Table of Contents

PIM Assert Mechanism

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:
  1. The PIM Forwarder is the router with the lowest administrative distance to the multicast source.
  2. 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.
  3. 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.
When you Configure IP Multicast, you can configure the interval at which the virtual router sends PIM Assert messages out an interface (the Assert interval). When you View IP Multicast Information, the
PIM Interface
tab displays the Assert interval for an interface.

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