Share source information between rendezvous points
An IPv4 PIM sparse-mode RP can learn local sources through PIM Register messages. MSDP lets it announce a source and group to peers using a Source-Active (SA) message over TCP port 639. The message includes the originating RP address, which peers can use for a peer-RPF check when deciding whether to accept and relay it.
A remote RP with receiver interest can use an accepted SA to join toward the source with PIM. MSDP distributes source discovery information. The multicast forwarding tree still carries the stream. Source-specific multicast receivers already identify the source and do not require MSDP discovery.
Try it in a lab
- On a lab platform supporting MSDP, configure two IPv4 PIM sparse-mode domains, each with its own RP. Peer the RPs over MSDP and allow multicast routing between the domains.
- Start an any-source multicast sender in the first domain and join its group from the second domain. Verify that the second RP learns the Source-Active record and that PIM builds state toward the source.
- Remove the receiver and confirm that the receiver branch of the forwarding tree is pruned. Source discovery can continue while the sender remains active.