[mrtg] Graph under reporting utilization

[Fred Weston]

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 192.168.198.1:
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 -- COLO-STACK

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.

I'm not sure why there's a discrepancy.  Am I doing something wrong or
missing something simple?

Thanks,
FW