Quickly verify what speed and duplex your interface negotiated and act on mismatches. 27.12.2025 | reading time: 2 min On a Linux host he can quickly discover the negotiated link speed and duplex between a network interface and its partner; this short guide shows the commands, a real output example and quick remedies. Hands-on example Run the command and read the Speed, Duplex and Auto-negotiation lines; example: ```sudo ethtool eth0 Settings for eth0: Supported ports: [ TP ] Supported link modes: 10baseT/Half 10baseT/Full 100baseT/Half 100baseT/Full 1000baseT/Full Supports auto-negotiation: Yes Advertised link modes: 10baseT/Full 100baseT/Full 1000baseT/Full Speed: 1000Mb/s Duplex: Full Port: Twisted Pair Auto-negotiation: on Link detected: yes ``` Look for `Speed`, `Duplex` and whether `Auto-negotiation` is on; those three tell what the NIC and its partner agreed on. Interpretation in practice If you see a lower speed than expected first check cable type and switch port; mismatched duplex used to cause slow or erratic performance but today autoneg usually prevents it; if you must force a setting use `sudo ethtool -s eth0 speed 100 duplex full autoneg off` but be cautious because forcing can break connectivity with the link partner. Edge cases to watch Optical SFP modules or certain 10G cards sometimes report "Unknown!" for Speed and require vendor tools or check `dmesg` for PHY details; for wireless use `iw dev wlan0 link` or `iwconfig` to see the current bitrate rather than ethtool. Quick extra checks Inspect interface statistics with `ip -s link` and vendor NIC counters with `ethtool -S`, verify switch port configuration if available and consult `journalctl -k` or `dmesg` for PHY messages when negotiation fails. Final thought Knowing how to read link negotiation is a small skill that prevents long troubleshooting sessions and gives immediate clues about cabling, switch or NIC problems; explore deeper networking topics and consider formalizing skills with courses and exams such as CompTIA Linux+ or LPIC-1 via bitsandbytes.academy. Join Bits & Bytes Academy First class LINUX exam preparation. network troubleshooting utilities
