Basic Cisco IOS Skills

Configuring the Router from a PC

Understanding Command Modes

Getting Help

Enable Secret and Enable Passwords

Entering Global Configuration Mode

Using Commands

Abbreviating Commands

Command-Line Error Messages

Undoing Commands

Saving Configuration Changes

Using Debug Commands

Where to Go Next

Basic Cisco IOS Skills

This chapter describes what you need to know before you begin configuring your router with Cisco IOS software (the software that runs your router).

Note Cisco recommends that inexperienced network administrators use the Cisco 800 Fast Step application to configure their routers.

This chapter contains the following sections:

Understanding these concepts saves you time when you are configuring your router. If you have never used 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 IOS software, you can proceed to the configuration chapter that is appropriate for your network.

Configuring the Router from a PC

If you are configuring your router from a PC (not a dumb terminal), you need a type of communications software called terminal emulation software. The PC uses this software to send commands to your router. Table 2-1 lists some common names for this software, based on the type of PC you are using.

Table 2-1: Terminal Emulation Software

PC Operating System Software

Windows 95, Windows 98, Windows NT

HyperTerm (included with Windows software)

Windows 3.1

Terminal (included with Windows software)

Macintosh

ProComm, VersaTerm

PC Operating System	Software
Windows 95, Windows 98, Windows NT	HyperTerm (included with Windows software)
Windows 3.1	Terminal (included with Windows software)
Macintosh	ProComm, VersaTerm

You can use the terminal emulation software to change settings for the type of device that is connected to the PC, in this case a router. Configure the software to the following settings so that your PC can communicate with your router:

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

You can now configure your router using your PC.

Understanding Command Modes

This section describes the IOS command mode structure. Each command mode supports specific IOS commands. For example, the interface type number command is used only from global configuration mode.

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

User EXEC
Privileged EXEC
Global configuration

From global configuration mode, you can access the following modes:

Interface configuration
Router configuration
Line configuration

Table 2-2 lists the command modes that are used in this guide, how to access each mode, the prompt you see in that mode, and how to exit to a previous mode or enter the next mode.

Table 2-2: Command Modes Summary

Mode Access Method Prompt Exit/Entrance Method About This Mode¹

User EXEC

Begin a router session.

router>
To exit router session, enter logout.

Use this mode to:

Change terminal settings

Perform basic tests

Display system information

Privileged EXEC

Enter enable from user EXEC mode.

router#
To exit to user EXEC mode, enter disable.

To enter global configuration mode, enter configure.

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, access to this mode should be protected with a password.

Global configuration

Enter configure from privileged EXEC mode.

router
(config)#
To exit to privileged EXEC mode, enter exit or end or press Ctrl-Z.

To enter interface configuration mode, enter interface.

Use this mode to configure parameters that apply to your router as a whole.

Interface configuration

Enter interface (with a specific interface) from global configuration mode.

router
(config-if)#
To exit to global configuration mode, enter exit.

To exit to privileged EXEC mode, enter end or press Ctrl-Z.

To enter subinterface configuration mode, specify a subinterface with the interface command.

Use this mode to configure parameters for the router Ethernet and ISDN interfaces.

Router configuration

Enter your router command followed by the appropriate keyword from global configuration mode.

router (config-
router)#
To exit to interface configuration mode, enter exit.

To exit to privileged EXEC mode, enter end or press Ctrl-Z.

Use this mode to configure an IP routing protocol.

Line configuration

Specify a line with the line vty command from global configuration mode.

router (config-
line)#
To exit to router configuration mode, enter exit.

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

Use this mode to configure parameters for the terminal line.

¹For any of the modes, you can see a comprehensive list of the available commands by entering a question mark (?) at the prompt.

Mode	Access Method	Prompt	Exit/Entrance Method	About This Mode¹
User EXEC	Begin a router session.	router>	To exit router session, enter logout.	Use this mode to: Change terminal settings Perform basic tests Display system information
Privileged EXEC	Enter enable from user EXEC mode.	router#	To exit to user EXEC mode, enter disable. To enter global configuration mode, enter configure.	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, access to this mode should be protected with a password.
Global configuration	Enter configure from privileged EXEC mode.	router (config)#	To exit to privileged EXEC mode, enter exit or end or press Ctrl-Z. To enter interface configuration mode, enter interface.	Use this mode to configure parameters that apply to your router as a whole.
Interface configuration	Enter interface (with a specific interface) from global configuration mode.	router (config-if)#	To exit to global configuration mode, enter exit. To exit to privileged EXEC mode, enter end or press Ctrl-Z. To enter subinterface configuration mode, specify a subinterface with the interface command.	Use this mode to configure parameters for the router Ethernet and ISDN interfaces.
Router configuration	Enter your router command followed by the appropriate keyword from global configuration mode.	router (config- router)#	To exit to interface configuration mode, enter exit. To exit to privileged EXEC mode, enter end or press Ctrl-Z.	Use this mode to configure an IP routing protocol.
Line configuration	Specify a line with the line vty command from global configuration mode.	router (config- line)#	To exit to router configuration mode, enter exit. To enter privileged EXEC mode, enter end 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 at that command mode, 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.

Enable Secret and Enable Passwords

Because many privileged EXEC commands are used to set operating parameters, you should use passwords to protect these commands.

You can use two commands to do this:

enable secret password (a very secure, encrypted password)
enable password (a less secure, unencrypted password)

You must enter enable secret password 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 variable can contain from 1 to 25 uppercase and lowercase alphanumeric characters. An enable password variable 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.

If you lose or forget your enable password, refer to Chapter 9, "Troubleshooting."

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:

Step 1 After your router boots ups, answer no when the following question displays:

Would you like to enter the initial configuration dialog [yes]: no

Step 2 Enter the enable command, and if you have configured your router with an enable password, enter the enable password when you are prompted for it.

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

router> enable
Password: enable password
router#
Enable mode is indicated by the # in the prompt. You can now make changes to your router configuration.

Step 3 Enter the configure terminal command to enter global configuration mode, indicated by (config)# in the prompt:

router# configure terminal
router (config)#

You can now make changes to your router configuration.

Using Commands

This section provides some tips about entering 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

Command-Line Error Messages

Table 2-3 lists common error messages that you might encounter while using the CLI to configure your router.
Table 2-3: 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 of the keywords or values required by this command.

Re-enter the command followed by a question mark (?) with a 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 of the commands that are available in this command mode.

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 of the keywords or values required by this command.	Re-enter the command followed by a question mark (?) with a 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 of the commands that are available in this command mode.

Undoing Commands

If you want to disable a feature or undo a command you entered, you can 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 RAM (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
Building configuration...

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

[OK]
router #

Using Debug Commands

You can use the debug commands to troubleshoot any configuration problems that you might be having on your network. Debug commands provide extensive, informative displays to help you interpret any problems.

Table 2-4 contains important information about debug commands.

Caution Debugging is assigned a high priority in your router CPU process, and it can render your router unusable. For this reason, use debug commands only to troubleshoot specific problems.The best time to use debug commands is during periods of low network traffic and few users, to decrease the likelihood that the debug command processing overhead affects network users.

Table 2-4: Important Information about Debug Commands

What Information

Additional documentation

You can find additional information and documentation about the debug commands in the Debug Command Reference document on the Documentation CD-ROM that came with your router.

Disabling debugging

To turn off any debugging, enter the undebug all command.

Telnet sessions

If you want to use a debug command during a Telnet session with your router, you must first enter the terminal monitor command.

What	Information
Additional documentation	You can find additional information and documentation about the debug commands in the Debug Command Reference document on the Documentation CD-ROM that came with your router.
Disabling debugging	To turn off any debugging, enter the undebug all command.
Telnet sessions	If you want to use a debug command during a Telnet session with your router, you must first enter the terminal monitor command.

Where to Go Next

Now that you have learned IOS software basics, you can begin to configure your router.

Remember the following key points:

You can use the question mark (?) and arrow keys to get help for the 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.
If you want 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 that they are not lost if there is a system reload or power outage.

Proceed to any one of the configuration chapters to begin configuring your router.

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Basic Cisco IOS Skills