Refer to the exhibit.

MSDP is required because the design connects separate PIM-SM multicast domains and needs multicast source discovery across domain boundaries. In each autonomous system, receivers join a multicast group through the local rendezvous point. Without MSDP, an RP in one domain does not automatically learn about active sources registered to an RP in another domain. MSDP allows RPs to exchange Source-Active information so that receivers in AS65773 can learn about VoD sources in AS5373 and build the required multicast forwarding state. Auto-RP and BSR are mechanisms for discovering or advertising rendezvous point information inside a PIM domain; they do not solve interdomain source discovery between separate multicast domains. PIM SSM can work for source-specific applications, but the question states that R2 has been selected as RP, which indicates an ASM/PIM-SM design. With BGP already established between offices, MSDP complements the interdomain routing design by exchanging multicast source reachability information. Reference topics: MSDP, PIM-SM, interdomain multicast, Source-Active messages, rendezvous point resilience.