NFE setting manually to 1000baseT and half duplex

Jon Radel jon at
Sun Jun 8 11:02:03 UTC 2008

Gelsema, P (Patrick) - FreeBSD wrote:
> Hi List,
> I am using the nfe driver on Freebsd 7.0R and I am unable to change 
> the NIC driver manually to 1000baseT with half-duplex. I believe I am
> not getting the max out of my network connection and want to see if 
> changing the duplex will help.

Even if you do have hardware that supports half-duplex gigabit ethernet
on both ends, the need to do carrier extension for any frame shorter
than 512 bytes so that CSMA/CD actually works on a reasonable sized
cable, does horrible things to your throughput if you've got lots of
small frames. (In other words, at gigabit speeds, frames smaller than
512 bytes zip down the wire so quickly that you can no longer reliably
detect collisions, so the frames all get padded.) I'm having trouble 
wrapping my head around any circumstances other than horribly, horribly 
broken hardware or software where half-duplex would increase your 
performance over full-duplex.

That said, most (an imprecise way saying "every time I've looked this
has been the case, but I generally no longer bother looking") gigabit
ethernet hardware I've ever touched has been incapable of doing
half-duplex when it's being used at gigabit speeds.  The specs for doing
it exist more for theoretical completeness than out of practical
utility.  See, for example

for a discussion on this and related topics.

My suggestion would be to let both sides auto-detect if they're both
capable of gigabit ethernet.

--Jon Radel

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