Full duplex

Full duplex means 2 channels where each channel goes in the opposite direction of the other. What this means is that at 100 Mbps full duplex you can send 100 Mbps of data and receive 100 Mbps of data for an aggregate of 200 Mbps. Collisions in full duplex cannot happen. Ethernet switches are full-duplex most of the time.

Forcing an interface into full duplex mode

In BSD you can see the media modes of an interface with the ifconfig command:

$ ifconfig -m $xl0
xl0: flags=8843<UP,BROADCAST,RUNNING,SIMPLEX,MULTICAST> mtu 1500
       lladdr 00:60:08:5a:86:82
       media: Ethernet 100baseTX full-duplex
       status: no carrier
       supported media:
               media none
               media 10baseT
               media 10baseT mediaopt full-duplex
               media 100baseTX
               media 100baseTX mediaopt full-duplex
               media autoselect
       inet 172.16.2.2 netmask 0xfffffe00 broadcast 172.16.3.255
       inet6 fe80::260:8ff:fe5a:8682%xl0 prefixlen 64 scopeid 0x2

To set it to full duplex you would type:

$ ifconfig $xl0 media 100baseTX mediaopt full-duplex

In Linux you'd use mii-tool to see the duplex setting of an interface.