[mrtg-developers] Re: Measurement Mismatch between MRTG reports and PacketShaper reports.

Alex van den Bogaerdt alex at slot.hollandcasino.nl
Thu Dec 27 13:24:38 MET 2001

Anuj wrote:

> Problem Description: 
> I have MRTG running in my network and it is measuring the traffic on WAN interfaces of the cisco 7200 series router.
> Just after the Ethernet port of the router we have deployed PacketShaper bandwidth management/shaping device.
> I can get the bandwidth utilization reports form PacketShaper as well as MRTG.
> The main problem is that the Peak Utilization reports from both are not matching.
> I have got the following document from the PacketShaper site regarding the measurement technique:


> For all Packetwise enabled devices, for both current and peak rate calculations, we use a proprietary algorithm (EWMA) - exponential weighted moving average. The interval for the EWMA is approximately one second.
> For average rate, we use EWMA to get the average rate for that second. We take x samples during that second and use EWMA to get the average for that second. We calculate the average rate for 1 minute interval using EWMA on all one second data. For intervals longer than 1 minute, the Average Rate is calculated by applying the EWMA formula to the 1 minute averages. So the average rate always remains the average during the interval.

Keywords: proprietary, interval ... one second, ...take x samples during that second ...

MRTG has an entirely different sampling interval, its main use is average rates.
MRTG takes two samples, 5 minutes apart, and calculates the average.  There's no
way that MRTG can know about one-second intervals (without hacking it).

Measuring multiple times in one second is something you do not want
to try over the network, you need hardware for this.  The resulting
numbers can then be transfered over the network.

 / alex at slot.hollandcasino.nl                  alex at ergens.op.het.net \
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