[mrtg] Graph under reporting utilization

Pavel Ruzicka pavouk at pavouk.org
Wed Nov 5 00:25:00 CET 2014

Hi Fred,

it is because you use 32bit counters in SNMPv1. You must switch MRTG
to SNMPv2 which automatically try to read 64bit counters instead 32.
There are ifHCInOctets and ifHCOutOctets OIDs.

You can add :::::2 to end of your target.
Something like this:
Target[colo-metro-ethernet-utilization]: 50:public at

Please read MRTG documentation about SNMP options.
You can also automatically generate MRTG config with "cfgmaker".

Best regards,

> I am running MRTG 2.16.2, which I realize is quite old.  I haven't had
> occassion or need to upgrade it in quite some time.
> I use it to monitor several network links and it works very well for
> all but one.  I am using it to monitor the interface utilization of a
> 1 Gb/s ethernet port on a Cisco 3850 switch and while MRTG does get
> data and creates a graph, I don't believe the data it is presenting is
> accurate.
> Here is the config snippet from my mrtg.cfg:
> Target[colo-metro-ethernet-utilization]: 50:public at
> SetEnv[colo-metro-ethernet-utilization]: MRTG_INT_IP=""
> MRTG_INT_DESCR="GigabitEthernet1/0/48"
> MaxBytes[colo-metro-ethernet-utilization]: 125000000
> Title[colo-metro-ethernet-utilization]: Traffic Analysis for 50 --
> Here is an example of the daily graph that is generated by MRTG:
> https://lpga.box.com/shared/static/vefa3l9dxhmdgifgpw6f.png
> The problem is that these numbers do not seem correct to me (they seem
> to be significantly under reporting utilization).
> That seems to be confirmed by what the switch itself says:
> 5 minute input rate 46968000 bits/sec, 14177 packets/sec
> 5 minute output rate 306860000 bits/sec, 28679 packets/sec
> This was taken from the switch around the same time the graph above
> was generated.  The switch seems to indicate the 5 min utilization
> rate is about 58.5 MB/s while MRTG is only showing about 18.2 MB/s.

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