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 foreth0:
	Supportedports: [MII ]
	Supportedlink modes:   10baseT/Half 10baseT/Full 
	                        100baseT/Half 100baseT/Full 
	                        1000baseT/Half 1000baseT/Full 
	Supports auto-negotiation: Yes
	Advertisedlink 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
	Currentmessage level: 0x000000ff (255)
	Linkdetected: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

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