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== Multicast RPF ==
Multicast RPF, typically denoted simply as RPF, is used in conjunction with a multicast routing protocol such as [[Multicast Source Discovery Protocol|MSDP]], [[Sparse multicast|PIM-SM]] and [[Dense multicast|PIM-DM]] to ensure loop-free forwarding of multicast packets. In multicast routing, the decision to forward traffic is based upon source address and not on destination address as in unicast routing. It does this by utilizing either a dedicated multicast routing table or alternatively the router's unicast routing table.
In IP multicast routing, the router forwards the packet away from the source to make progress along the distribution tree and prevent routing loops. The router's multicast forwarding state runs more logically by organizing tables based on the reverse path, from the receiver back to the root of the distribution tree. This process is known as reverse-path forwarding (RPF).
When a multicast packet enters a router's interface, it will look up the list of networks that are reachable via that interface i.e., it checks the reverse path of the packet. If the router finds a matching routing entry for the source IP address of the multicast packet, the RPF check passes and the packet is forwarded to all other interfaces that are participating in multicast for that multicast group. If the RPF check fails, the packet will be dropped. As a result, the forwarding of the packet is decided based upon the reverse path of the packet rather than the forward path. RPF routers only forward packets that come into the interface that also holds the routing entry for the source of the packet, thus breaking any loop.
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