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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.
When a multicast packet enters a router's interface, the router looks up the list of networks that are reachable via that interface (i.e., it checks the paths by which the packet could have arrived). 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 that multicast group. If the RPF check fails, the packet is
This is critically important in redundant multicast topologies. Because the same multicast packet could reach the same router via multiple interfaces, RPF checking is integral in the decision to forward packets or not. If the router forwarded all packets that come in interface A to interface B and it also forwarded all packets coming in interface B to interface A and both interfaces receive the same packet, this will create a classic [[routing loop]] where packets will be forwarded in both directions until their IP [[Time to live|TTL]]s expire. Even considering TTL expiry, all types of routing loops are best avoided as they involve at least temporary network degradation.
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