Cisco IOS Software Basics

This chapter describes what you need to know about the Cisco IOS software before you configure the router using the command-line interface (CLI). This chapter includes the following:

Understanding these concepts will save you time and aggravation later. If you have never used the Cisco IOS software or need a refresher, take a few minutes to read this chapter before you proceed to the next chapter.

If you are already familiar with the Cisco IOS software, proceed to Chapter 3, "Configuring with the Command-Line Interface."

Getting Help

Use the question mark (?) and arrow keys to help you enter commands:

For a list of available commands, enter a question mark:

Router> ?

To complete a command, enter a few known characters followed by a question mark (with no space):

Router> s?

For a list of command variables, enter the command followed by a space and a question mark:

Router> show ?

To redisplay a command you previously entered, press the up arrow key. You can continue to press the up arrow key for more commands.

Understanding Command Modes

The Cisco IOS user interface is divided into different modes. Each command mode permits you to configure different components on your router. The commands available at any given time depend on which mode you are currently in. Entering a question mark (?) at the prompt displays a list of commands available for each command mode. Table 2-1 lists the most common command modes.

Command Mode	Access Method	Router Prompt Displayed	Exit Method
User EXEC	Log in.	`Router>`	Use the logout command.
Privileged EXEC	From user EXEC mode, enter the enable command.	`Router#`	To exit to user EXEC mode, use the disable, exit, or logout command.
Global configuration	From the privileged EXEC mode, enter the configure terminal command.	`Router (config)#`	To exit to privileged EXEC mode, use the exit or end command, or press Ctrl-z.
Interface configuration	From the global configuration mode, enter the interface type number command, such as interface serial 0/0.	`Router (config-if)#`	To exit to global configuration mode, use the exit command. To exit directly to privileged EXEC mode, press Ctrl-z.

Table 2-1: Common Command Modes

Command Mode Access Method Router Prompt Displayed Exit Method

User EXEC

Log in.

Router>

Use the logout command.

Privileged EXEC

From user EXEC mode, enter the enable command.

Router#

To exit to user EXEC mode, use the disable, exit, or logout command.

Global configuration

From the privileged EXEC mode, enter the configure terminal command.

Router (config)#

To exit to privileged EXEC mode, use the exit or end command, or press Ctrl-z.

Interface configuration

From the global configuration mode, enter the interface type number command, such as interface serial 0/0.

Router (config-if)#

To exit to global configuration mode, use the exit command.

To exit directly to privileged EXEC mode, press Ctrl-z.

TimeSaver
Each command mode restricts you to a subset of commands. If you are having trouble entering a command, check the prompt, and enter the question mark (?) for a list of available commands. You might be in the wrong command mode or using the wrong syntax.

In the following example, notice how the prompt changes after each command to indicate a new command mode:

Router> enable
Router# configure terminal
Router(config)# interface serial 0/0
Router(config-if)# line 0
Router(config-line)# controller t1 0
Router(config-controller)# exit
Router(config)# exit
Router#
%SYS-5-CONFIG_I: Configured from console by console

: The last message is normal and does not indicate an error. Press Return to get the Router# prompt.

Note You can press Ctrl-z in any mode to immediately return to enable mode (Router#), instead of entering exit, which returns you to the previous mode.

Undoing a Command or Feature

If you want to undo a command you entered or disable a feature, enter the keyword no before most commands; for example, no ip routing.

Saving Configuration Changes

You need to enter the copy running-config startup-config command to save your configuration changes to nonvolatile random-access memory (NVRAM), so the changes are not lost if there is a system reload or power outage. For example:

Router# copy running-config startup-config
Building configuration...

It might take a minute or two to save the configuration to NVRAM. After the configuration has been saved, the following appears:

[OK]
Router#

Where to Go Next

Now that you have learned some Cisco IOS software basics, you can begin to configure the router using the CLI.

Remember that:

You can use the question mark (?) and arrow keys to help you enter commands.
Each command mode restricts you to a set of commands. If you have difficulty entering a command, check the prompt and then enter the question mark (?) for a list of available commands. You might be in the wrong command mode or using the wrong syntax.
To disable a feature, enter the keyword no before the command; for example, no ip routing.
You need to save your configuration changes to NVRAM so the changes are not lost if there is a system reload or power outage.

Proceed to Chapter 3, "Configuring with the Command-Line Interface," to begin configuring the router.

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