4-18
Multimedia Traffic Control with IP Multicast (IGMP)
How IGMP Operates
Forced Fast-Leave IGMP
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 4-3, 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
“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.
Setting Fast-Leave and Forced Fast-Leave from the CLI
In previous software versions, Fast-Leave and Forced Fast-Leave options for
a port were set exclusively through the MIB
. The following commands now
allow a port to be configured for Fast-Leave or Forced Fast-leave operation
from the CLI. Note that these commands must be executed in a VLAN context
To view the IGMP Fast-Leave status of a port use the show running-config or
show configuration commands.
Syntax: [no] ip igmp fastleave <port-list>
Enables IGMP Fast-Leaves on the specified ports in the VLAN
(the default setting). In the Config context, use the VLAN
specifier, for example, vlan < vid > ip igmp fastleave <port-list>.
The “no” form disables Fast-Leave on the specified ports.
[no] ip igmp forcedfastleave <port-list>
Forces IGMP Fast-Leaves on the specified ports in the VLAN,
even if they are cascaded.
