3.2.a Troubleshoot reverse path forwarding

they got this right…

An RPF check examines the source address of an incoming packet and checks it against the router’s unicast routing table to see what interface should be used to get back to the source network. If the incoming multicast packet is using that interface, the RPF check passes, and the packet is forwarded. If the multicast packet comes in a different interface, the RPF check fails, and the packet is discarded.