Default MSDP peer
Sometimes, BGP (or MBGP) is not run between AS or within an AS, and if MSDP is required to join multiple PIM domains, MSDP can be set up using default peer(s). If a single MSDP peer is configured, a router will accept all SA messages advertised to it from that peer.
Consider the following scenario-
RouterA ------ RouterB ----- RouterC
RouterB has a connection each to RouterA and RouterC. They do not run BGP between each other. However, RouterB requires MSDP sessions to be set up with RouterA and RouterC. So, the dependency on BGP can be disabled using ip msdp default-peer command. Peer-RPF checks are not done since there is only one path.
By default, the first peer configured on the router (considering the session is UP) will become default-peer and so SA messages are only accepted from this default-peer; SA messages are not not accepted from other peers unless the first peer fails. If the first peer fails, SA messages are accepted from the other peer.
To cause the RP to accept SA messages from multiple peers whilst avoiding the dependency on BGP, prefix-lists are used. The peer then accepts SA messages from all the peers allowed by the prefix-list.
To configure this, use
ip msdp default-peer <peer-ip-address> prefix-list <prefix-list-name>