If this is a Solaris system, check for a script in /etc/rc2.d called S68net-tune. That seems to be the 'recommended' way of doing it.
Otherwise,
example: forced 1000 Mbit/s on an UTP
# ndd -set /dev/ce instance 0 <-- settings for ce0
# ndd -set /dev/ce link_master 0 <-- for switch connection
# ndd -set /dev/ce adv_autoneg_cap 0
# ndd -set /dev/ce adv_1000fdx_cap 1
# ndd -set /dev/ce adv_1000hdx_cap 0
# ndd -set /dev/ce adv_100fdx_cap 0
# ndd -set /dev/ce adv_100hdx_cap 0
# ndd -set /dev/ce adv_10fdx_cap 0
# ndd -set /dev/ce adv_10hdx_cap 0
This is the
link