All things related to multicast

[David Meyer <>]

        Folks, I've put a document on 

        ftp://ftpeng.cisco.com/ipmulticast/msdp-and-logical-rp.txt

        (included below) that describes some of the issues with
        logical RP and MSDP. Please let me know if you have any
        questions or comments.

        Dave

-------

#
#     msdp-and-logical-rp.txt
#
#
#       David Meyer
#       

#       Tue Jul  6 14:50:38 1999
#
#       $Header: /ftp/ipmulticast/RCS/msdp-and-logical-rp.txt,v 1.1 
1999/07/06 21:52:22 dmm Exp $ 
#


      Consider the following scenario:


      R1, R2, and R3 form a logical RP for an AS. C1 and C2 are 
      customer routers (and the BGP session not multi-hop); RP is C1's
      RP. P1 is a peer router.  


           MSDP/MBGP        Logical RPs
      C1 ============= R1----------------R2 
       |  SA(S,G,RP)    \                /  
       |                 \              /
       |  MBGP/MSDP       \ MSDP  MSDP /
       |                   \          /
       |                    \        /
      C2 SA(S',G',RP')       \      /
                              \    /
                                R3 ============== P1
                                     SA(S,G,RP)
                                      ------->
          

      Internal MSDP Peering
      =====================
      When R1 sees SA(S,G,RP) from C1, it sees the next-hop toward
      prefix covering RP is through C1, so R1 accepts the SA. R1 will
      forward SA(S,G,RP) to R2 and R3, which will only accept
      SA(S,G,RP) if R1 is announcing  (and is) the next-hop towards
      the originating RP each SA message. Note that operationally,
      this means next-hop needs to be the same as the MSDP
      connect-source.

      This implies that if you want to pass on an SA internally,
      you have to be announcing the next-hop towards the AS that
      originates the prefix covering the originating RP. Note that
      the MSDP connect-source has to be the interface that is
      configured with the address of the next-hop.
      
      Now, if C2 tries MSDP peer with R1 directly (cutting out
      transit provider C1), then C2's SA RPF fails at R1, because
      R1 expects SA message to come from a MSDP peer in the next
      AS in the AS-PATH towards the originating RP, which in this
      case would C1. 
      
      RULE: An internal MSDP peer will accept an SA message from
            another internal peer iff that peer is the next-hop
            towards the prefix covering the RP which originated
            the SA.  


      External MSDP Peering
      =====================
      External peer P1 will accept an SA from R3 iff R3 comes from the 
      next AS in the path. This breaks, for example, if P1 
      peers with C1.

      RULE: An external MSDP peer will accept an SA message from
            another peer iff the peer is in the next AS in the path
            towards the AS originating the prefix covering the
            RP in the SA message.


      Mesh Groups
      ===========
      First, it is important to note that mesh-groups don't change the
      above rules. What mesh-groups avoid is the RPF check if one of
      the members of the mesh-group originates the SA.