Cisco Systems 78-11741-02 Wireless Office Headset User Manual


 
Configuring IP Multicast Routing
IGMP Features Configuration Task List
IPC-414
Cisco IOS IP Configuration Guide
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-leave group-list command 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:
To change the values of the LMQC, use the following commands in interface configuration mode:
Command Purpose
Router(config-if)# ip igmp static-group
group-address
Configures the router as a statically connected member of a group.
Command Purpose
Router(config-if)# ip igmp
last-member-query-interval interval
Configures the interval at which the router sends IGMP
group-specific or group-source-specific (with IGMPv3) query
messages.
Command Purpose
Router(config-if)# ip igmp
last-member-query-count lmqc
Configures the number of times that the router sends IGMP
group-specific or group-source-specific (with IGMPv3) query
messages.