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| 1.4 |  | Basic Configuration of the Switch |
| 1.4.9 |  | Defining link speed and line mode on a switch |

Setting the port speed and line mode (duplex) are among the most common configuration requirements for a network engineer. It can be frustrating to see messages constantly appear on the console screen, complaining about speed or duplex mismatches. Although autonegotiation is generally supposed to take care of speed configuration, frequently the setting must be hard-coded. Much frustration can be avoided by proactively configuring port speed rather than to depend on autonegotiation.

The order of configuration for speed and duplex is important. First, configure the speed for a port. Next, follow with the duplex configuration.

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.

The following guidelines can serve as a reference when configuring speed and the duplex settings on IOS-based switches:

- Gigabit Ethernet ports should always be set to 1000 Mbps, but they can negotiate full-duplex with the attached device. To connect to a remote Gigabit Ethernet device that does not autonegotiate, disable autonegotiation on the local device, and set the duplex parameters to be compatible with the other device. Gigabit Ethernet ports that do not match the settings of an attached device lose connectivity and do not generate statistics.
- To connect to a remote 100BASE-T device that does not autonegotiate, set the duplex setting to full-duplex or half-duplex, and set the speed setting to autonegoriation. Autonegotiation for the speed setting selects the correct speed even if the attached device does not autonegotiate, but the duplex setting must be explicitly set.
- 10/100/1000 ports can operate at 10 or 100 Mbps when they are set to half-duplex or full-duplex, but they operate in the full-duplex mode only when set to 1000 Mbps.
- Gigabit Interface Converter (GBIC) module ports operate only at 1000 Mbps.
- 100BASE-FX ports operate at only 100 Mbps in full-duplex.

On a Cisco IOS-based switch, the speed of the port is set using the speed {10|100|1000|auto} command from the interface mode.

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

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

To view the configuration changes, use the show interface mod/port and show running-configuration commands. -