Fast leaves and forced fast leaves

The fast leave and forced fast leave functions can help to prune unnecessary multicast traffic when an MLD host issues a leave request from a multicast address. Fast leave is enabled by default, and forced fast leave is disabled by default. Both functions are applied to individual ports.

Which function to use depends on whether a port has more than one node attached to it, as follows:

  • If a port has only one node attached to it, when the switch sees a leave request from that node (an MLD host) it knows that it does not need to send any more multicast traffic for that multicast address to the host's port.

  • If fast leave is enabled (the default setting), the switch stops sending the multicast traffic immediately.

  • If fast leave is disabled, the switch continues to look for join requests from the host in response to group-specific queries sent to the port.

  • The interval during which the switch looks for join requests is brief and depends on the forced fast leave setting:

  • If forced fast leave is enabled for the port, it is equal to the "forced fast leave interval" (typically several seconds or less).

  • If forced fast leave is disabled for the port, the period is about 10 seconds (governed by the MLD standard).

  • When this process has completed, the multicast traffic for the group will stop (unless the switch sees a new join request).

  • If a single port has multiple nodes attached to it, a leave request from one of those nodes (an MLD host) does not provide enough information for the switch to stop sending multicast traffic to the port. In this situation, the fast leave function does not operate. The switch continues to look for join requests from any MLD hosts connected to the port in response to group-specific queries sent to the port.As in the case described above for a single-node port that is not enabled for fast leave, the interval during which the switch looks for join requests is brief and depends on the forced fast leave setting:

  • If forced fast leave is enabled for the port, it is equal to the "forced fast leave interval" (typically several seconds or less).

  • If forced fast leave is disabled for the port, the period is about 10 seconds (governed by the MLD standard).

  • When this process has completed, the multicast traffic for the group will stop unless the switch sees a new join request. This reduces the number of multicast packets forwarded unnecessarily.