MSDP-speaking routers in a PIM-SM domain have MSDP peering relationship with MSDP peers 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 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 message and sends it to all MSDP peers.
The source-active message is flooded (after an RPF check) by each peer to its MSDP peers until the source-active 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 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 also may join the shortest path tree to the source.
The MSDP speaker periodically sends source-active messages that include all sources.