MSDP provides a mechanism that allows RPs to share information about active sources. When RPs in remote domains learn about the active sources, they pass on that information to the local receivers and multicast data is forwarded between the domains. MSDP allows each domain to maintain an independent RP that does not rely on other domains but enables RPs to forward traffic between domains. PIM-SM is used to forward the traffic between the multicast domains.
Using PIM-SM, multicast sources and receivers register themselves with their local RP and this registration is performed by the closest multicast router to which the source and receiver are connected. The RP maintains information about the sources and receivers for a specific group. RPs in other domains do not have any knowledge about sources located in other domains.
MSDP is required to provide inter-domain multicast services using Any Source Multicast (ASM). Anycast RP for MSDP enables fast convergence when should an MSDP/PIM PR router fail by allowing receivers and sources to rendezvous at the closest RP.