In an interdomain multicast deployment scenario, RP1 is in AS1 and RP2 is in AS2. MSDP is
configured between RP1 and RP2 A source in AS1 and a receiver in AS2 have just become
active. What initially triggers RP1 to send source-active messages (SAs) to RP2?
A join-to-RP message is sent from RP2 to RP1.
A join-to-source message is sent from RP2 to RP1
A register message is received on RP1.
A register message is received on RP2.