4-15
Multimedia Traffic Control with IP Multicast (IGMP)
How IGMP Operates
Automatic Fast-Leave IGMP
IGMP Operation Presents a “Delayed Leave” Problem. Where 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 IGMP device
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 means that the switch continues
to transmit unnecessary multicast traffic through the port until the Querier
renews the multicast group status.
Fast-Leave IGMP. Depending on the switch model, Fast-Leave is enabled
or disabled in the default configuration.
Table 4-2.IGMP: Data-Driven and Non-Data Driven Behavior
On switches that do not support Data-Driven IGMP, unregistered multicast
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
Switch Model or
Series
Data-
Driven
IGMP
Included?
IGMP Fast-
Leave Setting
Default IGMP Behavior
Switch 8212zl
Switch 6400cl
Switch 6200yl
Switch 5400zl
Switch 5300xl
Switch 4200vl
Switch 3500yl
Switch 3400cl
Switch 2800
Switch 2810
Switch 2510G
Switch 2510-48
Switch 2500
Yes Always
Enabled
Drops unjoined multicast traffic except for
always-forwarded traffic toward the
Querier or multicast routers, and out of
IGMP-forward ports. Selectively forwards
joined multicast traffic.
Switch 2510-24
Switch 2600
Switch 2600-PWR
Switch 4100gl
Switch 6108
No Disabled in
the Default
Configuration
IGMP Fast-Leave disabled in the default
configuration. Floods unjoined multicast
traffic to all ports. Selectively forwards
joined multicast traffic.
