Reverse path forwarding (RPF) is used in conjection with a Multicast routing protocol to build a loop-free shortest-path tree.
In normal unicast IP forwarding, the router reads the destination address of the packet, and looks it up in the forwarding table to decide the port to send the packet out from. In multicast IP forwarding, the destination address corresponds to a group of ports on the router.
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.
Consider the following example which describe the Reverse Path Forwarding procedure -
The source node sends a multicast packet to the multicast group. Router A is directly attached to the source of the multicast traffic. Router A also connected to router B and C so it sends a multicast packet to each (router B and C).
Router B receives the multicast packet on link 1. This is a parent link. It duplicates the packet and sends to all other routers (C, D and E) which is directly connected with it.
Router C receives the packet on link 2, which is a parent link. It also receives a multicast packet on link 3 from router B. This is a child link for C. So C discards the packet. Router C also duplicates the packet and sends to the other routers directly connected with it.
Router D receives the packet on link 4, which is its parent link. It also duplicates the packet and send to router E.
Router E receives the multicast packet on links 5, 6, 8 and 9. Link 5 is a parent link and other links are child links. So router E accepts the packet on link 5 and discards the rest.
Router F receives the multicast packet on links 7 and 9. It accepts the packet only on link 7 because it is the parent link.
To optimize this example, let’s assume that B learned that link 3 is a child link for C. So when B receives a mutlicast packet, it will send it out of all links except links 1 and 3.
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