Force network speed on a network card to 100 MBit Full Duplex and in the same time disable auto negotiation

# ethtool -s eth0 speed 100 duplex full autoneg off

These settings are active until you reboot the machine or restart the network.

In order to keep this change on each system reboot, append the following line in the following Redhat file /etc/sysconfig/network-scripts/ifcfg-eth0

ETHTOOL_OPTS="speed 100 duplex full autoneg off"

An alternative tool to use to change the settings is mii-tool.