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Page 60 | AlliedWare™ OS How To Note: IGMP
How clients leave groups: queries and timers > Comparing the Querier and Snooper timers
z To calculate the timer, the Snooper takes the LMQI value that it receives from the
Querier and multiplies it by the Snooper’s own LMQC.
z The 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
281-03 and
2.9.1
In software versions earlier than 281-03, the Snooper reset the group timer for all its ports
when it received a Specific Query. Since versions 28
1
-03 and 2.9.
1
, each port has a separate
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.