Autonegotiation lets connected interfaces automatically agree on compatible speed and duplex settings. Cisco IOS enables it by default, and it usually works well for network connections. section 8.1.2 section 8.2
Each interface advertises its capabilities, and both sides select the best mode they support. Higher speeds take priority, followed by full duplex over half duplex. section 8.2
Router-to-switch negotiation
A router capable of 1 Gbps connects to a switch supporting only 100 Mbps. Autonegotiation makes both interfaces operate at 100 Mbps/full duplex, the best shared mode.
Use these Cisco IOS commands to configure or verify negotiation:
- Use speed auto to enable speed autonegotiation, or speed 10, speed 100, or speed 1000 to force a speed in Mbps. section 8.1.2
- Use duplex auto to enable duplex autonegotiation, or select duplex full or duplex half for a manual setting. section 8.1.3
Leave autonegotiation enabled for end devices such as PCs because their supported speeds vary. Engineers may manually configure infrastructure links, but inconsistent settings and human error can create problems. section 8.2
Duplex mismatch
When one side autonegotiates and the other side is manually configured, the negotiating side may sense the speed but select half duplex. The link remains up, yet performance drops and collisions, late collisions, Runts, or CRC errors may increase. section 8.3.2
