# vi /etc/sysconfig/network-scripts/ifcfg-eth0

Append the line as follows to disable Auto-Negotiation, enable Full Duplex and set up Speed to 100 Mb/s :

ETHTOOL_OPTS=”speed 100 duplex full autoneg off”

Restart the interface to apply changes :

# ifdown eth0 && ifup eth0 

temporary change or live OS can be done using ethtool.

# ethtool -s eth0 speed 100 duplex full autoneg off

http://www.shellhacks.com/en/HowTo-Change-Speed-and-Duplex-of-Ethernet-card-in-Linux