3.2 Basic Configuration of the Switch
3.2.7 Defining line mode on a switch

Full-duplex is the simultaneous action of transmitting and receiving data by two devices. This operation is achievable only if the devices on each end support full-duplex.

Full-duplex links not only double potential throughput, but also eliminate collisions and the need for each station to wait until the other station finishes transmitting. If reads and writes on a full-duplex link are symmetric, data throughput can be theoretically doubled. However, in reality, bandwidth improvements are more modest.

Full-duplex links are particularly useful for server-to-server, server-to-switch, and switch-to-switch connections.

To set the duplex mode of an interface on a Cisco IOS software-based switch, enter the following command in interface configuration mode:

Switch(config-if)#duplex auto | full | half

Parameter Definition
auto Sets the 100BASE-TX port into auto-negotiation mode; this is the default for the 100BASE-TX port; this argument is valid on 100BASE-T ports only
full Forces the 10BASE-T or 100BASE-TX port into full-duplex mode
half Forces the 10BASE-T or 100BASE-TX port into half-duplex mode; this is the default for a 10BASE-T port

Note: Use the auto argument only for fixed Fast Ethernet TX ports. In auto-negotiation mode, the switch attempts to negotiate full-duplex connectivity with the connecting device. If negotiation is successful, the port operates in full-duplex mode. If the connecting device is unable to operate in full duplex, the port operates in half duplex. This process is repeated whenever there is a change in link status.

The example in Figure shows that the fixed port FastEthernet 0/2 is configured for full-duplex mode. To return the duplex parameter to the default setting, enter the no duplex command in the interface configuration mode. 

To set the port duplex mode on a set-based switch, enter the following command in privileged mode:

Switch> (enable) set port duplex mod/port full | half

where

  • Half-duplex mode is the default for 10-Mbps ports.
  • Full-duplex mode is the default for 100-Mbps ports.


Note: You cannot change the duplex mode of ports configured for auto-negotiation.

Use the show port command to verify your configuration. The example in Figure shows that the 10/100 Ethernet module 6 port 1 is connected and is operating in full-duplex mode. It is important to note that sometimes ports are not activated by default. To activate a port, enter the set port enable mod/port command in privileged mode.