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Limiting the Number of IGMP Multicast Group Joins on Logical Interfaces

The group-limit statement enables you to limit the number of IGMP multicast group joins for logical interfaces. When this statement is enabled on a router running IGMP version 2 (IGMPv2) or version 3 (IGMPv3), the limit is applied upon receipt of the group report. Once the group limit is reached, subsequent join requests are rejected.

• Each any-source group (*,G) counts as one group toward the limit.
• Each source-specific group (S,G) counts as one group toward the limit.
• Groups in IGMPv3 exclude mode are counted toward the limit.
• Multiple source-specific groups count individually toward the group limit, even if they are for the same group. For example, (S1, G1) and (S2, G1) would count as two groups toward the configured limit.
• Combinations of any-source groups and source-specific groups count individually toward the group limit, even if they are for the same group. For example, (*, G1) and (S, G1) would count as two groups toward the configured limit.
• Configuring and committing a group limit on a network that is lower than what already exists on the network results in the removal of all groups from the configuration. The groups must then request to rejoin the network (up to the newly configured group limit).
• You can dynamically limit multicast groups on IGMP logical interfaces using dynamic profiles.

Beginning with Junos OS 12.2, you can optionally configure a system log warning threshold for IGMP multicast group joins received on the logical interface. It is helpful to review the system log messages for troubleshooting purposes and to detect if an excessive amount of IGMP multicast group joins have been received on the interface. These log messages convey when the configured group limit has been exceeded, when the configured threshold has been exceeded, and when the number of groups drop below the configured threshold.

The group-threshold statement enables you to configure the threshold at which a warning message is logged. The range is 1 through 100 percent. The warning threshold is a percentage of the group limit, so you must configure the group-limit statement to configure a warning threshold. For instance, when the number of groups exceed the configured warning threshold, but remain below the configured group limit, multicast groups continue to be accepted, and the device logs the warning message. In addition, the device logs a warning message after the number of groups drop below the configured warning threshold. You can further specify the amount of time (in seconds) between the log messages by configuring the log-interval statement. The range is 6 through 32,767 seconds.

You might consider throttling log messages because every entry added after the configured threshold and every entry rejected after the configured limit causes a warning message to be logged. By configuring a log interval, you can throttle the amount of system log warning messages generated for IGMP multicast group joins.

1. Access the logical interface at the IGMP protocol hierarchy level.
   [edit]user@host# edit protocols igmp interface interface-name
2. Specify the group limit for the interface.
   [edit protocols igmp interface interface-name]user@host# set group-limit limit
3. (Optional) Configure the threshold at which a warning message is logged.
   [edit protocols igmp interface interface-name]user@host# set group-threshold value
4. (Optional) Configure the amount of time between log messages.
   [edit protocols igmp interface interface-name]user@host# set log-interval seconds

To confirm your configuration, use the show protocols igmp command. To verify the operation of IGMP on the interface, including the configured group limit and the optional warning threshold and interval between log messages, use the show igmp interface command.