Sorry if this is a rudimentary question...
When using PIM-DM, if a router performs an RPF check to prevent looping multicast packets by looking at the unicast routing table to determine the interface it would send to reach the source, what happens if the router has multiple equal cost routes to the source? Wouldn't the router receive the packets on all those interfaces when it is flooded, and each packet would pass the RPF check, and then the router would flood it back out the other interfaces. Obviously, this behaviour would stop when the packet reaches the upstream routers again, but it just seems like an inefficient check. For instance, dual core design connecting to two building distribution routers (meshed) - host in building A sources a packet destined for a multicast group. Building A router floods packets to both cores - each packet passes RPF check. Both cores then send the packet to building B router (which has equal cost paths to the two cores). Router in building B then performs RPF check on both packets, and they both pass? Or does the router choose one route from the unicast routing table as the "best" for PIM? If both packets pass, then Building B router forwards the packet that it received from core1 to core2, which then fails RPF, and building B router also forwards packet received from core2 to core1, which then fails RFP on core1? I know that PIM-DM is inefficient, but that would seem to be a really bad design in a large campus. THanks in advance.
