HP ProCurve Series 4100GL Management And Configuration Manual page 314

Multimedia Traffic Control with IP Multicast (IGMP)
How IGMP Operates
13-14
group members exist on the same port. This means that the switch continues
to transmit unnecessary multicast traffic through the port until the Querier
renews multicast group status.
Fast-Leave IGMP Defaults to Disabled. On HP ProCurve switches that
support Data-Driven IGMP ("Smart" IGMP), when unregistered multicasts are
received the switch will automatically filter them. Thus, the sooner the IGMP
Leave is processed, the sooner this multicast traffic stops flowing.
On switches that do not support Data-Driven IGMP, such as the switches
covered in this guide, unregistered multicasts groups are flooded to the VLAN
rather than pruned. In this scenario, Fast-Leave IGMP can actually increase
the problem of multicast flooding by removing the IGMP group filter before
the Querier has recognized the IGMP Leave. The Querier will continue to
transmit the multicast group during this short time, and because the group is
no longer registered the switch will then flood the multicast group to all ports.
For this reason, the IGMP FastLeave feature is disabled by default on all HP
ProCurve switches that do not support Data-Driven IGMP, including the
switches covered in this guide. The feature can be enabled on these switches
via an SNMP set of the IgmpPortFastLeaveState.< vid >.< port number > object.
However, this is not recommended as this will increase the amount of multi-
cast flooding during the period between the client's IGMP Leave and the
Querier's processing of that Leave.
Automatic Fast-Leave Operation. If a switch port is:
a. Connected to only one end node
b. The end node currently belongs to a multicast group; i.e. is an IGMP
client
c. 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.)