MSDP is an interdomain multicast protocol used to connect multiple multicast routing domains that run Protocol Independent Multicast-Sparse Mode (PIM-SM).
MSDP speakers are configured on each PIM-SM domain. These speakers establish a peering relationship with other MSDP speakers through secured TCP connections. When the source sends traffic, the MSDP speaker learns about the source through its Rendezvous Point (RP). In turn, the RP advertises the source to its peers through Source Active (SA) messages. The peers receive these advertisements and inform their RPs about the presence of the active source in the other PIM-SM domain, which triggers the normal PIM operation in the corresponding domains.
For example, as businesses expand and networks grow in size, it might become necessary to connect PIM domains to allow multicast applications to reach other offices across the network. MSDP simplifies this process by providing a mechanism to connect those multicast routing domains without reconfiguring existing domains. Each PIM domain remains separate and has its own RP. The RP in each domain establishes an MSDP peering relationship over a TCP connection either with RPs in other domains or with border routers leading to other domains. When an RP learns about a new multicast source in its own domain (using the normal PIM registration process), it then sends a SA message to all of its MSDP peers, letting them know about the new stream. In this way, the network can receive multicast traffic from all over the network without having to reconfigure each existing PIM domain.