What is an RPF check?
Multicast routing adds a whole new complexity to normal unicast routing. The router doesn’t just have to worry about which interfaces it must transmit the packets out on. It also has to consider the source IP of the packet and which interface it arrives on to prevent loops. It does this by performing an RPF, or Reverse Path Forwarding check. Normally this means looking up the source IP in the router’s unicast routing table and seeing if it has a route to the source going out the interface that the packet came in on. If not then the packet didn’t arrive on the most efficient path between source and the router and should be discarded. There are other criteria that RPF checks can be based on, but this gets complicated quickly.
