ethtool is used for querying settings of an ethernet device and changing them. For more information on ethtool, you go to Using ethtool to check and change Ethernet Card Settings and Forcing NIC to operate at Full Duplex using Ethtool
To use ethtool to set NIC to operate at Full Duplex and 100Mb and autonegotiate off, you can use the following commands
# ethtool -s eth0 speed 100 duplex full autoneg off
To force the NIC to use full duplex, 100Mbps, and autonegotiate off and make it permanent, you can put this in /etc/sysconfig/network-scripts/ifcfg-eth0
ETHTOOL_OPTS="speed 100 duplex full autoneg off"
To verify that the settings is correct,do
# ethtool eth0 (or eth1 depending the NIC you are using)