IPv6 Configuration Guide K/KA/KB.15.15

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.
Current MLD status
The following information is shown for each VLAN that has MLD snooping enabled:
• VLAN ID number and name
• Querier address
IPv6 address of the device acting as querier for the VLAN.
• Querier up time
Length of time in seconds that the querier has been acting as querier.
• Querier expiry time
If this switch is the querier, this is the amount of time until the switch sends the next general
query. If this switch is not the querier, this is the amount of time in seconds until the current
querier is considered inactive (after which a new querier election is held).
Current MLD status 83