If you’re experiencing slow speeds on a Fastnet provided Leased Line there is a test you can run that will enable us to help troubleshoot further.
Fastnet will typically provide you with a managed Cisco router which connects to the fibre ONT that the carrier installs. Behind the router you may have a Fastnet provided firewall and wireless equipment, or this may be equipment that you have provided yourselves or via a third party. If this is the case, depending on where you’re connecting into the network, it could be a switch, firewall or wireless equipment causing the slow speeds rather than the circuit itself. The aim here is to see what speeds the circuit can achieve. We can then work back plugging the firewall back in etc to see where a bottleneck in the network could be.
If you’ve run a test here: [insert Fastnet speed test link] and you’re expected to achieve much higher than what is displayed, the next best step would be to run an iPerf test.
An iPerf test is used to measure the performance of a network connection, especially its bandwidth (throughput).
In simple terms, it works like this:
- You run iPerf on two devices (a client and a server).
- The client sends traffic to the server (or vice versa).
- iPerf measures how much data can be transferred over the network in a given time.
What it tells you:
- Maximum achievable bandwidth
- Throughput consistency
- Packet loss (for UDP tests)
- Latency/jitter (mainly with UDP)
Why it’s used:
- To check network speed between two points
- To troubleshoot slow networks
- To validate performance of links like LAN, WAN, or VPN connections
In short: iPerf simulates network traffic to show how well your network performs.
You will need a laptop / node that can connect directly via an Ethernet cable to one of the Cisco LAN ports. Please ensure you have installed iPerf; you can download it here: https://iperf.fr/iperf-download.php
Once installed and connected to the Cisco LAN (please ensure all of the other LAN ports are empty before you proceed), you will need to set some IP credentials. If you’re unsure of the IP addresses to use, please contact the Fastnet technical department for assistance.
Please open the command prompt feature (run as administrator) and run the following command. This is based on a 1Gb service:
iperf3 -c iperfpublic-lon.fastnet.co.uk -u -R -b 900m -t 30
This test connects to our London iPerf server and runs a 30‑second test where the server sends UDP traffic back to you at 900 Mbps, so you can measure your download performance (including packet loss and jitter).
Once complete, please send the results over to help@fastnet.co.uk, which will open a ticket with our technical team. We can then diagnose further and come back to you with the next steps.