enable duplex

[Jacob Friis Larsen <jfl@list.idg.dk>]

I have a HP ProLiant DL140 with Integrated Dual Broadcom 10/100/1000 NICs.

Am I correct that duplex means that I can use the two jacks as one?

If so, do I enable it like this:
ethtool -s eth0 speed 100 duplex full autoneg off

What happens if one of the jacks suddenly is not connected?

Thanks,
Jacob