Multicast Forwarding
Reverse Path Forwarding (RPF)
What is RPF?
A router forwards a multicast datagram only if received on the up
stream interface to the source (i.e. it follows the distribution tree).
The RPF Check
The source IP address of incoming multicast packets are
checked against a unicast routing table.
If the datagram arrived on the interface specified in the
 routing table for the source address; then the RPF check
  succeeds.
Otherwise, the RPF Check fails.