Reverse Path Forwarding (RPF)

Reverse Path Forwarding (RPF)
RPF works in tandem with the routing protocols, but it is described briefly here. As you have
seen in Figures 19.13 and 19.14, the traffic goes only to the multicast group receivers. We also
indicated that bidirectional distribution eliminates the need to forward data upstream. You
might ask, “How do you define upstream?” It is easy to clarify. By means of the routing protocols,
routers are aware of which interface leads to the source(s) of the multicast group. That
interface is considered upstream.
The Reverse Path Forwarding process is based on the upstream information. After
receiving an incoming multicast packet, the router verifies that the packet came in on an
interface that leads back to the source. The router forwards the packet if the verification is
positive; otherwise, the packet is discarded. This check stops potential loops. To avoid
increased overhead on the router’s processor, a multicast forwarding cache is implemented
for the RPF lookups.