Set 1000Mbps full-duplex, enter:

  ifconfig <interface-name> <IP_address> media 1000baseTX mediaopt full-duplex

For example, set interface em0 with IP 10.10.1.2 to 100Mbps full duplex, enter:

   ifconfig em0 10.10.1.2 media 100baseTX mediaopt full-duplex

If the interface is currently forced to 100 full duplex, in order to change to half duplex you must type the following command:

  ifconfig em0 10.10.1.2 media 100baseTX -mediaopt full-duplex

The -mediaopt option disable the specified media options (full-duplex) on the interface i.e. go back to half duplex.