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

See ethtool --help or the very long ethtool manual page for details.