autoneg

Autonegotiation, often shortened to autoneg, is a communication process used in computer networking to automatically determine the transmission speed and duplex mode for a network link. It is defined by the IEEE 802.3 standard. When two network devices, such as a network interface card (NIC) and a switch port, are connected, they can use autonegotiation to establish the optimal settings for their communication.

The process involves both devices advertising their supported speeds and duplex modes to each other. They then compare these advertised capabilities and agree on the highest common speed and the appropriate duplex setting (half-duplex or full-duplex). For example, if one device supports 10 Mbps half-duplex and 100 Mbps full-duplex, and the other supports 100 Mbps full-duplex and 1 Gbps full-duplex, they would agree on 100 Mbps full-duplex.

Autonegotiation simplifies network configuration by eliminating the need for manual setup of these parameters. It helps prevent mismatches that can lead to poor performance or a complete lack of connectivity. While generally reliable, autonegotiation can sometimes fail, especially when connecting older or non-standard equipment, or in the presence of network errors. In such cases, manual configuration of speed and duplex settings may be necessary. Modern networking equipment overwhelmingly supports and uses autonegotiation.