Speed / Duplex Auto-Negotiation

Interfaces that can run at different speeds (10/100 or 10/100/1000) have default settings of speed auto and duplex auto.

Interfaces 'advertise' their capabilities to the neighboring device, and they negotiate the best speed and duplex settings they are both capable of.

What if autonegotiation is disabled on the device connected to the switch?

Speed

The switch will try to sense the speed that the other device is operating at.

If it fails to sense the speed, it will the slowest supported speed.

Duplex

If the speed is 10 or 100 Mbps, the switch will use half duplex.

If the speed is 1000 Mbps or greater, use full duplex.