How to measure available network bandwidth

This is an issue I see from time to time. A server appears to be “slow” on the network. When you run into this kind of an issue it can be difficult to troubleshoot as there are a considerable number of variables that may be causing the “slowness”. One of the first tests I like to perform is to test the available network bandwidth. It is fairly easy to test and will cut the possible culprits in half.

Below is a step-by-step on how to test, accompanied by a diagram. A note on testing. In the diagram below the Problem server and Working server have the same network path back to the Client PC. This allows an apples-to-apples comparison to be made.

iperf bandwidth diagram

Download the Iperf utility. You can get it from here. Copy it to the Client PC, Problem server and Working server.
Extract the Iperf zip file on the Client PC, Problem server and Working server.
On the Client PC, open an administrative command prompt and navigate to the folder with the extracted Iperf binaries. Run the following command*: Iperf3.exe -s
On the Problem server, open an administrative command prompt and navigate to the folder with the extracted Iperf binaries. Run the following command**: Iperf3.exe -c 192.168.1.9 -w 2m -t 30s -i 1s
Repeat step 4 on the Working server

The Working server and Problem server can now be compared directly. If the speed and amount of data transferred are very close, then the network can be virtually eliminated as a cause of the “slowness”. Also it is unlikely there is a network card hardware or driver issue.
However if the Problem server transferred significantly less data or the transfer speed was significantly lower, then the network needs a closer look. To rule out the network switch, swap the network switch ports of the two servers, and repeat step 4 and 5 above. To rule out the network cable, swap the network cables of the two server and repeat step 4 and 5 above. If problems continue, then update the network card driver and firmware on the problem server. If at this point the issue remains, it is time to take a closer look at the operating system on the Problem server.

*Iperf runs on port 5201 by default. If a firewall is enabled on the Client PC, the firewall will need to be disabled or an inbound rule created
**Change the IP address to match the system you ran the iperf.exe -s command on.