Users Guide

Adjusting Timers
The following sections describe viewing and adjusting timers.
To view the current value of all IGMP timers, use the following command.
• View the current value of all IGMP timers.
EXEC Privilege mode
show ip igmp interface
For more information, refer to the example shown in Viewing IGMP Enabled Interfaces.
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 the illustration in IGMP Version 2). 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.
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 period between queries.
INTERFACE mode
ip igmp query-interval
• Adjust the maximum response time.
INTERFACE mode
ip igmp query-max-resp-time
• Adjust the last member query interval.
INTERFACE mode
ip igmp last-member-query-interval
Preventing a Host from Joining a Group
You can prevent a host from joining a particular group by blocking specific IGMP reports using an extended access list containing the
permissible source-group pairs.
NOTE:
For rules in IGMP access lists, source is the multicast source, not the source of the IGMP packet. For IGMPv2,
use the keyword any for source (as shown in the following example) because the IGMPv2 hosts do not know in advance
who the source is for the group in which they are interested.
To apply the access list, use the following command.
• Apply the access list.
INTERFACE mode
ip igmp access-group access-list-name
Dell Networking OS Behavior: Do not enter the ip igmp access-group command before creating the access-list. If you do, after
entering your first deny rule, the Dell Networking OS clears the multicast routing table and re-learns all groups, even those not covered by
the rules in the access-list, because there is an implicit deny all rule at the end of all access-lists. Therefore, configuring an IGMP join
request filter in this order might result in data loss. If you must enter the ip igmp access-group command before creating the
access-list, prevent the Dell Networking OS from clearing the routing table by entering a permit any rule with a high sequence number
before you enter any other rules.
Internet Group Management Protocol (IGMP)
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