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Autonegotiation

Full duplex, half duplex and auto-negotiation have the following meanings.
 

• Full Duplex - Logic that enables concurrent sending and receiving. This is usually desirable and enabled when your computer is connected to a switch.
   
• Half Duplex - This logic requires a card to only send or receive at a single point of time. When your machine is connected to a Hub, it auto-negotiates itself and uses half duplex to avoid collisions.
   
• Auto-negotiation - This is the process of deciding whether to work in full duplex mode or half duplex mode. An ethernet card supporting autonegotiation will decide for itself which mode is the optimal one depending on the network it is attached to.

There are three useful command to get information about interface using ethtool:

• ethtool eth0 displays setting.

  root@nti01:~ # ethtool eth1
  Settings for eth1:
          Supported ports: [ TP ]
          Supported link modes:   10baseT/Half 10baseT/Full
                                  100baseT/Half 100baseT/Full
                                  1000baseT/Full
          Supports auto-negotiation: Yes
          Advertised link modes:  Not reported
          Advertised auto-negotiation: No
          Speed: 100Mb/s
          Duplex: Full
          Port: Twisted Pair
          PHYAD: 1
          Transceiver: internal
          Auto-negotiation: off
          Supports Wake-on: d
          Wake-on: d
          Link detected: yes

• ethtool -a eth0 displays information about autonegotiation
• ethtool -S eth0 displays many parameters
• mii-tool -v 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'