31.4.4 Determining Forwarder

If multiple PIM-SM routers are connected on the same LAN, multicast packets might be duplicated and forwarded on the network.

If multiple PIM-SM routers exist on the same LAN, and two or more of these routers forward multicast packets to the LAN, the PIM-SM routers use PIM-Assert messages to compare the multicast path preferences and metrics, and choose the most appropriate router on the sender network as the forwarder.

Only the router that becomes the forwarder forwards multicast packets on the LAN, in order to prevent duplicate multicast packet forwarding.

The flow for determining the forwarder by PIM-Assert messages is as follows:

Preferences are compared, and the router with the lowest value becomes the forwarder.
If the preferences are the same, the metrics are compared, and the router with the lowest value becomes the forwarder.
If the metrics are the same, the IP addresses of the routers are compared, and the router with the highest IP address becomes the forwarder.

The Switch sends PIM-Assert messages with the multicast path preference fixed at 101, and the metric fixed at 1024. However, for direct connections with a sender, PIM-Assert messages are sent with the preference and metric both fixed at 0. You can also obtain the distance and metric of the path from the unicast information in configuration mode, and send them as the preference and metric of PIM-Assert messages.

The following figure shows how the forwarder is determined.

Figure 31-14: Decision Forwarder