Autonegotation

Feature that allows Network Switch or Router to communicate with other device to consider the Transmission Mode for the connection.

How it works

Autonegotation works as a protocol, it works if both devices enable autonegotation protocol.

The protocol will automatically find the best Transmission Mode and speed according to the highest that's available.

For example:

• If device A has tranmission mode 10/100/1000 but only 1000 Mbps is full-duplex. Whereas device B has 10/100/1000 and they're all support full-duplex. Autonegotation process will choose 1000 Mbps / Full duplex

When would autonegotiation fail

Autonegotation often fail if the the other device does not turn on Autonegotation. Or there is a autonegotiation conflict.

Autonegotation conflict happens when device is setting in different duplex mode (e.g 100/Full duplex vs 100/Half duplex

• This is because for half-duplex, TX needs to wait for RX to finish before can transmit
• However if the other device is full-duplex, it will not monitor RX and keeps sending TX. Therefore the half-duplex will never reply

[!important]
Because of this, make sure they're configured the same way

What happen after autonegotiation fail

After autonegiation fail, a process called Parallel Detection will run in order to find the best speed (not the transmission mode). And then you need to set the duplex mode manually

Configuring autonegotiation on Cisco

1. You need to config the speed first by speed ?
2. Once you setting the speed, you can set duplex mode to auto, full or half: duplex ?