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

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.

To view the IGMP Fast-Leave status of a port use the show running-configor show configuration commands.

4-18