[mrtg] Re: MRTG to monitor Gigabit-ethernet with counter32 and interval: 1

Alex van den Bogaerdt alex at ergens.op.het.net
Tue May 27 13:13:19 MEST 2003

On Tue, May 27, 2003 at 12:54:47PM +0200, Michael Markstaller wrote:
> Just a quick question, I have to monitor a 3Com Superstack 4900 GE-switch which seems to have no SNMP-v2c at all and so no 64Bit-counters ifHC*Octets. 
> So far so bad, I understand the whole problem; when traffic hits 114 MBit, counters wrap etc..

The problem is slightly different:  above 114 Mbps and with a 5-minute interval, the wrapping occurs
so often that mrtg doesn't find a wrap.

counter increase real
value   seen     increase

12      2        2+2^32
14      2        2+2^32

By polling more often, this happens:

counter increase real
value   seen     increase

11+2^31 1+2^31   1+2^31			(extra poll)
12      1+2^31   1+2^31
13+2^31 1+2^31   1+2^31			(extra poll)
14      1+2^31   1+2^31

Polling more often introduces more error.
Suppose the timing accuracy is 1 second.  Normally this
results in an error of ~ 1:300.

If you're going to poll every 1 second then this results
in an error of 1:1.  Not good.

Specifying an interval of 300 seconds in rrdtool and polling
every minute (or so) will work and should result in better
handling of the frequent wraps but do keep the error in mind.


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