What is auto-negotiation / speed sensing?
Auto-negotiation and speed sensing allows the port of a network device to automatically detect the speed and duplex setting of the link (10/100/1000 and half or full duplex) and negotiate link with the connected device. Due to a history of consistency and reliability issues with 10/100 Auto-negotiation network administrators and engineers often configure links that include 10/100 devices to 100 Full Fixed. If two Auto-enabled devices fail to properly establish link they will typically default to a lower speed half duplex setting – serious performance issues on the link result. Also, if an Auto enabled device is connected to a device with Fixed settings they will sometimes initially establish link (and sometimes not) but will often fail to re-establish link of a port “flaps” on one of the endpoints or either of them is rebooted.
