing information. 213 It is normal in a Autonomous System to have BGP speakers that do not 214 understand the concept of Route-Reflectors (let us call them as 215 conventional BGP speakers). The Route-Reflector Scheme allows such 216 conventional BGP speakers to co-exist. Conventional BGP speakers 217 could be either members of Non-Client group or Client group. This 218 allows for an easy and gradual migration from the current IBGP model 219 to the Route Reflection model. One could start creating clusters by 220 configuring a single router as the designated RR and configuring 221 other RRs and their clients as normal IBGP peers. Additional clusters 222 can be created gradually. 224 6. Redundant RRs 226 Usually a cluster of clients will have a single RR. In that case, the 227 cluster will be identified by the ROUTER_ID of the RR. However, this 228 represents a single point of failure so to make it possible to have 229 multiple RRs in the same cluster, all RRs in the same cluster must be 230 configured with a 4-byte CLUSTER_ID so that an RR can discern routes 231 from other RRs in the same cluster. 233 7. Avoiding Routing Information Loops 235 As IBGP learned routes are reflected, it is possible through mis- 236 configuration to form route re-distribution loops. The Route 237 Reflection method defines the following attributes to detect and 238 avoid routing information loops. 240 ORIGINATOR_ID 242 ORIGINATOR_ID is a new optional, non-transitive BGP attribute of Type 243 code 9. This attribute is 4 bytes long and it will be created by a 244 RR. This attribute will carry the ROUTER_ID of the originator of the 245 route in the local AS. A BGP speaker should not create an 246 ORIGINATOR_ID attribute if one already exists If routing information 247 comes back to the originator, it must be ignored. 249 CLUSTER_LIST 251 Cluster-list is a new optional, non-transitive BGP attribute of Type 252 code 10. It is a sequence of CLUSTER_ID values representing the 253 reflection path that the route has passed. It is encoded as follows: 255 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 256 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 257 | Attr. Flags |Attr. Type Code| Length | value ... 258 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 260 Where Length is the number of octets. 262 When a RR reflects a route from its Clients to a Non-Client peer, it 263 must append the local CLUSTER_ID to the CLUSTER_LIST. If the 264 CLUSTER_LIST is empty, it must create a new one. Using this attribute 265 an RR can identify if the routing information is looped back to the 266 same cluster due to mis-configuration. If the local CLUSTER_ID is 267 found in the cluster-list, the advertisement will be ignored. 269 8. Implementation and Configuration Considerations 271 Care should be taken to make sure that none of the BGP path 272 attributes defined above can be modified through configuration when 273 exchanging internal routing information between RRs and Clients and 274 Non-Clients. This could result is looping of routes. 276 In some implementations, modification of the BGP path attribute, 277 NEXT_HOP is possible. For example, there could be a need for a RR to 278 modify NEXT_HOP for EBGP learned routes sent to its internal peers. 279 However, this must not be possible for an RR to set on reflected IBGP 280 routes as this breaks the basic principle of Route Reflection and 281 will result in potential black holes. 283 An RR should not modify any AS-PATH attributes (i.e. LOCAL_PREF, MED, 284 DPA)that could change consistent route selection. This could 285 resulting in potential loops. 287 The BGP protocol provides no way for a Client to identify itself 288 dynamically as a Client to an RR configured BGP speaker and the 289 simplest way to achieve this is by manual configuration. 291 9. Security 293 Security considerations are not discussed in this memo. 295 10. Acknowledgments 297 The authors would like to thank Dennis Ferguson, Enke Chen, John 298 Scudder, Paul Traina and Tony Li for the many discussions resulting 299 in this work. This idea was developed from an earlier discussion 300 between Tony Li and Dimitri Haskin. 302 11. References 304 [1] Rekhter, Y., and Li, T., "A Border Gateway Protocol 4 (BGP-4)", 305 RFC1771, March 1995. 307 [2] Haskin, D., "A BGP/IDRP Route Server alternative to a full mesh 308 routing", RFC1863, October 1995. 310 [3] Traina, P. "Limited Autonomous System Confederations for BGP", 311 INTERNET-DRAFT, , April 1995. 313 12. Author's Addresses 315 Tony Bates 316 MCI 317 2100 Reston Parkway 318 Reston, VA 22091 320 phone: +1 703 715 7521 321 email: Tony.Bates@mci.net 323 Ravishanker Chandrasekeran 324 (Ravi Chandra) 325 cisco Systems 326 170 West Tasman Drive 327 San Jose, CA 95134 329 email: rchandra@cisco.com