HP 3500YL - Manual (Page 49)

Multimedia Traffic Control with IP Multicast (IGMP)
How IGMP Operates
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.
Note that 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 figure 2-3 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 confir­
mation 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.
Configuring Fast-Leave IGMP.
Syntax: [no] ip igmp fastleave < port-list >
Enables IGMP fast-leaves on the specified ports in the selected
VLAN. The
no form of the command disables IGMP fast-leave
on the specified ports in the selected VLAN. Use show running
to display the ports per-VLAN on which Fast-Leave is
disabled.
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 2-3, 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 group
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