If a switch port has the following characteristics, the fast-leave operation will apply:
• Connected to only one end node.
• The end node currently belongs to a multicast group, that is, is an IGMP client.
• The end node subsequently leaves the multicast group.
Then the switch does not need to wait for the Querier status update interval, but instead immediately removes the
IGMP client from its IGMP table and ceases transmitting IGMP traffic to the client. (If the switch detects multiple
end nodes on the port, automatic fast-leave does not activate—regardless of whether one or more of these end
nodes are IGMP clients.)
In the following figure, automatic fast-leave operates on the switch ports for IGMP clients "3A" and "5A," but not
on the switch port for IGMP clients "7A" and "7B," server "7C," and printer "7D."
Figure 2: Example of automatic fast-leave IGMP criteria
When client "3A" running IGMP is ready to leave the multicast group, it transmits a Leave Group message.
Because the switch knows that there is only one end node on port A3, it removes the client from its IGMP table
and halts multicast traffic (for that group) to port A3. If the switch is not the Querier, it does not wait for the actual
Querier to verify that there are no other group members on port A3. If the switch itself is the Querier, it does not
query port A3 for the presence of other group members.
Fast-leave operation does not distinguish between end nodes on the same port that belong to different VLANs.
Thus, for example, even if all of the devices on port A6 in the figure belong to different VLANs, fast-leave does not
operate on port A6.
Default (enabled) IGMP operation solves the "delayed leave" problem
Fast-leave IGMP is enabled by default. When fast-leave is disabled and multiple IGMP clients are connected to
the same port on an IGMP device (switch or router), if only one IGMP client joins a given multicast group, then
later sends a Leave Group message and ceases to belong to that group, the switch automatically retains that
IGMP client in its IGMP table and continues forwarding IGMP traffic to the IGMP client until the Querier triggers
confirmation that no other group members exist on the same port. This delayed leave operation means that the
switch continues to transmit unnecessary multicast traffic through the port until the Querier renews multicast
group status.
Forced fast-leave IGMP
When enabled, forced fast-leave IGMP speeds up the process of blocking unnecessary IGMP traffic to a switch
port that is connected to multiple end nodes. (This feature does not activate on ports where the switch detects
only one end node.) For example, in
Figure 2: Example of automatic fast-leave IGMP criteria
even if you configured forced fast-leave on all ports in the switch, the feature would activate only on port A6
(which has multiple end nodes) when a Leave Group request arrived on that port.
When a port having multiple end nodes receives a Leave Group request from one end node for a given multicast
group "X," forced fast-leave activates and waits a small amount of time to receive a join request from any other
Chapter 2 Multimedia traffic control with IP multicast (IGMP)
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