MSDP allows a domain to receive advertised messages of the availability of multicast sources from other domains and to connect with them.
When a rendezvous point (RP) in the Protocol Independent Multicast Sparse Mode (PIM-SM) domain learns of a new source, it constructs a source-active (SA) message and sends it to its MSDP peers. The MSDP peers use a Reverse Path Forwarding (RPF) check mechanism to find an appropriate MSDP peer to receive the SA message. All RPs, which aim to originate or receive SA messages, must establish MSDP peering with other RPs, either directly or through an intermediate MSDP peer.
When an RP receives the advertisements about the required multicast sources, PIM-SM uses a source-tree building mechanism to deliver multicast data over an interdomain distribution tree.
MSDP uses important communication information that is provided by Border Gateway Protocol (BGP) and uses Transmission Control Protocol (TCP) as its transport protocol.
MSDP is defined by Multicast Source Discovery Protocol (MSDP), RFC 3618 at https://tools.ietf.org/rfc/rfc3618.txt.