Reverse path forwarding
PIM makes use of unicast routing to determine neighbor routers and create multicast distribution trees.
The PIM router compares the ingress interface for each multicast packet with the unicast route to source. If the ingress interface is the correct RPF interface the packet is forwarded on the best matching multicast distribution tree. If the RPF check fails the packet is dropped. It is useful to note that PIM assumes unicast routes are symmetrical.