MSDP (Multicast Source Discovery Protocol) peers exchange messages to advertise active multicast sources. The peer with the higher IP address passively listens to a well-known port number and waits for the side with the lower IP address to establish a Transmission Control Protocol (TCP) connection on port 639. When a PIM-SM RP that is running MSDP becomes aware of a new local source, it sends an SA message over the TCP connection to its MSDP peer. When the SA message is received, a peer-RPF check is performed to make sure the peer is toward the originating RP. If so, the RPF peer floods the message further. If not, the SA message is dropped and the message is rejected.
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