Ethernet stuck at 100 Mbps: five things to check
A gigabit link uses all eight wires of the cable. A 100 Mbps link uses only four of them: pins 1, 2, 3 and 6. If one of the other four wires does not connect, the two devices usually settle on 100 Mbps and the connection keeps working, so the fault goes unnoticed. Check five things, in this order: the ports, the cable, the plugs, the condition of the cable and the adapter settings.
1. The ports at each end
Look at the network port at each end of the cable. Some televisions, older routers and other devices have a 100 Mbps port. Then the link cannot go faster, whatever cable joins them. The speed of each port is in the device's specifications.
2. Swap the cable
Connect the same two devices with a cable you know works at 1 Gbps. If the link now shows 1 Gbps, the first cable is the cause. If it still shows 100 Mbps, go back to the ports and check the adapter settings.
3. The plugs
If you fitted the plugs yourself, check that all eight wires reach the end of the plug and that both ends use the same wiring scheme, T568A or T568B. A cable tester lights each pin from 1 to 8. A pin that does not light, or lights only sometimes, points to that plug: cut it off and fit a new one. The steps are in the Installation Guide.
4. The condition of the cable
A cable that has been crushed, pierced or bent sharply can break a wire inside the jacket without any visible mark. If the tester shows the same fault after you fit new plugs, replace the cable.
5. The adapter settings
In the computer's network adapter settings, the speed should be set to automatic, not fixed at 100 Mbps.
Our cables
Mr. Tronic Cat 5e and Cat 6 cables are rated at 1 Gbps. A higher category does not fix a link stuck at 100 Mbps: the cause is one of the five checks above.
Cat 5e construction: 4 twisted pairs, 24 AWG. Conductor: CCA (copper-clad aluminium). Supports Gigabit Ethernet (1 Gbps).
Cat 6 construction: 4 twisted pairs, 24 AWG, cross separator. Conductor: CCA (copper-clad aluminium). Supports Gigabit Ethernet (1 Gbps).