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Network Working Group                                           T. BatesRequest for Comments: 1966                                 cisco SystemsCategory: Experimental                                        R. Chandra                                                           cisco Systems                                                               June 1996BGP Route ReflectionAn alternative to full mesh IBGPStatus of this Memo   This memo defines an Experimental Protocol for the Internet   community.  This memo does not specify an Internet standard of any   kind.  Discussion and suggestions for improvement are requested.   Distribution of this memo is unlimited.Abstract   The Border Gateway Protocol [1] is an inter-autonomous system routing   protocol designed for TCP/IP internets. BGP deployments are   configured such that that all BGP speakers within a single AS must be   fully meshed so that any external routing information must be re-   distributed to all other routers within that AS. This represents a   serious scaling problem that has been well documented with several   alternatives proposed [2,3].   This document describes the use and design of a method known as   "Route Reflection" to alleviate the the need for "full mesh" IBGP.1.  Introduction   Currently in the Internet, BGP deployments are configured such that   that all BGP speakers within a single AS must be fully meshed and any   external routing information must be re-distributed to all other   routers within that AS. This "full mesh" requirement clearly does not   scale when there are a large number of IBGP speakers as is common in   many of todays internet networks.   For n BGP speakers within an AS you must maintain n*(n-1)/2 unique   IBGP sessions. With finite resources in both bandwidth and router CPU   this clearly does not scale.   This scaling problem has been well documented and a number of   proposals have been made to alleviate this [2,3]. This document   represents another alternative in alleviating the need for a "full   mesh" and is known as "Route Reflection". It represents a change in   the commonly understood concept of IBGP and the addition of two newBates & Chandra               Experimental                      [Page 1]

RFC 1966                  BGP Route Reflection                 June 1996   optional transitive BGP attributes.2.  Design Criteria   Route Reflection was designed to satisfy the following criteria.           o Simplicity             Any alternative must be both simple to configure as well             as understand.           o Easy Migration             It must be possible to migrate from a full mesh             configuration without the need to change either topology             or AS. This is an unfortunate management overhead of the             technique proposed in [3].           o Compatibility             It must be possible for non compliant IBGP peers             to continue be part of the original AS or domain             without any loss of BGP routing information.   These criteria were motivated by operational experiences of a very   large and topology rich network with many external connections.3.  Route Reflection   The basic idea of Route Reflection is very simple. Let us consider   the simple example depicted in Figure 1 below.                        +------ +        +-------+                        |       |  IBGP  |       |                        | RTR-A |--------| RTR-B |                        |       |        |       |                        +-------+        +-------+                              \             /                          IBGP \   ASX     / IBGP                                \         /                                 +-------+                                 |       |                                 | RTR-C |                                 |       |                                 +-------+                         Figure 1: Full Mesh IBGPBates & Chandra               Experimental                      [Page 2]

RFC 1966                  BGP Route Reflection                 June 1996   In ASX there are three IBGP speakers (routers RTR-A, RTR-B and RTR-   C).  With the existing BGP model, if RTR-A receives an external route   and it is selected as the best path it must advertise the external   route to both RTR-B and RTR-C. RTR-B and RTR-C (as IBGP speakers)   will not re-advertise these IBGP learned routes to other IBGP   speakers.   If this rule is relaxed and RTR-C is allowed to reflect IBGP learned   routes, then it could re-advertise (or reflect) the IBGP routes   learned from RTR-A to RTR-B and vice versa. This would eliminate the   need for the IBGP session between RTR-A and RTR-B as shown in Figure   2 below.                        +------ +        +-------+                        |       |        |       |                        | RTR-A |        | RTR-B |                        |       |        |       |                        +-------+        +-------+                             \             /                         IBGP \   ASX     / IBGP                               \         /                                 +-------+                                 |       |                                 | RTR-C |                                 |       |                                 +-------+                      Figure 2: Route Reflection IBGP   The Route Reflection scheme is based upon this basic principle.4.  Terminology and Concepts   We use the term "Route Reflector" (RR) to represent an IBGP speaker   that participates in the reflection.  The internal peers of a RR are   divided into two groups:           1) Client Peers           2) Non-Client Peers   A RR reflects routes between these groups.  A RR along with its   client peers form a Cluster. The Non-Client peer must be fully meshed   but the Client peers need not be fully meshed. The Client peers   should not peer with internal speakers outside of their cluster.   Figure 3 depicts a simple example outlining the basic RR components   using the terminology noted above.Bates & Chandra               Experimental                      [Page 3]

RFC 1966                  BGP Route Reflection                 June 1996                      / - - - - - - - - - - - - -  -\                      |           Cluster           |                        +-------+        +-------+                      | |       |        |       |  |                        | RTR-A |        | RTR-B |                      | |Client |        |Client |  |                        +-------+        +-------+                      |      \             /        |                         IBGP \           / IBGP                      |        \         /          |                                +-------+                      |         |       |           |                                | RTR-C |                      |         |  RR   |           |                                +-------+                      |           /   \             |                      \ - - - - -/- - -\- - - - - - /                          IBGP  /       \  IBGP                       +-------+         +-------+                       | RTR-D |  IBGP   | RTR-E |                       |  Non- |---------|  Non- |                       |Client |         |Client |                       +-------+         +-------+                          Figure 3: RR Components5. Operation   When a route is received by a RR, it selects the best path based on   its path selection rule. After the best path is selected, it must do   the following depending on the type of the peer it is receiving the   best path from:           1) A Route from a Non-Client peer              Reflect to all other Clients.           2) A Route from a Client peer              Reflect to all the Non-Client peers and also to the              Client peers other than the originator. (Hence the              Client peers are not required to be fully meshed).            3) Route from an EBGP peer               Send to all the Client and Non-Client Peers.Bates & Chandra               Experimental                      [Page 4]

RFC 1966                  BGP Route Reflection                 June 1996   An Autonomous System could have many RRs. A RR treats other RRs just   like any other internal BGP speakers. A RR could be configured to   have other RRs in a Client group or Non-client group.   In a simple configuration the backbone could be divided into many   clusters.  Each RR would be configured with other RRs as Non-Client   peers (thus all the RRs will be fully meshed.). The Clients will be   configured to maintain IBGP session only with the RR in their   cluster.  Due to route reflection, all the IBGP speakers will receive   reflected routing information.   It is normal in a Autonomous System to have BGP speakers that do not   understand the concept of Route-Reflectors (let us call them   conventional BGP speakers). The Route-Reflector Scheme allows such   conventional BGP speakers to co-exist. Conventional BGP speakers ould   be either members of a Non-Client group or a Client group. This   allows for an easy and gradual migration from the current IBGP model   to the Route Reflection model. One could start creating clusters by   configuring a single router as the designated RR and configuring   other RRs and their clients as normal IBGP peers. Additional clusters   can be created gradually.6.  Redundant RRs   Usually a cluster of clients will have a single RR. In that case, the   cluster will be identified by the ROUTER_ID of the RR. However, this   represents a single point of failure so to make it possible to have   multiple RRs in the same cluster, all RRs in the same cluster must be   configured with a 4-byte CLUSTER_ID so that an RR can discern routes   from other RRs in the same cluster.7.  Avoiding Routing Information Loops   As IBGP learned routes are reflected, it is possible through mis-   configuration to form route re-distribution loops. The Route   Reflection method defines the following attributes to detect and   avoid routing information loops.   ORIGINATOR_ID   ORIGINATOR_ID is a new optional, non-transitive BGP attribute of Type   code 9.  This attribute is 4 bytes long and it will be created by a   RR. This attribute will carry the ROUTER_ID of the originator of the   route in the local AS. A BGP speaker should not create an   ORIGINATOR_ID attribute if one already exists.  A route reflector   must never send routing information back to the router specified in   ORIGINATOR_ID.Bates & Chandra               Experimental                      [Page 5]

RFC 1966                  BGP Route Reflection                 June 1996   CLUSTER_LIST   Cluster-list is a new optional, non-transitive BGP attribute of Type   code 10. It is a sequence of CLUSTER_ID values representing the   reflection path that the route has passed. It is encoded as follows:              0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+     |  Attr. Flags  |Attr. Type Code|   Length      | value ...     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+   Where Length is the number of octets.   When a RR reflects a route from its Clients to a Non-Client peer, it   must append the local CLUSTER_ID to the CLUSTER_LIST. If the   CLUSTER_LIST is empty, it must create a new one. Using this attribute   an RR can identify if the routing information is looped back to the   same cluster due to mis-configuration. If the local CLUSTER_ID is   found in the cluster-list, the advertisement will be ignored.8.  Implementation and Configuration Considerations   Care should be taken to make sure that none of the BGP path   attributes defined above can be modified through configuration when   exchanging internal routing information between RRs and Clients and   Non-Clients. This could result is looping of routes.   In some implementations, modification of the BGP path attribute,   NEXT_HOP is possible. For example, there could be a need for a RR to   modify NEXT_HOP for EBGP learned routes sent to its internal peers.   However, it must not be possible for an RR to set on reflected IBGP   routes as this breaks the basic principle of Route Reflection and   will result in potential black holeing of traffic.   An RR should not modify any AS-PATH attributes (i.e. LOCAL_PREF, MED,   DPA)that could change consistent route selection. This could result   in potential loops.   The BGP protocol provides no way for a Client to identify itself   dynamically as a Client to an RR configured BGP speaker and the   simplest way to achieve this is by manual configuration.9.  Security Considerations   Security issues are not discussed in this memo.Bates & Chandra               Experimental                      [Page 6]

RFC 1966                  BGP Route Reflection                 June 199610. Acknowledgments   The authors would like to thank Dennis Ferguson, Enke Chen, John   Scudder, Paul Traina and Tony Li for the many discussions resulting   in this work. This idea was developed from an earlier discussion   between Tony Li and Dimitri Haskin.11. References   [1]  Rekhter, Y., and T. Li, "A Border Gateway Protocol 4 (BGP-4)",RFC 1771, March 1995.   [2]  Haskin, D., "A BGP/IDRP Route Server alternative to a full mesh        routing",RFC 1863, October 1995.   [3]  Traina, P., "Limited Autonomous System Confederations for BGP",RFC 1965, June 1996.12. Authors' Addresses   Tony Bates   cisco Systems   170 West Tasman Drive   San Jose, CA 95134   Phone: +1 408 527 2470   EMail: tbates@cisco.com   Ravishanker Chandrasekeran   (Ravi Chandra)   cisco Systems   170 West Tasman Drive   San Jose, CA 95134   EMail: rchandra@cisco.comBates & Chandra               Experimental                      [Page 7]