When an RP in a PIM-SM domain first learns of a new sender, for example, by PIM register messages, it constructs an SA message and sends it to its MSDP peers. The SA message contains the following fields:
source address of the data source
group address the data source sends to
IP address of the RP
An RP that is not a designated router on a shared network does not originate SA messages for directly-connected sources on that shared network. It only originates SA messages in response to a register message received from the designated router.
Each MSDP peer receives and forwards the message away from the RP address in a peer-RPF flooding fashion. The notion of peer-RPF flooding is with respect to forwarding SA messages. The Multicast RPF Routing Information Base (MRIB) is examined to determine which peer toward the originating RP of the SA message is selected. Such a peer is called an RPF peer.
If the MSDP peer receives the SA message from a non-RPF peer toward the originating RP, it drops the message. Otherwise, it forwards the message to all its MSDP peers (except the peer from which it received the SA message).
When an MSDP peer that is also an RP for its own domain receives a new SA message, it determines if there are any group members in the domain interested in any group described by an (S,G) entry within the SA message. That is, the RP checks for a (*,G) entry with a non-empty outgoing interface list. This implies that a system in the domain is interested in the group. In this case, the RP triggers an (S,G) join event toward the data source as if a join/prune message was received and addressed to the RP. This sets up a branch of the source-tree to this domain. Subsequent data packets arrive at the RP by this tree branch and are forwarded down the shared tree inside the domain. If leaf routers choose to join the source-tree they have the option to do so according to existing PIM-SM conventions. If an RP in a domain receives a PIM join message for a new group G, the RP must trigger an (S,G) join event for each active (S,G) for that group in its SA cache.
This procedure is called flood-and-join because if an RP in the domain is not interested in the group, the SA message can be ignored when there are no receivers or members interested in that domain; otherwise, the RP joins a distribution tree.