After MSDP receives an SA message, it forwards the message to all its peers. To prevent SA messages from being circularly forwarded between MSDP peers, MSDP performs RPF check on the received SA message. MSDP strictly controls the inbound of the SA message. It directly discards the SA message that does not comply with the RPF rules.
MSDP has the following RPF rules:


1.If the peer that sends the SA message is the sources RP (the RP that constructs the SA message), the SA message is accepted and forwarded to other peers.


2.The SA message sent by a static RPF peer is accepted. A device can set up MSDP peer relationship with multiple devices at the same time. Users can select one or multiple peers from these remote peers and set it (or them) as a static RPF peer (or RPF peers).


3.If a device has only one remote MSDP peer, the remote peer automatically becomes the RPF peer. The device accepts the SA message sent by this remote peer. The PIM-SM domain that has only one remote MSDP peer outside the PIM-SM domain is called a Stub domain.


4.If the peer that sends the SA message and the local device belong to the same mesh group, the local device accepts the SA message. The SA message from the mesh group is not forwarded to the members of the mesh group but to all peers that do not belong to the mesh group.


5.If the peer that sends the SA message is the next-hop "route" to the sources RP, the SA message is accepted and forwarded to other peers. The "route" includes the MBGP route, multicast static route, and unicast route (BGP or IGP).


6.If the route that reaches the sources RP spans multiple ASs, the SA message sent by the peer of the next-hop AS is accepted. If the next-hop AS has multiple remote MSDP peers, the SA message sent by the remote peer with the highest IP address is accepted.

informative doc.

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helpful post