RPF (Reverse Path Forwarding) is used to check the source address of an incoming packet. With unicast traffic, this can be used as a security check. When the router receives a packet, it checks if the source is known and/or the interface that it received the packet on is used to reach the source. If not, packets can be dropped. In multicast, RPF is used to prevent routing loops. These lessons explain topics where RPF is used in one way or another.