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Cisco ip igmp query-max-response-time
From: Christopher Maynard <christopher.maynard () gtech com>
Date: Fri, 13 Apr 2012 18:21:03 +0000 (UTC)

One of the Cisco IOS IP commands is "ip igmp query-max-response-time", described
here:
http://www.cisco.com/en/US/docs/ios/12_2/ipmulti/command/reference/1rfmult1.html#wp1018684.

The range of values one can use is not listed, but it is (or at least was the
last time I checked) 1-25.  Some time ago, a colleague & I discovered
empirically what each value in the range of 1-25 actually corresponds to in real
time.  It was a simple matter of setting it to each value in the range of 1-25
and then examining the value of the Max Response Time field of the IGMP query
packet (See http://tools.ietf.org/html/rfc2236#section-2.2).

For the 1st 12 values, it's 1-1, but then it increases exponentially.  Here's
the full table:

query-max-response-time    actual response time (seconds)
1                          1
2                          2
3                          3
4                          4
5                          5
6                          6
7                          7
8                          8
9                          9
10                         10
11                         11
12                         12
13                         14.4
14                         22.4
15                         35.2
16                         51.2
17                         83.2
18                         128
19                         192
20                         307.2
21                         460.8
22                         716.8
23                         1126.4
24                         1638.4
25                         2662.4

I don't remember the exact version of IOS we tested with, and I don't know if
this behavior was intentional or not on the part of Cisco or if it was a bug
that was corrected in later versions or what.

I just thought that for anyone using this command, this information might be
useful to you, particularly when using values above 12.  If anyone thinks this
information is useful, it could be added to http://wiki.wireshark.org/IGMP.

Well, for what it's worth ...
- Chris


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