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.
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 but may fail to re-establish link if a port goes up and down or if endpoints reboot.