Autonegotiation

Autonegotiation is a feature of Ethernet networks that allows two connected devices to automatically exchange information about their capabilities and to select the best common combination of speed and duplex for the link. It is defined in IEEE 802.3 and is typically used on copper twisted-pair Ethernet connections.

During link establishment, each device advertises its supported capabilities, such as available speeds (for example 10/100/1000 Mbps), duplex modes (half or full), and, where supported, flow control options. The two devices exchange these advertisements and negotiate the highest speed and full duplex that both ends can support. If no common capability exists, the link may fail to come up or fall back to a lower, pre-configured setting.

Autonegotiation may also encompass other negotiated parameters on certain media, and it is usually enabled by default on compliant devices. In practice, autonegotiation minimizes configuration errors and supports plug-and-play operation between switches, network adapters, and routers. Some networks disable autonegotiation to use fixed settings for stability or to enforce specific configurations.

If problems occur, common causes include mismatched autonegotiation settings, bad cabling, or hardware failure. Troubleshooting typically involves verifying that both ends are set to autonegotiation or to the same fixed speed and duplex, checking link status indicators, and testing with known-good cables.