no shutdown
Administratively enables a network interface. This command signals the Cisco IOS to power up the physical port hardware, begin transmitting keepalives, and attempt to bring the line protocol state to "Up".
Quick Reference
Router(config-if)#Syntax Breakdown
This is a standalone execution command with no trailing arguments.
By default, for security and topology stability, all interfaces on a Cisco Router are shipped in a shutdown state. You must explicitly authorize the interface to begin passing traffic by negating the shutdown state. Conversely, most Cisco Catalyst switchports default to an active state so they can be "plug and play" out of the box.
CLI Deployment Scenarios
Scenario 1: Activating a New Router Interface
You have just assigned an IP address to a router interface. Until you issue the 'no shutdown' command, the interface will not process packets.
Scenario 2: The "Bounce" Technique
A connected device is unresponsive, or an interface seems stuck. A common troubleshooting step is to "bounce" the port by shutting it down and bringing it back up.
CCNA Exam Gotchas
Status Code Translation
When reading a show ip interface brief output, you must know the difference between failure states.
If the status is "Administratively Down", an engineer typed shutdown (or forgot to type no shut).
If the status is "Down", the port is enabled in the software, but there is a Layer 1 physical problem (e.g., unplugged cable, dead switch on the other end, bad transceiver).
Recovering from Err-Disable
If port security is triggered (e.g., an unauthorized MAC address is detected), the switch will throw the port into an err-disable state. Typing no shutdown immediately will do absolutely nothing. To recover an err-disabled port, you must execute shutdown to clear the error state flag, and then execute no shutdown to bring it back online.