ethtool speed HowTo : Change Speed and Duplex of Ethernet card in Linux

To change Speed and Duplex of an ethernet card, we can use ethtool - a Linux utility for Displaying or Changing ethernet card settings.

1. Install ethtool

You can install ethtool by typing one of the following commands, depending upon your Linux distribution.

Install ethtool in Fedora, CentOS, RHEL etc. :
# yum install ethtool

Install ethtool in Ubuntu, Debian etc. :
# sudo apt-get install ethtool

2. Get the Speed, Duplex and other information for the interface eth0

To get speed, duplex and other information for the network interface eth0, type the following command as root.
# ethtool eth0
Sample output :

Settings for eth0:

	Supported ports: [ MII ]

	Supported link modes:   10baseT/Half 10baseT/Full

	                        100baseT/Half 100baseT/Full

	                        1000baseT/Half 1000baseT/Full

	Supports auto-negotiation: Yes

	Advertised link modes:  10baseT/Half 10baseT/Full

	                        100baseT/Half 100baseT/Full

	                        1000baseT/Half 1000baseT/Full

	Advertised auto-negotiation: Yes

	Speed: 100Mb/s

	Duplex: Half

	Port: Twisted Pair

	PHYAD: 1

	Transceiver: internal

	Auto-negotiation: on

	Supports Wake-on: g

	Wake-on: d

	Current message level: 0x000000ff (255)

	Link detected: yes

3. Change the Speed and Duplex settings

The following changes are temporary and they'll stop working after reboot. Read the next section, to make settings permanent.
The next command enables Auto-Negotiate feature :
# ethtool -s eth0 autoneg on
The next command disables Auto-Negotiation, enables Half Duplex and sets up Speed to 10 Mb/s :
# ethtool -s eth0 speed 10 duplex half autoneg off
The next command disables Auto-Negotiation, enables Full Duplex and sets up Speed to 100 Mb/s :
# ethtool -s eth0 speed 100 duplex full autoneg off

4. Change the Speed and Duplex settings Permanently on CentOS/RHEL

To make settings permanent, you need to edit /etc/sysconfig/network-scripts/ifcfg-eth0 file for eth0 interface. This file is used by RHEL, CentOS, Fedora etc.
# 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

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