Perhaps you can begin with a review of the man pages for ifconfig and for ndd.
If I recall correctly, an interface will default to its fastest designed link speed and mode,
which in this century would be an auto-negotiated 100Mbit,
or an auto-negotiated Gigabit, per the chipset and the drivers in your OS of choice.
Only a manual intervention would muck that up to something slower.
> Perhaps you can begin with a review of the man pages
> for ifconfig and for ndd.
I don't see information in either man page that would help someone find the right thing to type for speed of a network device.
If you just need to check and not set, then 'kstat' is often the easiest way.
# kstat -n eri0 | grep speed
ifspeed 100000000
(Speed is in bits per second).
--
Darren