Bug 29871

using:
xl0 driver
3com 900b 10BaseT full-duplex PCI adapter

 in 4.3-release:

#ifconfig xl0 media 10BaseT/UTP mediaopt full-duplex

ifconfig reports the media change, but the adapter remains at half-duplex mode physically.

History-
The adapter was working in full-duplex mode in the same machine with 3.2-RELEASE just prior to this. My boot sector got wiped and I did not have a copy of my disk label, so I formatted and installed 4.3. The adapter is connected to a switch that reports whether each node is running full-duplex. On system powerup, the switch reports the adapter is set for full-duplex. When BSD boots, and the devices are brought up - the device defaults to half-duplex mode through auto-select (this makes no sense to me, barring backwards compatibility concern). Again, the switch reflects the change in media, and ifconfig reflects the same. When ifconfig is run to change the media, as above, the kernel reports the media selection - but it never physically appears to happen. Peer-to-peer data transfer never exceeds 100KB/s, and it should be a little over 1MB so I know I'm not halucinating. I'm going to try reverting to 3.2 for the time being, but this may be something that needs looking into.

Fix: 

no solution
How-To-Repeat: not known exactly.

system specs-
processor- AMD k6-3 450 on EPOX MVP3G-M MB, 1MB cache model
RAM- 192M pc100
net adapter - 3COM 3c900b 10BaseT/UDP full-duplex capable