Multicast Source Discovery Protocol (MSDP) defines a protocol to exchange information about multicast sources across multiple PIM-SM domains. MSDP-speaking routers in a PIM-SM domain have an MSDP peering relationship with the MSDP-speaking peer in another domain. The peering relationship is made up of a TCP connection in which control information is exchanged. Each domain has one or more connections to this virtual topology.
When a PIM-SM rendezvous point (RP) learns about a new multicast source within its own domain from a standard PIM register mechanism, it encapsulates the first data packet in an MSDP source-active (SA) message and sends it to all MSDP peers.
The SA message is flooded (after a reverse path forwarding (RPF) check) by each peer to its MSDP peers until the SA message reaches every MSDP router in the interconnected networks. If the receiving MSDP peer is an RP and the RP has a (*.G) entry (receiver) for the group, the RP creates a state for the source and joins to the shortest path tree for the source. The encapsulated data is de-encapsulated and forwarded down the shared tree of that RP. When the packet is received by the last hop router of the receiver, the last hop router may also join the shortest path tree to the source.
The MSDP speaker periodically sends SA messages that include all sources.