To prevent Source Active (SA) messages from being circularly forwarded between MSDP peers, MSDP performs the Reverse Path Forwarding (RPF) check on the received SA messages. MSDP strictly controls the inbound of the SA message. MSDP directly discards an SA message that does not comply with the RPF rules.
MSDP has the following RPF rules:
Rule 1: If the peer that sends the SA message is the source Rendezvous Point (RP), the SA message is received and forwarded to other peers.
Rule 2: The SA message sent by the static RPF peer is received. A device can set up an MSDP peer relationship with multiple devices. Users can select one or multiple peers from these remote peers and set it as the static RPF peer.
Rule 3: If a device has only one remote MSDP peer, the remote peer automatically becomes the RPF peer. The device receives the SA message sent by the remote peer. The PIM-SM domain that has only one remote MSDP peer outside the PIM-SM domain is called STUB domain.
Rule 4: If the peer that sends the SA message and the local device belong to the same mesh group, the local device receives the SA message. The SA messages from the mesh group are not forwarded to the members of the mesh group, but to all the peers that do not belong to the mesh group.
Rule 5: If the peer that sends the SA message is the next hop of the route to the source RP or a route forwarder, the local device receives SA messages and forwards them to other peers. The route types can be BGP routes, static multicast routes, and Interior Gateway Protocol (IGP) routes.
Rule 6: If the route that reaches the source RP spans multiple autonomous systems (ASs), only the SA message received from the peer whose AS number is in the AS-path is accepted.