Allied Telesis AR400 manual Problem 1 Last Member Query Interval too short

Example of bad choices for timer values > Problem 1: Last Member Query Interval too short

Problem 1: Last Member Query Interval too short

The Last Member Query Interval was set to 5, using the command:

set ip igmp lmqi=5

This value is incredibly low—actually 5 tenths of a second (half a second). A Last Member Query Count of 2 (the default), gives your clients 1 second to get their Report back to the Querier before the Snooper and Querier stop sending the data stream. Using too low a Last Member Query Interval may mean that some or all clients briefly lose the multicast stream.

Problem 2: Query Response Interval short

The Query Response Interval was also set to 5, using the command:

set ip igmp queryresponseinterval=5

This value is also half a second. This value means that clients randomly pick a time between 0 and 0.5 seconds to respond (send a Report) to a Query. Using a short time here congests the Snoopers and Querier with Reports in a short space of time. This is not necessarily a problem if you only have a few clients, but 0.5 seconds is definitely a short window of time.

Problem 3: Default Timeout Interval too short

The Default Timeout Interval was set to 126, using the command:

set ip igmp timeout=126

There are two issues with having the Default Timeout Interval so short:

zThere is no longer any allowance for packet loss in the network.

The Querier sends a General Query every Default Query Interval seconds and waits for the Default Interval Timeout seconds for replies. Then it deletes any existing group entries that did not get refreshed. Therefore, if the Default Interval Timeout is less than twice the Default Query Interval, the Querier deletes entries if they do not reply to one General Query. A single dropped General Query causes multicasting to stop for the whole Layer 2 network.

zClients may not have time to reply.

In this example, the Default Timeout Interval (126 seconds) is only 1 second longer than the Default Query Interval (125). There is only 1 second for clients to receive the General Query and generate a Report and all Snoopers and the Querier to receive the Report.

Depending on the network, this may not be long enough. If a client Report does not get back to the Querier in time, the Querier deletes that port’s entry and multicasting stops briefly for the client.

RFC 2236 says that the Default Timeout Interval (the Group Membership Interval) must be:

(Robustness Variable * Default Query Interval) + one Query

Response Interval

For the settings in this example, that means (2*125) + 0.5 = 250.5 seconds.

Page 84 AlliedWare™ OS How To Note: IGMP