In this example, a device is connected through an IP router to a PIM-SM group source that is sending traffic for a multicast group. The device also is connected to a receiver for the group.
When PIM-SM traffic snooping is enabled, the device listens for PIM-SM Join and Prune messages and IGMP group membership reports. Until the device receives a Join message or a membership report, it forwards IP multicast traffic out on all ports. When the device receives a Join message or group membership report, the device forwards subsequent traffic for that group only on the ports from which the Join messages or reports were received.
In this example, the switch snoops for PIM SM Join and Prune messages. The router connected to the receiver for group 239.255.162.1 sends a Join message toward the group‘s source. Because PIM-SM traffic snooping is enabled on the device, the device examines the Join message to learn the group ID, then makes a forwarding entry for the group ID and the port connected to the receiver‘s router. The next time the device receives traffic for 239.255.162.1 from the group‘s source, the device forwards the traffic only on port 0/1, because that is the only port connected to a receiver for the group.