Cisco Tip: Setting Switch Port to Auto/Auto

If you want to change a hard-coded switch port back to its default setting of auto/auto (speed/duplex), set the port speed to auto. On a CatOS switch, duplex can either be coded as full or half, not auto.

Also, on 10/100-Mbps Fast Ethernet ports, you cannot change the duplex mode manually if the port speed is set to auto.