RPF Check Rules for SA Messages

To prevent source active (SA) messages from being cyclically transmitted between MSDP peers, MSDP peers perform a reverse path forwarding (RPF) check on received SA messages and discard any SA messages that fail the check.

RPF check rules for SA messages are as follows:

Rule 1: If an SA message is sent from an MSDP peer that functions as a source rendezvous point (RP) constructing the SA message, the receiving multicast device permits the SA message.
Rule 2: If an SA message is sent from an MSDP peer that is a static RPF peer, the receiving multicast device permits the SA message. A receiving multicast device can set up MSDP peer relationships with multiple other multicast devices. You can specify one or more MSDP peers as static RPF peers.
Rule 3: If the receiving multicast device has only one MSDP peer, the peer automatically becomes an RPF peer. The receiving multicast device permits SA messages sent from this peer.
Rule 4: If an SA message is sent from an MSDP peer that is in the same mesh group as the receiving multicast device, the receiving multicast device permits the SA message. The receiving multicast device does not forward the SA message to MSDP peers in the mesh group but forwards it to all MSDP peers outside the mesh group.
Rule 5: If an SA message is sent from an MSDP peer that is a route advertiser or the next hop of a source RP, the receiving multicast device permits the SA message. If a network has multiple equal-cost routes to a source RP, the receiving multicast device permits SA messages sent from all MSDP peers on the equal-cost routes.
Rule 6: If a network has inter-AS routes to a source RP, the receiving multicast device permits SA messages sent from MSDP peers whose AS numbers are recorded in the AS-path.

If an SA message matches any of rules 1 to 4, the receiving multicast device permits the SA message. The application of rules 5 and 6 depends on route types.

If a route to a source RP is a BGP or a Multicast Border Gateway Protocol (MBGP) route:
- If an MSDP peer is an External Border Gateway Protocol (EBGP) or MEBGP peer, rule 6 applies.
- If an MSDP peer is an Internal Border Gateway Protocol (IBGP) or MIBGP peer, rule 5 applies.
- If an MSDP peer is not a BGP or an MBGP peer and the route to the source RP is an inter-AS route, rule 6 applies. Rule 5 applies in other cases.
If a route to a source RP is not a BGP or an MBGP route:
- If IGP or multicast static routes exist, rule 5 applies.
- If no routes exist, the receiving multicast device discards SA messages sent from MSDP peers.

If an RP address is a local address, an RPF check fails.