Reference Guide

358 | Internet Group Management Protocol (IGMP)
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Figure 19-8. Viewing Static and Learned IGMP Groups
Adjusting Timers
View the current value of all IGMP timers using the show ip igmp interface command from EXEC
Privilege mode, as shown in Figure 19-6.
Adjusting Query and Response Timers
The querier periodically sends a general query to discover which multicast groups are active. A group must
have at least one host to be active. When a host receives a query, it does not respond immediately, but
rather starts a delay timer. The delay time is set to a random value between 0 and the Maximum Response
Time. The host sends a response when the timer expires; in version 2, if another host responds before the
timer expires, the timer is nullified, and no response is sent.
The Maximum Response Time is the amount of time that the querier waits for a response to a query before
taking further action. The querier advertises this value in the query (refer to Figure 19-1). Lowering this
value decreases leave latency but increases response burstiness because all host membership reports must
be sent before the Maximum Response Time expires. Inversely, increasing this value decreases burstiness
at the expense of leave latency.
• Adjust the period between queries using the
ip igmp query-interval command from INTERFACE mode.
• Adjust the Maximum Response Time using the command
ip igmp query-max-resp-time command
from INTERFACE mode.
When the querier receives a leave message from a host, it sends a group-specific query to the subnet. If no
response is received, it sends another. The amount of time that the querier waits to receive a response to the
initial query before sending a second one is the last member query interval (LMQI). The switch waits one
LMQI after the second query before removing the group from the state table.
• Adjust the Last Member Query Interval using the
ip igmp last-member-query-interval command from
INTERFACE mode.
Adjusting the IGMP Querier Timeout Value
If there is more than one multicast router on a subnet, only one is elected to be the querier, which is the
router that sends queries to the subnet.
1. Routers send queries to the all multicast systems address, 224.0.0.1. Initially, all routers send queries.
FTOS(conf-if-te-1/0)#do sho ip igmp groups
Total Number of Groups: 2
IGMP Connected Group Membership
Group Address Interface Uptime Expires Last Reporter
224.1.1.1 TengigabitEthernet 1/0 00:00:03 Never CLI
224.1.2.1 TengigabitEthernet 1/0 00:56:55 00:01:22 1.1.1.2