[mrtg] Two copies of MRTG disagree with each other about usage

[Zach White]

Hi Everyone,

I have an odd problem that I'm hoping someone can shed some light on.

I have MRTG 2.10.13 setup on an OpenBSD 3.7 machine. It polls my switch, a
catalyst 2924, every 5 minutes. Data coming from my switch seems to
be valid and agrees with what iptraf on my firewall says about current 
usage.

The ethernet drop from my ISP comes into port 1 on the switch, and port
2 goes to the firewall. These ports are in their own vlan so that data
from our network doesn't get comingled in.

My ISP uses MRTG 2.9.29, I don't know what OS. They poll the port that
the other end of my ethernet drop uses every 5 minutes.

The problem is that the data I'm seeing and the data my ISP is seeing
don't match up. The usage they're seeing is a lot higher than what I'm
seeing, but the curve of the graph matches up pretty well.

For example, looking at the weekly graph, with a 30 minute average,
my copy of MRTG reports a peak of 2800.0k. Their copy of MRTG is
reporting a peak in the same period of something around 23.0M. Comparing
more data shows about the same ratio between my data and their data.

I've been working with one of their network engineers and so far neither
of us have been able to figure out where the problem lies. I'm currently
working on getting them to let me monitor their switch directly and have
them monitor my switch directly to make sure each switch is reporting
the same data, but it's unclear as to whether they'll go for this.

So, ignoring that it may an issue with either my switch or theirs, does
anyone have any ideas as to where the problem may lie? A small difference
I'd understand, but the data they're showing is consistantly 8-16x higher
than what I show for the same period.

I've appended my config and their config (properly sanitized) below.

Thanks for any light you can shed on this issue.

-Zach

My Config:
Target[10.0.x.x_2]: 2:xxxxxxx at 10.0.x.x:
SetEnv[10.0.x.x_2]: MRTG_INT_IP="" MRTG_INT_DESCR="FastEthernet0/1"
MaxBytes[10.0.x.x_2]: 12500000
Title[10.0.x.x_2]: Traffic Analysis for 2 -- polestar-sac-sw1
PageTop[10.0.x.x_2]: <H1>Traffic Analysis for 2 -- polestar-sac-sw1</H1>
*snip rest of html*

Their Config:
Target[Polestar_UDS2-234]: 458:xxxxxx at 65.74.x.x:
SetEnv[Polestar_UDS2-234]: MRTG_INT_IP=""
MRTG_INT_DESCR="FastEthernet8/10"
Directory[Polestar_UDS2-234]: polestar
MaxBytes[Polestar_UDS2-234]: 12500000
Title[Polestar_UDS2-234]: Traffic Analysis for Polestar FE8/10 UDS2-234
-- Sac2-UDS2
PageTop[Polestar_UDS2-234]: <H1>Traffic Analysis for Polestar FE8/10
*snip rest of html*

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