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INTERNET-DRAFT                                                Tony Bates
<draft-ietf-idr-route-reflect-v2-01.txt>                    Ravi Chandra
                                                               Enke Chen
                                                           Cisco Systems
                                                              April 1999


                           BGP Route Reflection
                     An alternative to full mesh IBGP
                 <draft-ietf-idr-route-reflect-v2-01.txt>



Status of this Memo

   This document is an Internet Draft. Internet Drafts are working
   documents of the Internet Engineering Task Force (IETF), its Areas,
   and its Working Groups. Note that other groups may also distribute
   working documents as Internet Drafts.

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   Please check the I-D abstract listing contained in each Internet
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Abstract

   The Border Gateway Protocol [1] is an inter-autonomous system routing
   protocol designed for TCP/IP internets. Currently in the Internet 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.








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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.  For n BGP speakers within an AS that
   requires to maintain n*(n-1)/2 unique IBGP sessions.  This "full
   mesh" requirement clearly does not scale when there are a large
   number of IBGP speakers each exchanging a large volume of routing
   information, as is common in many of todays internet networks.

   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". This approach allows a BGP
   speaker (known as "Route Reflector") to advertise IBGP learned routes
   to certain IBGP peers.  It represents a change in the commonly
   understood concept of IBGP, and the addition of two new optional
   transitive BGP attributes to prevent loops in routing updates.


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 Transition

             It must be possible to transition 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




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   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 IBGP

   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 advertise IBGP
   learned routes to IBGP peers, 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




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   The Route Reflection scheme is based upon this basic principle.


4.  Terminology and Concepts

   We use the term "Route Reflection" to describe the operation of a BGP
   speaker advertising an IBGP learned route to another IBGP peer.  Such
   a BGP speaker is said to be a "Route Reflector" (RR), and such a
   route is said to be a reflected route.

   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, and may reflect routes
   among client peers.  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.  Figure 3 depicts a simple example outlining the
   basic RR components using the terminology noted above.

                      / - - - - - - - - - - - - -  -
                      |           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 Components




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5. Operation

   When a RR receives a route from an IBGP peer, 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 IBGP peer

              Reflect to all the Clients.

           2) A Route from a Client peer

              Reflect to all the Non-Client peers and also to the
              Client peers. (Hence the Client peers are not required
              to be fully meshed.)


   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 possible 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
   could 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 can be
   configured with a 4-byte CLUSTER_ID so that an RR can discard routes



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   from other RRs in the same cluster.


7.  Avoiding Routing Information Loops

   When a route is 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
   in reflecting a route.  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 router
   should ignore a route received with its ROUTER_ID as the
   ORIGINATOR_ID.

   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, it must prepend 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 received will be ignored.


8. Implementation 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



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   Non-Clients. Their modification could potential result in routing
   loops.

   In addition, when a RR reflects a route, it should not modify the
   following path attributes: NEXT_HOP, AS_PATH, LOCAL_PREF, and MED.
   Their modification could potential result in routing loops.


9. Configuration and Deployment Considerations

   The BGP protocol provides no way for a Client to identify itself
   dynamically as a Client of an RR.  The simplest way to achieve this
   is by manual configuration.

   One of the key component of the route reflection approach in
   addressing the scaling issue is that the RR summarizes routing
   information and only reflects its best path.

   Both MEDs and IGP metrics may impact the BGP route selection.
   Because MEDs are not always comparable and the IGP metric may differ
   for each router, with certain route reflection topologies the route
   reflection approach may not yield the same route selection result as
   that of the full IBGP mesh approach. A way to make route selection
   the same as it would be with the full IBGP mesh approach is to make
   sure that route reflectors are never forced to perform the BGP route
   selection based on IGP metrics which are significantly different from
   the IGP metrics of their clients, or based on incomparable MEDs. The
   former can be achieved by configuring the intra-cluster IGP metrics
   to be better than the inter-cluster IGP metrics, and maintaining full
   mesh within the cluster. The latter can be achieved by:

          o setting the local preference of a route at the border router
            to reflect the MED values.

          o or by making sure the AS-path lengths from different ASs are
            different when the AS-path length is used as a route
            selection criteria.

          o or by configuring community based policies using which the
            reflector can decide on the best route.

   One could argue though that the latter requirement is overly
   restrictive, and perhaps impractical in some cases.  One could
   further argue that as long as there are no routing loops, there are
   no compelling reasons to force route selection with route reflectors
   to be the same as it would be with the full IBGP mesh approach.

   To prevent routing loops and maintain consistent routing view, it is



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   essential that the network topology be carefully considered in
   designing a route reflection topology. In general, the route
   reflection topology should congruent with the network topology when
   there exist multiple paths for a prefix. One commonly used approach
   is the POP-based reflection, in which each POP maintains its own
   route reflectors serving clients in the POP, and all route reflectors
   are fully meshed. In addition, clients of the reflectors in each POP
   are often fully meshed for the purpose of optimal intra-POP routing,
   and the intra-POP IGP metrics are configured to be better than the
   inter-POP IGP metrics.


10.  Security

   This extension to BGP does not change the underlying security issues
   inherent in the existing IBGP.


11. Acknowledgments

   The authors would like to thank Dennis Ferguson, 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.

   In addition, the authors would like to acknowledge valuable review
   and suggestions from Yakov Rekhter on this document, and helpful
   comments from Tony Li, Rohit Dube, and John Scudder on Section 9.


12. References


   [1]  Rekhter, Y., and Li, T., "A Border Gateway Protocol 4 (BGP-4)",
        RFC1771, March 1995.

   [2]  Haskin, D., "A BGP/IDRP Route Server alternative to a full mesh
        routing", RFC1863, October 1995.

   [3]  Traina, P. "Limited Autonomous System Confederations for BGP",
        RFC1965, June 1996.










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13. Author's Addresses

        Tony Bates
        Cisco Systems
        170 West Tasman Drive

        email: tbates@cisco.com


        Ravishanker Chandrasekeran
        (Ravi Chandra)
        Cisco Systems
        170 West Tasman Drive
        San Jose, CA 95134

        email: rchandra@cisco.com


        Enke Chen
        Cisco Systems
        170 West Tasman Drive
        San Jose, CA 95134

        email: enkechen@cisco.com



























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