Chapter 3 Installing the Router
Cisco IOS User Interface
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Cisco 12006 and Cisco 12406 Router Installation and Configuration Guide
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Global Configuration Mode

The router can be configured manually using the global configuration mode
through the Cisco IOS command line interface (CLI). This method requires you
to enter configuration commands on a line-by-line basis at the system console,
without being prompted by the setup command configuration script.
Cisco IOS User Interface
The Cisco IOS software provides a command line interface by which you can
configure and manage your router. If you are unfamiliar with the Cisco IOS
command line interface, refer to the “Using the Command Line Interface” chapter
in the Cisco Configuration Fundamentals Configuration Guide, which discusses
different command modes, context-sensitive help, and editing features.

User Interface Command Modes

The Cisco IOS user interface is organized into many different modes. The
commands that are available depend on which mode you are currently in. Entering
a question mark (?) at the system prompt displays a list of commands available for
the current command mode.
When you start a session on the router, you begin in user mode, often called EXEC
mode. Only a limited subset of commands are available in EXEC mode. To have
access to all commands, enter privileged EXEC mode, which requires that you
enter a password. From privileged EXEC 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
configuration status, 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 and can be used
when you reboot your router. Starting at global configuration mode, you can enter
interface configuration mode, subinterface configuration mode, and other
protocol-specific modes.