Principles of RPF check on the AR router

In unicast routing and forwarding, unicast packets are transmitted along a P2P path. The router only needs to consider the destination address, so it can learn the outbound interface through which a packet can be forwarded to the destination. Multicast routing and forwarding are different. The destination address of multicast packets is the multicast address that just identifies a group of receivers. That is, the receiver's position cannot be found based on the destination address. However, the source address of multicast packet is determined. Therefore, the correctness of forwarding path of multicast packets depend on the source address.
When a router receives a multicast packet, it searches the unicast routing table for the route to the source address of the packet. After finding the route, the router checks whether the outbound interface of the route is the same as the inbound interface of the multicast packet. If they are the same, the router considers that the multicast packet is received from a correct interface. This ensures correctness and uniqueness of the entire forwarding path. This process is called RPF check.
The correct interface is called an RPF interface.

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