[mrtg] Re: Spike when counter rolls over?

Alex van den Bogaerdt alex at ergens.op.het.net
Wed Dec 17 12:36:41 MET 2003

On Wed, Dec 17, 2003 at 02:19:32AM -0800, John Oliver wrote:
> I got a huge spike on the LAN graph for my DI-604.  I'm 99.9% certain
> that this is because the counter rolled over, not because there was a
> sudden burst of traffic just between the two PCs connected to it... :-)
> The thing is, though, that MaxBytes and AbsMax is set pretty high,
> because it's a 100 Mb/s interface.  Is there any way to protect against
> false spikes other than by setting these values lower?

I don't think a true roll over will do this.  A reset may.

If you're using RRDtool as your back end, you can change the
data source type from counter into derive.  Set the minimum
allowed value to zero and it should work.

Another way of doing what you want is to use a script to preprocess
the values.  Basically you fetch the values yourself (using SNMP),
do some checking and then decide to hand them over to MRTG.  Use
the external program interface.
Possible checks include:
- value _has_ to be higher than the previous value
- the uptime of the device needs to be larger than the interval

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