MSDP is a mechanism that allows rendezvous points to share information about active sources. When RPs in remote domains hear about the active sources, they can pass on that information to the local receivers and multicast data can be 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 with their local RP by the closest multicast router. The RP maintains information about the sources and receivers for any particular 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.