IGMP - fast leave

In a Multicast implementation, the IGMP snooping fast leave feature allows for a quicker departure of a host from a multicast group.

In a multicast network, when a host wants to join or leave a multicast group, it sends an IGMP report to the router. The router then forwards the multicast traffic to the appropriate hosts. By default, when a host wants to leave a multicast group, it sends an IGMP leave message to the router. The router then initiates a process called "group-specific query" to check if there are any other hosts still interested in the multicast traffic for that group. If no other hosts respond, the router stops forwarding the traffic to the group.

With fast leave processing enabled, the router doesn't initiate the group-specific query process when it receives an IGMP leave message. Instead, it immediately stops forwarding the multicast traffic for that group, reducing the delay associated with the leave process. This feature is particularly useful in scenarios where the multicast traffic is no longer needed, such as when a user stops streaming a video.

To enable fast leave, use the following command:

ip igmp snooping fast-leave

On the 7600 series of devices, this command can be applied in interface configuration mode, enabling it on a per interface basis. On the 9300 platform, it is a command issued in global configuration mode, and is globally enabled.

Keep in mind that fast leave processing can cause issues in networks with multiple hosts interested in the same multicast traffic because the router might stop forwarding traffic even if other hosts are still interested. Therefore, it is recommended to use fast leave processing in networks where there is only one host per multicast group, or in situations where quick leave processing is more important than maintaining multicast traffic for other hosts.