How is MSDP used to connect multiple PIM-SM domains?

MSDP (Multicast Source Discovery Protocol) is designed to connect multiple PIM-SM (Protocol Independent Multicast - Sparse Mode) domains by enabling Rendezvous Points (RPs) in different domains to discover active multicast sources. The correct answer states that MSDP allows an RP to dynamically discover active sources outside its domain. This is achieved through Source-Active (SA) messages, which RPs send to their MSDP peers to advertise active sources within their own PIM-SM domain. Other RPs receiving these SA messages learn about sources in other domains. Incorrect options: "MSDP SA request messages are used to request a list of active sources for a specific group." While there are SA messages, the primary mechanism for discovery is advertising, not requesting a list for a specific group. "MSDP depends on BGP or multiprotocol BGP for interdomain operation." MSDP peers are typically established using TCP, and while BGP can be used to exchange reachability information for MSDP peers, MSDP itself does not depend on BGP for its core interdomain source discovery function. "MSDP messages are used to advertise active sources in a domain." This is partially true, as SA messages advertise sources. However, the key function in the context of connecting multiple domains is the discovery by RPs in other domains, which the correct answer highlights.