PIM uses an Assert mechanism to elect a PIM Forwarder
of multicast packets on a multiaccess network, which prevents forwarding
of duplicate multicast packets.
To prevent routers on a multiaccess network from forwarding
the same multicast traffic to the same next hop (which would cause
redundant traffic and wasted bandwidth), PIM uses the Assert mechanism
to elect a single PIM Forwarder for the multiaccess network.
If the virtual router receives a multicast packet from a source
on an interface that the virtual router already associates as the
outgoing interface for the same (S,G) pair identified in the packet,
that means this is a duplicate packet. Consequently, the virtual
router sends an Assert message containing its metrics to the other
routers on the multiaccess network. The routers then elect a PIM
Forwarder in this manner:
The PIM Forwarder is the router with the lowest administrative
distance to the multicast source.
In the event of a tie for lowest administrative distance, the
PIM Forwarder is the router with the best unicast routing metric
to the source.
In the event of a tie for best metric, the PIM Forwarder is
the router with the highest IP address.
Routers that are not elected as the PIM Forwarder will stop forwarding
traffic to the multicast group identified in the (S,G) pair.