On a given switch, I attempt to assign 'duplex' and 'speed' to gigabit switchports and some ports accept the command and others don't. I run show cdp neig and the respective ports which does not accept is not connected to anything.
speed and duplex commands are accepted only on ports using a GBIC that provides RJ-45 electrical signals:
>> media type is 10/100/1000BaseTX
ports with fiber based GBICs and ports like your g0/23 with a missing GBIC
>> Auto-duplex, Auto-speed, link type is auto, media type is Not Present <<
don't accept these commands.
fiber based GBICs are 1000 full fixed.
Hope to help