If we had a host on a subnet with two PIMv2-SM routers, how do the routers decide which one will forward multicast traffic to the hosts on that subnet?
Let's assume that we have two PIMv2-SM routers, that they are PIMv2 neighbors, and that each PIMv2-SM router is connected to the same upstream router. Let's also assume that we are running a recent Cisco switch (3750) and that it has IGMP snooping enabled. Also, let's assume that an RP has been manually configured on all routers, and there is correct routing to it.
Both routers would send PIMv2 hello messages (IP protocol 103) to multicast address 224.0.0.13 (all-PIM-routers multicast address), and the switch would see these PIM packets and add the interfaces receiving these PIM packets to its router interface list. A host running IGMPv2 would send a membership report with a destination address of the group. The switch would see these IGMP packets and add the multicast MAC address of the multicast group to the MAC address table for to the switch interface connected to the router interfaces (so that the routers will receive traffic sent to the multicast address), and the switch would do the same thing for the MAC address table for the switch interface that received the IGMP packet.
Once a host sends an IGMP join, both routers would receive the IGMP join message. In sparse mode, they would both do an RPF lookup on the RP and send a PIMv2 join message towards the RP. Both routers would then receive multicast traffic for the same host on the same subnet.
I've read about the PIM assert mechanism, but that only takes effect once the router "hears" the other router's multicast traffic sent towards the host. The question of which router processes the IGMP join in the first place is still there.
