Source: http://www.cyberciti.biz/faq/linux-change-the-speed-and-duplex-settings-of-an-ethernet-card/
Under Linux use mii-tool or ethtool package which allows a Linux sys admin to modify/change and view the negotiated speed of network interface card (NIC) i.e. it is useful for forcing specific Ethernet speed and duplex settings.
Depending on which type of Ethernet card is installed on the system you need to use either mii-tool or ethtool. I recommend installing both and use one of the tool, which will work with your card.
Task: Install mii-tool and ethtool tools
If you are using Debian Linux you can install both of these package with following command:
# apt-get install ethtool net-tools
If you are using Red Hat Enterprise Linux you can install both of these package with following command:
# up2date ethtool net-tools
If you are using Fedora Core Linux you can install both of these package with following command:
# yum install ethtool net-tools
Task: Get speed and other information for eth0
Type following command as root user:
# ethtool eth0
Output:
Settings for eth0:
Supported ports: [ TP MII ]
Supported link modes: 10baseT/Half 10baseT/Full
100baseT/Half 100baseT/Full
Supports auto-negotiation: Yes
Advertised link modes: 10baseT/Half 10baseT/Full
100baseT/Half 100baseT/Full
Advertised auto-negotiation: Yes
Speed: 100Mb/s
Duplex: Full
Port: MII
PHYAD: 32
Transceiver: internal
Auto-negotiation: on
Supports Wake-on: pumbg
Wake-on: d
Current message level: 0x00000007 (7)
Link detected: yes
Or use mii-tool command as follows:
# mii-tool eth0
Output:
eth0: negotiated 100baseTx-FD flow-control, link ok
Task: Change the speed and duplex settings
Setup eth0 negotiated speed with mii-tool
Disable autonegotiation, and force the MII to either 100baseTx-FD, 100baseTx-HD, 10baseT-FD, or 10baseT-HD:
# mii-tool -F 100baseTx-HD
# mii-tool -F 10baseT-HD
Setup eth0 negotiated speed with ethtool
# ethtool -s eth0 speed 100 duplex full
# ethtool -s eth0 speed 10 duplex half
To make these settings permanent you need to create a shell script and call from /etc/rc.local (Red Hat) or if you are using Debian create a script into the directory /etc/init.d/ directory and run update-rc.d command to update the script.
Read man page of mii-tool and ethtool for more information.
