MSDP is the correct solution for multicast across different autonomous systems when RPs must discover active sources outside their local domain. In PIM Sparse Mode, each multicast domain can have its own rendezvous point. Without a mechanism to exchange source information, receivers in one domain may not know about active sources registered with an RP in another domain. Multicast Source Discovery Protocol solves that problem by allowing RPs to establish peer relationships and exchange Source-Active information.
Cisco multicast design uses MSDP with PIM-SM when separate domains need to share multicast source reachability while relying on the unicast routing table for connectivity between RPs. SSM does not use an RP and depends on receivers joining a specific source and group. PIM-SM by itself works inside a domain but does not provide interdomain source discovery between RPs. PIM-DM is flood-and-prune and is not suitable for this sparse, interdomain design. Therefore, the organization should deploy MSDP between the RP devices or RP domains. Reference topics: MSDP, PIM-SM, interdomain multicast, Source-Active messages, RP-to- RP source discovery.