Figure 1 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 Figure 1 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.
Configuring fast-leave IGMP
For information about fast-leave IGMP, see “Automatic fast-leave IGMP” (page 15).
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.
Default: Enabled
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 connect ed to multiple end nodes. (This feature does not activate on ports
where the switch detects only one end node.) For example, in Figure 1 (page 17), 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 "X" member on that port. If the port does not receive a join
request for that group within the forced-leave interval, the switch then blocks any further group "X"
traffic to the port.
How IGMP operates 17
To Next Page
Loading...
Need help?
General