Allied Telesis AR400 manual Comparing the Querier and Snooper timers

How clients leave groups: queries and timers > Comparing the Querier and Snooper timers

zTo calculate the timer, the Snooper takes the LMQI value that it receives from the Querier and multiplies it by the Snooper’s own LMQC.

zThe Snooper only reduces the timer if it receives a Leave message followed by a Specific Query—one of the messages is not enough.

Changes with In software versions earlier than 281-03, the Snooper reset the group timer for all its ports 281-03 and when it received a Specific Query. Since versions 281-03 and 2.9.1, each port has a separate 2.9.1 timer and the Snooper only changes those which exceed the threshold. The flow chart above

describes the new behaviour.

Comparing the Querier and Snooper timers

By default, the Querier and Snooper port-specific group timers have the same value (2 seconds). This is because the LMQC is the same for the Querier and the Snoopers.

Consequences for high-loss and high-lag networks

If packet loss or lag time is an issue in your network, we recommend increasing the

Robustness Variable on the Snoopers and the Querier.

On Allied Telesis Snoopers and Queriers, LMQC = Robustness Variable. For Snoopers, not all vendors make these counters the same. RFC 2236 requires that LMQC and Robustness Variable have the same value on Queriers, but the IGMP timer rules for IGMP Snoopers are less well-defined.

Increasing the Querier LMQC (or Robustness Variable) increases the number of Specific Queries that the Querier sends. This increases the probability that an interested client will receive a Query.

Increasing the Snooper LMQC (or Robustness Variable) increases the length of time that the Snooper waits before aging out the port. This gives the client more time to reply to the Queries.

For example, if you increase the LMQC to 5 (the maximum) on the Querier and the Snooper, then the Querier sends 5 Queries and the Snooper waits for 5 * LMQI, which is 5 seconds with the default LMQI.

Make sure that the values on the Querier and the Snoopers match, so that the Snooper has time to forward all the Queries. For example, if you changed the Querier’s Robustness Variable to 5 but left the Snooper unchanged, the Querier would send out 5 Queries

1 second apart but the Snooper would age out the group entry after only the first 2 Queries.

For more information about setting the Robustness Variable, and the consequences of this, see "Robustness Variable" on page 75.

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