Multicast RPF

Right then, I'm happy with RPF as a principle and that the router checks the interface on which it receives multicast traffic and consults its routing table to see if that interface would be used to reach the multicast source.

What I didn't know (or at least I hadn't remembered) until now is that when there are equal-cost paths to the multicast source (e.g. OSPF, EIGRP etc) the router must pick one of them for Multicast RPF. Which one does it pick? It picks the one with highest neighbouring router ID.

For example, let's say that the multicast RP is located on 192.168.1.0/24 network. You downstream router receives two equal-cost routes for that subnet, one from R1 with a router-id of 1.1.1.1 and the other from R2 with a router-id of 2.2.2.2. The router will pick the interface connected to R2 as it has the highest router-id.

You can frig this by using tunnelling but that is a whole new ball game and one I'm not going into right now.