MBONED Working Group                            Mike McBride
Internet Draft                                  John Meylor
                                                Cisco Systems
                                                David Meyer
Category                                        Best Current Practice
draft-ietf-mboned-msdp-deploy-00.txt            February, 2002

        Multicast Source Discovery Protocol Deployment Scenarios


1. Status of this Memo

   This document is an Internet-Draft and is in full conformance with
   all provisions of Section 10 of RFC 2026.

   Internet-Drafts are working documents of the Internet Engineering
   Task Force (IETF), its areas, and its working groups.  Note that
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   and may be updated, replaced, or obsoleted by other documents at any
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   The list of current Internet-Drafts can be accessed at

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2. Abstract

   This document describes best current practices for intra-domain and
   inter-domain MSDP deployment.

3. Copyright Notice

   Copyright (C) The Internet Society (2002).  All Rights Reserved.

4. Introduction

   The Multicast Source Discovery Protocol [MSDP] is a mechanism to
   connect multiple PIM-SM [RFC2117] domains together. Each PIM-SM
   domain uses its own independent Rendezvous Point, or RP, and does not
   have to depend  on RPs in other domains. Current best practice for
   MSDP deployment utilizes Protocol Independent Multicast (Sparse Mode)
   and the Border Gateway Protocol With multi-protocol extensions
   [RFC2858,NICKLESS]. This document outlines how these protocols work
   together to provide Intra-domain and Inter-domain Any Source
   multicast (ASM) service. In addition, this document describes how
   MSDP can provide a PIM-SM domain with RP redundancy and load
   balancing using the Anycast RP mechanism [ANYCAST-RP].

5. Inter-domain MSDP peering scenarios

   The following sections describe the different inter-domain MSDP
   peering possibilities and their deployment options.

5.1. Peering between PIM border routers (Single hop peering)

   In this case, the MSDP peers within the domain each have their own RP
   located within a bounded PIM domain. In addition, a domain has it's
   own Autonomous Number (AS) and BGP speakers. The domain may also have
   multiple MSDP speakers. Each router has an MSDP and BGP peering with
   its peer routers. These deployments typically configure the BGP
   peering and MSDP peering using the same directly connected next hop
   peer IP address or another IP address from the same router. Typical
   deployments of this type are providers who have a direct peering with
   other providers or with providers who use their edge router to
   MSDP/MBGP peer with customers.

   For a direct peering inter-domain environment to be successful, the

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   first AS in the BGP best path to the originating RP must be the same
   as the AS of the MSDP peer [MSDP]. As an example, consider the
   following  topology:

                |    /
                |   /
                |  /

   In this case, AS4 receives an Source Active SA Message (SA),
   originated by AS1 via AS2, which also has an BGP peering with AS4.
   The BGP first hop AS from AS4, in the best path to the originating
   RP, is AS2. The origin AS of the sending MSDP peer is also AS2. The
   peer-RPF (Reverse Path Forwarding) check passes and the SA message is

   A peer-RPF failure will occur in this topology when the BGP first-hop
   AS in the best path to the originating RP is AS2 while the origin AS
   of the sending MSDP peer is AS3. An MSDP peering between AS2 and AS4
   would prevent this failure from occurring.

5.2. Peering between non border routers (Multi-hop peering)

   While the eBGP peer is typically directly connected between border
   routers, it is common for the MSDP peer to be located deeper into the
   transit providers AS. However, MSDP scalability is sacrificed if a
   provider must maintain BGP and MSDP peerings with all their edge
   routers so that they can BGP and MSDP peer with customer routers.
   Alternatively, providers commonly choose a few dedicated routers
   within their core network for the inter-domain MSDP peerings to their
   customers. These core MSDP routers will also typically be in the
   providers intra-domain MSDP mesh [MSDP] group and configured for
   Anycast RP. All multicast routers in the providers AS should
   statically point to the Anycast RP address. AutoRP and BSR mechanisms
   could be used to disseminate RP information within the provider's

   For an SA message to be accepted in this (multi-hop peering)
   environment, the MSDP peer address must be in the same AS as the AS
   of the MBGP peer and must be advertised via MBGP. For example, using
   the diagram below, if customer R1 router is MBGP peering with AS2
   provider's R2 router and if R1 is MSDP peering with R3 router, then
   R2 and R3 must be in the same AS. R1 also must have the MSDP peer

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   address of R3 in its BGP table.

        +--+    +--+    +--+
        +--+    +--+    +--+
         AS1     AS2     AS2

5.3. MSDP peering without BGP

   In this case, an enterprise maintains its own RP and has an MSDP
   peering with their service provider, but does not BGP peer with them.
   MSDP relies upon BGP path information to learn the MSDP topology for
   the SA peer-RPF check. MSDP can be deployed without BGP, however, and
   as a result there are some special cases where the requirement to
   perform an peer-RPF check on the BGP path information is suspended.
   In this case (when there is only a single MSDP peer connection) a
   default peer (default MSDP route) is configured and either the
   originating RP is directly connected or a mesh group is used. An
   enterprise will also typically configure a unicast default route from
   their border router to the provider's border router and then MSDP
   peer with the provider's border router. If internal MSDP peerings are
   also used within the enterprise, then an MSDP default peer will need
   to be configured on the border router pointing to the provider. In
   this way, all external multicast sources will be learned and internal
   sources can be advertised.

5.4. MSDP peering between mesh groups

   Mesh groups which are within different PIM domains can MSDP peer with
   one another to exchange information about active sources. An RP
   within AS1's mesh group may MSDP peer with an RP which is within
   AS2's mesh group. However, there should be no mesh group in common
   between PIM domains. It is important to note however, that mesh
   groups that span PIM domains is not recommended, as SA forwarding
   loops can develop. As an example, consider the following topology:

                / \
               /   \
              /     \
             /       \
            /         \
         AS1 -------- AS3

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   If each AS had their own intra-domain MSDP mesh group, and if there
   was an inter-domain MSDP mesh group between AS1-AS2, AS1-AS3, and
   AS2-AS3 then an SA loop would be created. Since there is no RPF check
   between mesh groups, the SAs would loop around from one PIM domain to

5.5. MSDP peering at a Multicast Exchange

   Multicast exchanges allow multicast providers to peer at a common IP
   subnet and share MSDP SA updates. Each provider will MSDP and BGP
   peer with each others directly connected exchange IP address. Each
   exchange router will send/receive SAs over the exchange fabric. They
   will then be able to forward SAs throughout their domain to their
   customers and any direct provider peerings.

6. Intra-domain MSDP peering scenarios

   The following sections describe the different intra-domain MSDP
   peering possibilities and their deployment options.

6.1. Peering between routers configured for both MSDP and MBGP

   The next hop IP address of the iBGP peer (that is MSDP is advertising
   as the next hop toward the originating RP) is used for the peer-RPF
   check. This is different from the inter-domain BGP/MSDP case, where
   AS path information is used for the peer-RPF check. For this reason,
   it is necessary  for the IP address of the MSDP peer connection be
   the same as the internal BGP peer connection whether or not the
   MSDP/MBGP peers are directly connected. A successful deployment would
   be similar to the following:

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                                | Rb |
                              / +----+
         AS1          AS2    /     |
        +---+         +--+  /      |
        |RP1|---------|Ra|         |
        +---+         +--+         |        |
                            \      |
                             \     |
                              \ +-----+
                                | RP2 |

   Where RP2 MSDP and MBGP peers with Ra using and with Rb using When the MSDP SA update arrives on RP2 from Ra, the MSDP RPF
   check for passes because RP2 receives the SA update from which is the correct BGP next hop for

   When RP2 receives the same SA update from MSDP peer, the BGP
   lookup for shows a next hop of so RPF correctly
   fails, preventing a loop.

   This deployment would also fail on an update from Ra to RP2 if RP2
   was BGP peering to an address other than on Ra. Intra-domain
   deployments should have MSDP and MBGP peering addresses which match.

6.2. MSDP peer is not BGP peer (or no BGP peer)

   This is a common MSDP intra-domain deployment in environments where
   few routers are running BGP or where the domain is not running BGP.
   The problem here is that the MSDP peer address needs to be the same
   as the BGP peer address. To get around this requirement, the intra-
   domain MSDP RPF rules have been relaxed in certain as follows:

     o By configuring the MSDP peer as a mesh group peer,
     o By having the MSDP peer be the only MSDP peer,
     o By configuring a default MSDP peer, or
     o By peering with the originating RP.

   The common choice around the intra-domain BGP peering requirement,
   when more than one MSDP peer is configured, is to deploy MSDP mesh
   groups. When a MSDP mesh group is deployed, there is no RPF check on
   arriving SA messages when received from a mesh group peer.
   Subsequently, SA messages are always accepted from mesh group peers.

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   MSDP mesh groups are helpful in reducing the amount of SA traffic in
   the network since SAs are not flooded to other mesh group peers.

7. MSDP and Anycast RPs

   A network with can achieve RP load sharing and redundancy by using
   the Anycast RP mechanism in conjunction with MSDP mesh groups
   [ANYCAST-RP]. This mechanism is a common deployment technique used by
   service providers, who commonly deploy several RPs within their
   domain. These RPs will all have the same IP address configured on a
   Loopback interface (making this the anycast addresses). These RPs
   will MSDP peer with each other using a separate loopback interface
   and are part of the same MSDP mesh group. This second Loopback
   interface will typically also be used for the MBGP peering. All
   routers within the provider's domain will learn of the Anycast RP
   address either through AutoRP, BSR, or a static RP assignment. Each
   designated router in the domain will send source registers and group
   joins to the Anycast RP address. Unicast routing will direct those
   registers and joins to the nearest Anycast RP. If a particular
   Anycast RP router fails, unicast routing will direct subsequent
   registers and joins to the nearest Anycast RP. That RP will then
   forward an MSDP update to all peers within the global MSDP mesh
   group. Each RP will then forward (or receive) the SAs to (from)
   external customers and providers.

7.1. Hierarchical Mesh Groups

   Hierarchial Mesh Groups are typically deployed in intra-domain
   environments where there are a large number of MSDP peers. Allowing
   multiple mesh groups to forward to one another can reduce the number
   of MSDP peerings per router and hence reduce router load. A good
   hierarchical mesh group implementation (one which prevents looping)
   contains a core mesh group in the backbone and these core routers
   serve as mesh group aggregation routers:

                       /  \
                      /    \
                     /      \
                    /        \
                   /          \
                  /            \
                 /              \

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   In this example, R1, R2, R3 are in MSDP mesh group A (the core mesh
   group) and each serves as MSDP aggregation routers for their mesh
   groups 1, 2, and 3. Since SA messages received from a mesh group peer
   are not forwarded to peers within that same mesh group, SA messages
   will not loop. In particular, do not create topologies which connect
   mesh-groups in a loop. In the above example for instance, "second
   tier" mesh-groups 1, 2, and 3 must not directly exchange SA message.

7.2. MSDP and Route Reflectors

   BGP requires all iBGP speakers that are not route-reflector clients
   or confederation members be fully meshed. This requirement does not
   scale when there are large number of iBGP speakers.  In the route-
   reflector environment, MSDP requires that the route reflector clients
   peer with the route reflector. For example, consider the following

                / | \
               A  B  C

   Ra is forwarding MSDP SAs to the route reflector RR. Routers A, B,
   and C also MSDP peer with RR. When RR forwards the SA to A, B, and C,
   these RR clients  will accept the SA because RR is the iBGP next hop
   for the originating  RP address.

   An SA will peer-RPF fail if Ra MSDP peers directly with Routers A, B,
   and C because the iBGP next hop for RR's clients is RR, but the SA
   update came from Ra. Proper deployment is to have RR's clients MSDP
   peer with RR.

7.3. MSDP Filtering

   Typically there is a fair amount of (S,G) state in a PIM-SM domain
   that is local to the domain. However, without proper filtering, SA-
   messages containing these local (S,G) announcements may be advertised
   to the global MSDP infrastructure. Examples of this includes domain
   local applications that use global IP multicast addresses and sources
   that use RFC 1918 addresses [RFC1918]. To improve on the scalability
   of MSDP and to avoid global visibility of domain local (S,G)
   information, the following external SA filter list is recommended to
   help prevent unnecessary creation, forwarding, and caching of some of

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   these well-known ³domain local³ sources [IANA].      Local application packets
         (packets from any application which are intended to stay
          adminstratively scoped, but use global addressing. The
          current list of applications which could be filtered
          is dynamic and subject to individual policy.  See WG
          mail group for latest recommendations)       AutoRP Announce       AutoRP Discovery      Admin. Scoped       private addresses [RFC1918]      private addresses [RFC1918]    private addresses [RFC1918]   private addresses [RFC1918]      Default SSM-range

8. Author's Addresses

   Mike McBride
   Cisco Systems

   John Meylor
   Cisco Systems

   David Meyer
   Email: dmm@sprint.net

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   [ANYCAST-RP] D. Meyer et. al, "Anycast RP mechanism using PIM and
                MSDP", draft-ietf-mboned-anycast-rp-08.txt, May, 2001.

   [NICKLESS]  Bill Nickless, "IPv4 Multicast Best Current Practice",
               draft-nickless-ipv4-mcast-bcp-01.txt, February 2002.

   [IANA]       http://www.iana.org

   [MSDP]       D. Meyer and Bill Fenner (Editors), "The Multicast
                Source Discovery Protocol (MSDP)", draft-ietf-msdp-spec-13.txt,
                November 2001.

   [RFC1771]    Rekhter, Y., and T. Li, "A Border Gateway Protocol 4
                (BGP-4)", RFC 1771, March 1995.

   [RFC1918]    Y. Rekhter, R. Moskowitz, D. Karrenberg, G. de Groot, E. Lear,
                "Address Allocation for Private Internets", 02/29/1996.

   [RFC2117]    D. Estrin et. al,  "Protocol Independent Multicast-Sparse
                Mode (PIM-SM): Protocol Specification",  RFC 2117,
                June, 1997.

   [RFC2362]    D. Estrin, et. al., "Protocol Independent Multicast -
                Sparse Mode (PIM-SM): Protocol Specification", RFC 2362,
                June, 1998.

   [RFC2858]    T. Bates, Y. Rekhter, R. Chandra, D. Katz, "Multiprotocol
                Extensions for BGP-4", RFC 2858,  June 2000.

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10. Full Copyright Statement

   Copyright (C) The Internet Society (2002).  All Rights Reserved.

   This document and translations of it may be copied and furnished to
   others, and derivative works that comment on or otherwise explain it
   or assist in its implementation may be prepared, copied, published
   and distributed, in whole or in part, without restriction of any
   kind, provided that the above copyright notice and this paragraph are
   included on all such copies and derivative works.  However, this
   document itself may not be modified in any way, such as by removing
   the copyright notice or references to the Internet Society or other
   Internet organizations, except as needed for the purpose of develop-
   ing Internet standards in which case the procedures for copyrights
   defined in the Internet Standards process must be followed, or as
   required to translate it into languages other than English.

   The limited permissions granted above are perpetual and will not be
   revoked by the Internet Society or its successors or assigns.

   This document and the information contained herein is provided on an

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