The status of duplex mode on this port.
When linkbeat is not present, halfduplex is always
reported.
When linkbeat is present, the result of full duplex
auto-negotiation is reported if c2900PortDuplexState is
set to auto-negotiate, otherwise it reports fullduplex
if c2900PortDuplexState is set to fullduplex or
halfduplex if c2900PortDuplexState is set to
halfduplex.