There are two ways to force duplex & speed on a Solaris 10 box - via the driver, and via ndd. First, via the driver:
# cat
<<EOD > /platform/`uname -i`/kernel/drv/bge.conf adv_autoneg_cap=0
adv_1000fdx_cap=0 adv_1000hdx_cap=0 adv_100fdx_cap=1 adv_100hdx_cap=0
adv_10fdx_cap=0 adv_10hdx_cap=0; EOD
Note that this sets all instances of bge to 100Mbit Full Duplex. If you wish to be more selective, you can do this:
# cat < /etc/init.d/net-tune
#!/bin/sh # Force to 100FDX NIC=bge for i in 0 1 2 3; do /usr/sbin/ndd -set
/dev/${NIC}${i} adv_1000fdx_cap 0 /usr/sbin/ndd -set /dev/${NIC}${i}
adv_1000hdx_cap 0 /usr/sbin/ndd -set /dev/${NIC}${i} adv_100fdx_cap 1
/usr/sbin/ndd -set /dev/${NIC}${i} adv_100hdx_cap 0 /usr/sbin/ndd -set
/dev/${NIC}${i} adv_10fdx_cap 0 /usr/sbin/ndd -set /dev/${NIC}${i}
adv_10hdx_cap 0 /usr/sbin/ndd -set /dev/${NIC}${i} adv_autoneg_cap 0 done EOD
# chmod 755 /etc/init.d/net-tune # ln -s /etc/init.d/net-tune /etc/rc2.d
/S68net-tune
You have to reboot for the kernel config file to take effect, but you can run the net-tune script at any time to make it work. You can change the 0 1 2 3 in the for do loop above to set the instances you need.