Right now, I am running a backup job over the network, its running at 1000Kbits/s - The full speed of the switch.
The switch confirms the speed, the backup program confirms the speed, and the increasing file size confirms the speed. All report the network is running at 1000Mbits/s
But PRTG (Traffic Sensor) says 400Mbit, and I sort of know why...
This type of behaviour only happens with scans at the interval of 60 seconds. If I change it down to 30 seconds then it starts to report 1000Mbits/s. If I use "Scan Now" over and over again, it reports correct too.
If I set the Interval down to 5 minutes, then it says its running about 100Mbits/s
The switch in question is a D-Link DGS-1224, but it may happen with other switches.
This is either a bug, or incompatibility. Sure it has its work around, but still hard to spot. My guess is some limit on the 32bit counter used on the switch, but maybe PRTG can detect this and advise to set the interval down so it does not misread this? False data is 100 times worse than no data at all. Happy to discuss.
Anyway, Hope this helps other people, and hope it can be investigated also.