If the Duplex value shows a red asterisk (*) behind the label, it indicates that the duplex setting could not be read from the device because the device does not support RFC 2665.

In this case, the duplex setting is estimated based on the presence or absence of collision error counters on the interface. I.e. If an interface is shown as Full Duplex, and then at a later time the interface encounters collisions, the duplex setting on the interface would change to Half Duplex.

If Interfaces show as “Half” without any asterisk, then that’s the exact setting that the switch is telling us. If it is telling us incorrect information, then this might be a problem with the Device.

If the interface shows “Auto” as the duplex setting on the device, the interface may “automatically” select half-duplex if it detects that the far-end device can only do half-duplex. It may also “automatically” incorrectly detect the remote device’s duplex setting. This may be why “Auto” ports show up as half-duplex.