Allied Telesis AR400 Default Timeout Interval, Synchronisation of timers, What this timer does

Configurable IGMP timers and counters > Default Timeout Interval

Default Timeout Interval

The Default Timeout Interval is referred to as the Group Membership Interval in RFC 2236.

What this timer does

The Default Timeout Interval specifies the length of time before the router or switch deletes a group from its multicast group database after the router or switch last receives a Membership Report for that group. All IGMP routers and switches in a network use this interval to maintain their group membership databases, not just the Querier.

The Querier also uses this interval to close down multicasting if it receives no replies to all General Queries for a group. If the Querier sends a General Query and does not receive any Membership Reports in response, it continues to send any existing multicast streams. In the meanwhile, the Default Timeout Interval counts down until it hits zero, at which point the Querier stops propagating the multicast streams through the LAN.

Potential problems with changing this timer

Make the Default Timeout Interval too short has serious consequences. You remove the network’s ability to cope with losing a General Query and you may not give enough time for client responses to reach the Querier. These problems would cause multicasting to stop for some or all clients. For more information, see "Example of bad choices for timer values" on page 83.

Synchronisation of timers

The Default Timeout Interval is a function of several other timers, according to the following formula from RFC 2236:

Default Timeout Interval = (Robustness Variable * Default Query Interval) +

one Query Response Interval in seconds

Since software versions 281-03 and 2.9.1, the Default Timeout Interval changes automatically if you change any of the Robustness Variable, the LMQC, the Default Query Interval, or the Query Response Interval. You can override the Default Timeout Interval value, but the router or switch displays a warning message if you do so.

For earlier versions, you need to calculate and change the interval yourself.

The following examples show how changing the timers changes the Default Timeout Interval.

Page 78 AlliedWare™ OS How To Note: IGMP