Filter
Top Products
Page 84 | AlliedWare™ OS How To Note: IGMP
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
1
26, using the command:
set ip igmp timeout=126
There are two issues with having the Default Timeout Interval so short:
z There 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.
z Clients may not have time to reply.
In this example, the Default Timeout Interval (
1
26 seconds) is only
1
second longer than
the Default Query Interval (
1
25). 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*
1
25) + 0.5 = 250.5 seconds.