Reverse Path Forwarding
Multicast routers identify upstream ports (pointing to the server or RP) and
downstream ports (pointing to other receivers) for each multicast group. The
upstream port is found using Reverse Path Forwarding (RPF). RPF involves
looking at the routing table to see which interface the router uses to send
unicast traffic to that server or RP. That interface is the upstream port, or
RPF port, for the multicast group. The RFP check is done every five
seconds. It is used in this way:
�¡ If a multicast packet arrives on the RPF port, the router forwards the
packet out to the interfaces listed in the outgoing interface list of a
multicast routing table.
�¡ If the packet does not arrive on the RPF port, the packet is discarded
to prevent loops.