Dec 3 2008
RPF Check
To prevent loops in a multicast network, an RPF check is carried out on all multicast traffic.
When a multicast packet enters a router, the router carries out an RPF check on the source address of the packet using the the unicast routing table.
Lets examine the toplogy below.
Now lets suppose the source sends a multicast packet to R1 who then forwards it to R2. R2 checks the source address of the multicast packet against the unicast routing table. If the exit interface in the unicast routing table for that source does not match the interface on which the multicast packet was received the RPF check fails and the multicast packet is dropped.