Here is a quick how to force your network interface speed and duplex on RHEL/CentOS Linux.
Make sure you have the network tools installed :
yum install ethtool net-tools
To apply the configuration without having to restart the network service or reboot your system, simply do this command with your settings :
ethtool -s <INTERFACE> speed <100|1000|10000> duplex <half|full> autoneg <on|off>
Example :
ethtool -s eth0 speed 1000 duplex full autoneg off
The connectivity will be briefly interrupted. You may have to log into your server after applying it.
To make those changes permanent and survive to reboot, edit your network interface config file (this example assume your interface is eth0) :
/etc/sysconfig/network-scripts/ifcfg-eth0
Add this line at the end of your configuration file :
ETHTOOL_OPTS="speed <100|1000|10000> duplex <half|full> autoneg <on|off>"
Again, an example with the values we used above :
ETHTOOL_OPTS="speed 1000 duplex full autoneg off"