Multicast PIM-assert forwarder election
When implementing PIM-assert, an election of the PIM forwarder takes place. Refer to the following topology:
When the source sends multicast traffic, it will reach the 192.168.234.0/24 network segment, and both R2 and R3 will "see" each other's multicast traffic on their Fa0/1 interfaces. This will trigger a negotiation between the two routers. This is part of the PIM assert mechanism. They exchange information, including their own locally significant administrative distance to the source of the multicast stream as it appears in the local routing table of each router.
If this is the same, they then compare their IP addresses.