Peer Reverse Path Forwarding flooding

When the MSDP device (also the RP) in domain 2 receives the Source Active message from the peer in domain 1, the MSDP device in domain 2 forwards the message to all other peers. This propagation process is sometimes called "peer Reverse Path Forwarding (RPF) flooding". In Figure 7, the MSDP device floods the Source Active message it receives from the peer in domain 1 to peers in domains 3 and 4.

The MSDP device in domain 2 does not forward the Source Active back to the peer in domain 1, because that is the peer from which the device received the message. An MSDP device never sends a Source Active message back to the peer that sent it. The peer that sent the message is sometimes called the "RPF peer". The MSDP device uses the unicast routing table for its Exterior Gateway Protocol (EGP) to identify the RPF peer by looking for the route entry that is the next hop toward the source. Often, the EGP protocol is Border Gateway Protocol (BGP) version 4.

The MSDP routers in domains 3 and 4 also forward the Source Active message to all peers except the ones that sent them the message. Figure 7 does not show additional peers.