Probably done by adding an option to /etc/sysconfig/network-scripts/ifcfg-ethX. 
I think the option might be driver dependant as to whether it will support it.

man ifconfig:

       media type
              Set the physical port or medium type to be used  by
              the  device.   Not all devices can change this set-
              ting, and those that can vary in what  values  they
              support.  Typical values for type are 10base2 (thin
              Ethernet), 10baseT (twisted-pair 10Mbps  Ethernet),
              AUI  (external transceiver) and so on.  The special
              medium type of auto can be used to tell the  driver
              to  auto-sense  the  media.  Again, not all drivers
              can do this.


Technomage wrote:
> 
> I have the same question. I have a 100BaseT card here and it defaults to
> 10BaseT on me as well (its an intel ether-express pro).
> 
> it would be nice to change this without having to go through a dos level
> reset of the device
> 
> technomage
> 
> On Monday 04 February 2002 15:38, you wrote:
> > Do anyone know of a way of permanently setting 100 MB/s - Full Duplex on a
> > NIC in RedHat?  I have to manually change the settings upon every reboot
> > using the mii-tool command.  Is there a config file to set?  Would an rc
> > script be appropriate?  Any advice would be appreciated.
> >
> > Thanks,
> >
> > Brian
> > ________________________________________________
> > See http://PLUG.phoenix.az.us/navigator-mail.shtml if your mail doesn't
> > post to the list quickly and you use Netscape to write mail.
> >
> > PLUG-discuss mailing list  -  PLUG-discuss@lists.plug.phoenix.az.us
> > http://lists.PLUG.phoenix.az.us/mailman/listinfo/plug-discuss
> ________________________________________________
> See http://PLUG.phoenix.az.us/navigator-mail.shtml if your mail doesn't post to the list quickly and you use Netscape to write mail.
> 
> PLUG-discuss mailing list  -  PLUG-discuss@lists.plug.phoenix.az.us
> http://lists.PLUG.phoenix.az.us/mailman/listinfo/plug-discuss