Cisco IOS Software Basic Skills

Understanding how to use Cisco IOS software can save you time when you are configuring your router. This appendix contains the following sections which provide basic information:

If you are already familiar with Cisco IOS software, go to the following chapter:

Configuring the Router from a PC

You can configure your router from a PC that is connected through the console port by using terminal emulation software. The PC uses this software to send commands to your router. Table A-1 lists some common types of terminal emulation software that you can use, depending on the operating system that you are running.

 

Table A-1 Types of Terminal Emulation Software
PC Operating System
Terminal Emulation Software

Windows 95, Windows 98, Windows 2000, Windows NT, Windows XP

HyperTerm (included with Windows software), ProComm Plus

Windows 3.1

Terminal (included with Windows software)

Macintosh

ProComm, VersaTerm

You can use the terminal emulation software to change settings for the router that is connected to the PC. Configure the software to the following standard VT-100 emulation settings so that your PC can communicate with your router:

  • 9600 baud
  • 8 data bits
  • No parity
  • 1 stop bit
  • No flow control

These settings should match the default settings of your router. To change the router baud, data bits, parity, or stop bits settings, you must reconfigure parameters in the ROM monitor. For more information, see the “ROM Monitor” section. To change the router flow control setting, use the flowcontrol command in global configuration mode.

For information on how to enter global configuration mode so that you can configure your router, see the “Entering Global Configuration Mode” section.

Understanding Command Modes

This section describes the Cisco IOS command mode structure. Each command mode supports specific Cisco IOS commands. For example, you can use the interface type number command only from global configuration mode.

The following Cisco IOS command modes are hierarchical. When you begin a router session, you are in user EXEC mode.

  • User EXEC
  • Privileged EXEC
  • Global configuration

Table A-2 lists the command modes that are used in this guide, describes how to access each mode, shows the prompt for each mode, and explains how to exit to a mode or enter another mode. Because each mode configures different router elements, you might need to enter and exit modes frequently. You can see a list of available commands for a particular mode by entering a question mark (?) at the prompt. For a description of each command, including syntax, see the Cisco IOS Release 12.3 documentation set.

 

Table A-2 Command Modes Summary

Mode
Access Method
Prompt
Mode Exit and Entrance
About This Mode

User EXEC

Begin a session with your router.

Router>

To exit a router session, enter the logout command.

Use this mode to:

  • Change terminal settings.
  • Perform basic tests.
  • Display system information.

Privileged EXEC

Enter the enable command from user EXEC mode.

Router#

  • To exit to user EXEC mode, enter the disable command.
  • To enter global configuration mode, enter the configure command.

Use this mode to:

  • Configure your router operating parameters.
  • Perform the verification steps shown in this guide.

To prevent unauthorized changes to your router configuration, protect access to this mode by using a password as described in the Enable Secret Passwords and Enable Passwords.

Global configuration

Enter the configure command from privileged EXEC mode.

Router (config)#

  • To exit to privileged EXEC mode, enter the exit or end command or press Ctrl-Z.
  • To enter interface configuration mode, enter the interface command.

Use this mode to configure parameters that apply to your router globally.

From this mode, you can access the following modes:

  • Interface configuration
  • Router configuration
  • Line configuration

Interface configuration

Enter the interface command (with a specific interface, such as interface atm 0 ) from global configuration mode.

Router (config-if)#

  • To exit to global configuration mode, enter the exit command.
  • To exit to privileged EXEC mode, enter the end command or press Ctrl-Z.
  • To enter subinterface configuration mode, specify a subinterface by using the interface command.

Use this mode to configure parameters for the router Ethernet and serial interfaces or subinterfaces.

Router configuration

Enter one of the router commands followed by the appropriate keyword—for example router rip —from global configuration mode.

Router (config- router)#

  • To exit to global configuration mode, enter the exit command.
  • To exit to privileged EXEC mode, enter the end command or press Ctrl-Z.

Use this mode to configure an IP routing protocol.

Line configuration

Enter the line command with the desired line number and optional line type, for example, line 0, from global configuration mode.

Router (config- line)#

  • To exit to global configuration mode, enter the exit command.
  • To exit to privileged EXEC mode, enter the end command or press Ctrl-Z.

Use this mode to configure parameters for the terminal line.

Getting Help

You can use the question mark (?) and arrow keys to help you enter commands.

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

Router> ?
access-enable Create a temporary access-list entry
access-profile Apply user-profile to interface
clear Reset functions
.
.
.
 

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

Router> sh?
* s=show set show slip systat
 

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

Router> show ?
.
.
.
clock Display the system clock
dialer Dialer parameters and statistics
exception exception information
.
.
.
 

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

Enable Secret Passwords and Enable Passwords

By default, the router ships without password protection. Because many privileged EXEC commands are used to set operating parameters, you should password-protect these commands to prevent unauthorized use.

You can use two commands to do this:

  • enable secret password —A very secure, encrypted password.
  • enable password —A less secure, unencrypted local password.

Both the enable and enable secret passwords control access to various privilege levels (0 to 15). The enable password is intended for local use and is thus unencrypted. The enable secret password is intended for network use; that is, in environments where the password crosses the network or is stored on a TFTP server. You must enter an enable secret or enable password with a privilege level of 1 to gain access to privileged EXEC mode commands.

For maximum security, the passwords should be different. If you enter the same password for both during the setup process, your router accepts the passwords but warns you that they should be different.

An enable secret password can contain from 1 to 25 uppercase and lowercase alphanumeric characters. An enable password can contain any number of uppercase and lowercase alphanumeric characters. In both cases, a number cannot be the first character. Spaces are also valid password characters; for example, two words is a valid password. Leading spaces are ignored; trailing spaces are recognized.

Entering Global Configuration Mode

To make any configuration changes to your router, you must be in global configuration mode. This section describes how to enter global configuration mode while using a terminal or PC that is connected to your router console port.

To enter global configuration mode, follow these steps:

Step 1 After your router boots up, enter the enable or enable secret command:

Router> enable
 

Step 2 If you have configured your router with an enable password, enter it when you are prompted.

The enable password does not appear on the screen when you enter it. This example shows how to enter privileged EXEC mode:

Password: enable_password
Router#
 

Privileged EXEC mode is indicated by the pound sign (#) in the prompt. You can now make changes to your router configuration.

Step 3 Enter the configure terminal command to enter global configuration mode:

Router# configure terminal
Router(config)#
 

You can now make changes to your router configuration.

 

Using Commands

This section provides some tips about entering Cisco IOS commands at the command-line interface (CLI).

Abbreviating Commands

You only have to enter enough characters for the router to recognize the command as unique. This example shows how to enter the show version command:

Router # sh v
 

Undoing Commands

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

Command-Line Error Messages

Table A-3 lists some error messages that you might encounter while using the CLI to configure your router.

 

Table A-3 Common CLI Error Messages

Error Message
Meaning
How to Get Help
% Ambiguous command:
"show con"

You did not enter enough characters for your router to recognize the command.

Re-enter the command, followed by a question mark (?) with no space between the command and the question mark.

The possible keywords that you can enter with the command are displayed.
% Incomplete command.

You did not enter all the keywords or values required by this command.

Re-enter the command, followed by a question mark (?) with no space between the command and the question mark.

The possible keywords that you can enter with the command are displayed.
% Invalid input detected at ‘^’ marker.

You entered the command incorrectly. The error occurred where the caret mark (^) appears.

Enter a question mark (?) to display all the commands that are available in this particular command mode.

Saving Configuration Changes

You must enter the copy running-config startup-config command to save your configuration changes to NVRAM so that they are not lost if there is a system reload or power outage. This example shows how to use this command to save your changes:

Router# copy running-config startup-config
Destination filename [startup-config]?
 

Press Return to accept the default destination filename startup-config or enter your desired destination filename and press Return.

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

Building configuration...
Router#
 

Summary

Now that you have reviewed some Cisco IOS software basics, you can begin to configure your router. Remember:

  • 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 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.
  • To disable a feature, enter the keyword no before the command; for example, no ip routing.
  • Save your configuration changes to NVRAM so that they are not lost if there is a system reload or power outage.

Where to Go Next

To configure your router, go to the “Basic Router Configuration” section.