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| 7.3 |  | Routing Multicast Traffic |
| 7.3.4 |  | Reverse Path Forwarding (RPF) |

RPF is a fundamental concept in multicast routing that enables routers to correctly forward multicast traffic down the distribution tree. RPF makes use of the existing unicast routing table to determine the upstream and downstream neighbors. A router will only forward a multicast packet if it is received on the upstream interface. This RPF check helps to guarantee that the distribution tree will be loop free.

RPF Check
When a multicast packet arrives at a router, the router will perform an RPF check on the packet. If the RPF check is successful, the packet will be forwarded. Otherwise it will be dropped.

For traffic flowing down a source tree, the RPF check procedure works as follows:

Step 1 | Router looks up the source address in the unicast routing table to determine if it has arrived on the interface that is on the reverse path back to the source.
Step 2 | If packet has arrived on the interface leading back to the source, the RPF check is successful and the packet will be forwarded.
Step 3 | If the RPF check in 2 fails, the packet is dropped.

Figure shows an example of an unsuccessful RPF check.

A multicast packet from source 151.10.3.21 is received on interface S0. A check of the unicast route table shows that the interface this router would use to forward unicast data to 151.10.3.21 is S1. Since the packet has arrived on S0 the packet will be discarded.

Figure is an example of a successful RPF check.

This time the multicast packet has arrived on S1. The router checks the unicast routing table and finds that S1 is the correct interface.