Duplex and Speed

Switch ports can be manually configured with specific duplex and speed settings. Use the duplex interface configuration mode command to manually specify the duplex mode for a switch port. Use the speed interface configuration mode command to manually specify the speed for a switch port. In Figure 1, port F0/1 on switch S1 and S2 are manually configured with the full keyword for the duplex command, and the 100 keyword for the speed command.

The default setting for both duplex and speed for switch ports on Cisco Catalyst 2960 and 3560 switches is auto. The 10/100/1000 ports operate in either half- or full-duplex mode when they are set to 10 or 100 Mb/s, but when they are set to 1000 Mb/s (1 Gb/s), they operate only in full-duplex mode. Cisco recommends only using the auto command for duplex and the speed command to avoid connectivity issues between devices. When troubleshooting switch port issues, the duplex and speed settings should be checked.

Note: Mismatched settings for the duplex mode and speed of switch ports can cause connectivity issues. Auto negotiation failure creates mismatched settings.

All fiber optic ports, such as 100BASE-FX ports, operate only at one preset speed and are always full-duplex.

Use the Syntax Checker in Figure 2 to configure port F0/1 of switch S1.