Two routers are configured as Anycast RPs sharing the same loopback address for redundancy. To ensure sources learned by one RP are advertised to the other RP so receivers can form SPTs, which additional protocol should be used between the RPs?

MSDP (Multicast Source Discovery Protocol) is essential here because it allows Anycast RPs to share information about active multicast sources. When a source registers with one RP, MSDP propagates that source's (S,G) information to other RPs in the Anycast RP set. This ensures that even if a receiver initially joins a group through a different RP, it can still learn about the source and build an SPT (Shortest Path Tree) directly to it. HSRP is a first-hop redundancy protocol, not for RP synchronization. Auto-RP is a discovery mechanism for RPs, not a synchronization protocol. IGMP is used by hosts to signal multicast group membership to their local router.