There are three useful setting that permit getting information using ethtool:

ethtool -i eth0 display the first NIC driver setting.
ethtool -a eth0 displays information about autonegotiation
ethtool -S eth0 display information about speed and other parameters

Setting specific interface parameters with ethtool
To set specific parameters usually option -s is used:

ethtool -s eth0 speed 100 duplex full autoneg off to force the first NIC to operate at fix and non-negotiable speed at 100Mbps full duplex.

Note! In order to retain the change on each system reboot, append the following line in file
Red Hat directory/file: /etc/sysconfig/network-scripts/ifcfg-eth0:

Parameter: ETHTOOL_OPTS="speed 100 duplex full autoneg off"
Suse directory/file: /etc/sysconfig/network (suse defines interfaces via MAC)

Parameter: ETHTOOL_OPTIONS='speed 100 duplex full autoneg off'