MSDP is the correct technology when a large multicast design uses multiple PIM Sparse Mode domains and needs to exchange active source information between those domains. Cisco describes Multicast Source Discovery Protocol as the mechanism used by RPs in different PIM-SM domains to learn about multicast sources outside their own domain. When a source becomes active, the local RP can advertise Source-Active information to MSDP peers, allowing receivers in other domains to discover and join that source. This supports domain separation and can help balance multicast control-plane scope in large networks. DVMRP and MOSPF are legacy multicast routing approaches and are not the design mechanism for modern PIM-SM domain interconnection. IGMP is used between hosts and local multicast routers for receiver membership signaling; it does not exchange source information between multicast domains. Therefore, a design with more than 100 routers, several multicast domains, and PIM-SM traffic balancing should include MSDP. The architect should also design RP placement, Anycast RP behavior, peer mesh scaling, and source filtering. Reference topics: MSDP, PIM-SM domains, Source-Active messages, interdomain multicast.