Cisco Systems 78-11741-02 manual Configuring Igmp Leave Latency, IPC-414

Configuring IP Multicast Routing

IGMP Features Configuration Task List

Command	Purpose
Router(config-if)#ip igmp static-group	Configures the router as a statically connected member of a group.
group-address

Configuring IGMP Leave Latency

In IGMPv2 and IGMPv3, hosts send IGMP messages to indicate that they do not wish to receive a particular group, source, or channel any more. The length of time between the host wanting to leave and the router stopping forwarding is called the IGMP leave latency. IGMP leave latency is only relevant when the last host on a subnet that was a member to a group, source, or channel intends to leave, because as long as there are still other interested members, the router still needs to forward the traffic.

When a router receives such a membership message that indicates a leave, by default, it needs to verify if there are still other members interested in the traffic. To do so, the IGMP querying router sends out a group-specific or group-source-specific query. This query contains the last member query interval (LMQI), which is the time within which other still interested hosts need to send a membership report or else the router will stop forwarding. Because IGMP messages may get lost between router and hosts, the router by default does not immediately stop forwarding after the LMQI has expired, but instead it repeats this process of sending the group or group-source-specific query and waiting for membership reports for a total of times specified by the last member query count (LMQC). Only thereafter will the router stop forwarding.

By default in Cisco IOS software and in the IGMPv2 and IGMPv3 RFCs, the LMQI is 1 second, and the LMQC is 2. Therefore, the default leave latency for individual leaves in Cisco IOS software is 3 seconds.

IGMPv3 explicit tracking allows to reduce the leave latency to approximately 0 for hosts that support IGMPv3. This feature is not available for hosts that support only IGMPv2 because of the protocol limitation.

In IGMPv2, if there is only one IP multicast receiving host connected to a subnet, the ip igmp immediate-leavegroup-listcommand can be configured so the router immediately stop forwarding traffic for the group, resulting in a leave latency of 0.

To change the values of the LMQI, use the following command in interface configuration mode:

Command	Purpose
Router(config-if)# ip igmp	Configures the interval at which the router sends IGMP
last-member-query-interval interval	group-specific or group-source-specific (with IGMPv3) query
	messages.

To change the values of the LMQC, use the following commands in interface configuration mode:

Command	Purpose
Router(config-if)# ip igmp	Configures the number of times that the router sends IGMP
last-member-query-count lmqc	group-specific or group-source-specific (with IGMPv3) query
	messages.

Cisco IOS IP Configuration Guide

IPC-414