CISCO IOS in a Nutshell

Basic Interface Configuration Commands

The following basic commands apply to just about any type of interface.

shutdown

The shutdown command disables an interface. The interface does not transmit packets after it has been shut down; all routing protocols are informed that the interface is unavailable.

interface serial0
  shutdown

Use the no shutdown command to restart an interface that has been shut down:

Router#config terminal
Router(config)#interface serial 0							
Router(config-if)#no shutdown

There can be side effects to an interface shutdown. The nature of these side effects depends on the interface type. Table 5-2 shows some of the possible side effects.

Table 5-2. Possible side effects of an interface shutdown

Interface	Side effect of a shutdown
Ethernet	Drops link-status indicator to the remote hub or switch
Serial	Drops DTR signal
FDDI	Activates optical bypass switch
Token ring	Removes interface from the token ring

The shutdown command can also be applied to a subinterface. In this case, it stops protocol processing on that subinterface without affecting the other subinterfaces or dropping the entire interface.

Interface Descriptions

An interface’s description shows up in the router’s configuration and in the output from the show interfaces command. This description is for informational purposes only; it helps you remember the configuration of the interface. For example:

interface serial0 description T1 Connection to Baltimore (Good place to document the circuit id!) ...