How do I check a dedicated server NIC’s link speed and duplex?
Link speed and duplex describe the local connection to the adjacent device, not end-to-end application throughput or the service’s bandwidth allowance. A slow transfer can still be limited by CPU, storage, congestion or provider policy. Inspect the correct physical interface before changing negotiation settings.
Before you start
- Authorised access to the dedicated server and the named inspection tools.
Instructions
1. Identify the actual interface
Use the current interface names rather than assuming eth0. A bond, bridge or VLAN can sit above the physical port.
ip -br linkFind the relevant physical NIC and its state. Record the higher-level interface carrying the server’s address as well.
2. Read negotiated link information
Replace enp1s0 with the identified physical interface. This reporting form does not force speed or duplex.
sudo ethtool enp1s0Review Link detected, Speed, Duplex and negotiation capabilities. Unsupported or unknown values can occur with some drivers or virtual devices.
Check the result
- The NIC and switch/provider port agree on the intended supported link, and workload throughput is evaluated separately under representative conditions.
Troubleshooting
- The link negotiated below expectations
- Check transceiver/cable support, switch configuration, NIC firmware and provider inventory.
- Forcing speed is suggested
- Coordinate both ends and model support; an unreviewed change can sever remote access or create a mismatch.
- The link is fast but transfers are slow
- Measure the full path and application bottleneck before blaming duplex.
Official references
Use the documentation for your installed version when a flag, service name or platform feature differs.