HOWTO lock a network card to 100MBit
If you have a network card capable of doing gigabit speeds or higher and you want to force it into a lower locked speed it's
ethtool to the rescue.
You can use
ethtool to low a network card to 100MBit with this fine command:
ethtool -s eth0 autoneg off speed 100
ethtool --help or the very long ethtool manual page for details.