Cisco IOS Software Fundamentals

This chapter describes what you need to know about Cisco IOS software before you configure your Cisco 6400 universal access concentrator using command-line interface (CLI) commands. If you have never used Cisco IOS software or need a refresher, take a few minutes to read this chapter before you begin to configure your access concentrator.

This chapter includes the following:

2.1 Getting Context-Sensitive Help

You can get context-sensitive help in any of the command modes. The help available in the Cisco IOS software describes the command syntax or displays the complete command name. To get context-sensitive help, use the question mark (?) and arrow keys to help you enter commands.

For a list of available commands for a particular command mode, enter a question mark:

Switch>?

For a list of commands that begin with a particular character string, enter a few known characters followed by a question mark (with no space):

Switch> sh?
show

To complete a partial command name, enter a few known characters followed by the Tab character (with no space):

Switch> sh<Tab>
Switch> show

For a list of a command's associated keywords, enter the command followed by a space and a question mark:

Switch> show?

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

Refer to the chapter "Configuring the User Interface" in the Configuration Fundamentals Configuration Guide for more information about any aspect of working with the user interface in the Cisco IOS software. The Configuration Fundamentals Configuration Guide is part of the Cisco IOS documentation in Cisco IOS Releases 11.1 and later.

2.2 Understanding Command Modes

The Cisco IOS user interface is divided into many different modes. The commands available to you at any given time depend on which mode you are currently in. Entering a question mark (?) at the system prompt allows you to obtain a list of commands available for each command mode.

When you start a session on the router, you begin in user mode, often called EXEC mode. Only a limited subset of the commands are available in EXEC mode. In order to have access to all commands, you must enter privileged EXEC mode. Normally, you must enter a password to enter privileged EXEC mode. From privileged mode, you can enter any EXEC command or enter global configuration mode. Most of the EXEC commands are one-time commands, such as show commands, which show the current status of something, and clear commands, which clear counters or interfaces. The EXEC commands are not saved across reboots of the router.

The configuration modes allow you to make changes to the running configuration. If you later save the configuration, these commands are stored across router reboots. In order to get to the various configuration modes, you must start at global configuration mode. From global configuration mode, you can enter interface configuration mode, subinterface configuration mode, and a variety of protocol-specific modes.

ROM monitor mode is a separate mode used when the router cannot boot properly. If your router or access server does not find a valid system image when it is booting, or if its configuration file is corrupted at startup, the system might enter read-only memory (ROM) monitor mode.

Summary of Main Command Modes

Table 2-1 summarizes the main command modes of the Cisco IOS software.

Table 2-1: Summary of Main Command Modes

Command Mode Access Method Prompt Exit Method

User EXEC

Log in.

Switch>

Use the logout command.

Privileged EXEC

From user EXEC mode, use the enable EXEC command.

Switch#

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

To enter global configuration mode, use the configure terminal privileged EXEC command.

Global configuration

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

Switch(config)#

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

To enter interface configuration mode, enter an interface configuration command.

Interface configuration

From global configuration mode, enter by specifying an interface with an interface command.

Switch(config-if)#

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

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

Subinterface configuration

From interface configuration mode, specify a subinterface with an interface command.

Switch(config-subif)#

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

To enter privileged EXEC mode, use the end command or press Ctrl-Z.

ROM monitor

From privileged EXEC mode, use the reload EXEC command. Press the Break key during the first 60 seconds while the system is booting.

>

rommon>

To exit to user EXEC mode, type continue.

Command Mode	Access Method	Prompt	Exit Method
User EXEC	Log in.	`Switch>`	Use the logout command.
Privileged EXEC	From user EXEC mode, use the enable EXEC command.	`Switch#`	To exit back to user EXEC mode, use the disable command. To enter global configuration mode, use the configure terminal privileged EXEC command.
Global configuration	From privileged EXEC mode, use the configure terminal privileged EXEC command.	`Switch(config)#`	To exit to privileged EXEC mode, use the exit or end command or press Ctrl-Z. To enter interface configuration mode, enter an interface configuration command.
Interface configuration	From global configuration mode, enter by specifying an interface with an interface command.	`Switch(config-if)#`	To exit to global configuration mode, use the exit command. To exit to privileged EXEC mode, use the exit command or press Ctrl-Z.
Subinterface configuration	From interface configuration mode, specify a subinterface with an interface command.	`Switch(config-subif)#`	To exit to global configuration mode, use the exit command. To enter privileged EXEC mode, use the end command or press Ctrl-Z.
ROM monitor	From privileged EXEC mode, use the reload EXEC command. Press the Break key during the first 60 seconds while the system is booting.	`>` rommon>	To exit to user EXEC mode, type continue.

For more information regarding command modes, refer to the "Using the Command-Line Interface" chapter of the Configuration Fundamentals Configuration Guide.

2.3 Saving Configuration Changes

Enter the copy system:running-config nvram:startup-config command to save your configuration changes to your startup configuration so that they will not be lost if there is a system reload or power outage. For example:

Switch# copy system:running-config nvram:startup-config
Building configuration...

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

[OK]
Switch#

2.4 Where to Go Next

Now that you have learned some Cisco IOS software basics, you are ready to configure your access concentrator using the command-line interface (CLI). For complete information regarding Cisco IOS software, refer to the section "Related Documentation" in the chapter "About This Guide."

Use the following helpful reminders when configuring your Cisco IOS software:

Use the question mark (?) and arrow keys to help you enter commands.
Each command mode restricts you to a set of commands. If you are having 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.
Enter the keyword no before a command if you want to disable a feature; for example, no ip routing.
When using the interface command, remember to use the two-bar (/) command syntax to identify slot, subslot, and port locations (slot/subslot/port). For example,the command interface atm 1/0/0 identifies an ATM interface at slot 1, subslot 0, port 0.
Save your configuration changes to NVRAM so they will not be lost if there is a system reload or power outage.

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