Modifying the IGMP Robustness Variable

The IGMP robustness variable provides fine-tuning to allow for expected packet loss on a subnet. The robust count automatically changes certain IGMP message intervals for IGMPv2 and IGMPv3. Increasing the robust count allows for more packet loss but increases the leave latency of the subnetwork.

On a shared network running IGMPv2 when the query router receives an IGMP leave message, it must send an IGMP group query message for a specified number of times. The number of IGMP group query messages sent is determined by the robust count.

The value of the robustness variable is also used in calculating the following IGMP message intervals:

• Group member interval—Amount of time that must pass before a multicast router decides that there are no more members of a group on a network. This interval is calculated as follows: (robustness variable x query-interval) + (1 x query-response-interval).
• Other querier present interval—The robust count is the amount of time that must pass before a multicast router decides that there is no longer another multicast router that is the querier. This interval is calculated as follows: (robustness variable x query-interval) + (0.5 x query-response-interval).
• Last-member query count—Number of group-specific queries sent before the router assumes there are no local members of a group. The default number is the value of the robustness variable.

For more information about the IGMPv2 robust count see RFC2236 - Internet Group Management Protocol, Version 2.

In IGMPv3 a change of interface state causes the system to immediately transmit a state-change report from that interface. In case the state-change report is missed by one or more multicast routers, it is retransmitted. The number of times it is retransmitted is the robust count minus one. In IGMPv3 the robust count is also a factor in determining the group membership interval, the older version querier interval, and the other querier present interval.

For more information about the IGMPv3 robust count see RFC 3376 - Internet Group Management Protocol, Version 3.

By default, the robustness variable is set to 2. You might want to increase this value if you expect a subnet to lose packets. To change the value of the robustness variable, include the robust-count statement:

You can include this statement at the following hierarchy levels:

• [edit protocols igmp]
• [edit logical-systems logical-system-name protocols igmp]

The number can be from 2 through 10.