speed [10|100|1000|auto]
Sets the speed of a Cisco switch or router interface to a specific value or auto-negotiation.
speed [10|100|1000|auto]When to Use This Command
- Manually setting a switch port to 100 Mbps full duplex to match an older device that doesn't support auto-negotiation.
- Forcing a router WAN interface to 1000 Mbps to ensure maximum throughput on a known gigabit link.
- Setting a port to 10 Mbps to intentionally limit bandwidth for traffic shaping or testing.
- Configuring auto-negotiation on a port to allow the device to automatically select the best speed with the connected device.
Command Examples
Setting interface speed to 100 Mbps
speed 100Switch(config-if)# speed 100 Switch(config-if)#
The command sets the interface speed to 100 Mbps. No output is shown if successful; the prompt returns.
Setting interface speed to auto-negotiate
speed autoSwitch(config-if)# speed auto Switch(config-if)#
The command enables auto-negotiation on the interface. The device will automatically negotiate speed with the connected device.
Understanding the Output
The 'speed' command does not produce any output on its own; it simply configures the interface. To verify the speed setting, use 'show interfaces [interface]' or 'show interfaces status'. In 'show interfaces', look for the line 'BW 100000 Kbit' for 100 Mbps or 'BW 1000000 Kbit' for 1000 Mbps. In 'show interfaces status', the 'Speed' column shows the configured or negotiated speed. A mismatch between the configured speed and the actual link speed indicates a problem, often due to duplex mismatch or faulty cabling.
CCNA Exam Tips
Remember that on Cisco switches, the 'speed' command is available in interface configuration mode, and the default is 'auto'.
Know that if you set speed manually, you should also set duplex manually to avoid mismatch; auto-negotiation handles both.
The CCNA exam may test that 'speed 1000' is only valid on Gigabit Ethernet interfaces; 10/100 ports cannot be set to 1000.
Be aware that 'speed auto' is the default and is recommended for most scenarios to avoid configuration errors.
Common Mistakes
Setting speed to 1000 on a FastEthernet (10/100) port, which will be rejected with an error.
Forgetting to set duplex when manually setting speed, leading to duplex mismatch and poor performance.
Assuming 'speed auto' will always result in the highest speed; it depends on the capabilities of both devices.
Related Commands
duplex [full|half|auto]
Sets the duplex mode on a switch or router interface to full, half, or auto-negotiation.
show interfaces
Displays detailed status and statistics for all interfaces or a specific interface, used to verify interface operational state, errors, and performance.
show interfaces status
Displays a summary of all switch interfaces including their status, VLAN, duplex, speed, and type, used to quickly verify interface connectivity and configuration.
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