IOS Basics

Study Material

Cisco® Internetwork Operating System software. The proprietary Cisco® software that provides common functionality, scalability, and security for Cisco® products.

CLI (Command Line Interface): The screen interface that allows the user to interact with the operating system by entering commands and optional arguments.

Command Modes

A hierarchical level of Cisco® IOS software. Each command mode permits you to configure different configuration components. For example, you configure global parameters in global configuration mode, interface parameters in interface configuration mode, and line parameters in line configuration mode. There are five command modes. Each mode is represented by a different prompt, as shown in the table below:

Command Mode	Prompt	Command to enter mode
User EXEC	Router1>	login
Privileged EXEC	Router1#	enable
Global configuration	Router1(config)#	configure terminal
Interface configuration	Router1(config-if)#	interface type number (from global configuration mode)
Sub interface configuration	Router1(config-subif)#	interface type number (to configure a sub interface from within interface configuration mode)
Router configuration	Router1(config-router)#	router routing_protocol (from global configuration mode)
Line configuration	Router1(config-line)#	line line_type line_number ending_line_number (from global configuration mode)

Table-1: Cisco® CLI Command Modes, respective prompts and commands.

Exec Mode

A console, modem, or Telnet command-line session to the router. There are two primary levels to the EXEC mode: user-level EXEC mode, and privileged EXEC mode. User-level EXEC is the initial mode entered upon logging into the router, and allows access only to basic monitoring commands. It is indicated by the > prompt:
Router1>
Privileged-level EXEC mode, sometimes called enable mode, provides access to configuration mode, which permits authorized users to configure and manage the router. To enter the privileged-level EXEC mode, enter the enable command, enter the enable password, and press Return. The prompt changes to the pound sign (#) as shown in the following example:

Router1> enable
Password: netserv3
Router1#

Global Configuration Mode

The commands entered in Global Configuration Mode affect the whole router. Here are the steps from Privileged EXEC mode:-

Router1# configure terminal

Router1# config t

The result will be:

Router1(config)#

Interface Mode

The commands entered in this mode affect the current interface only. Here are the steps from Global Configuration mode:-

Router1(config)# interface serial 0

Router1(config)# int serial 0

The result will be:

Router1(config-if)#

Line Mode

The router has three types of terminal lines: one console line, one auxiliary line, and five virtual (network) lines:-

Router1(config)# line con 0

Router1(config)# line vty 0 4

The result will be:

Router1(config-line)#

Other configuration modes will be explored in their respective configuration.

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Context Sensitive Help

To list all commands available for a particular command mode, enter a question mark (?) at the system prompt.
To obtain a list of commands that begin with a particular character string, enter the abbreviated command entry immediately followed by a question mark (?). This form of help is called word help, because it lists only the keywords or arguments that begin with the abbreviation you entered. e.g.

Router# co?
configure connect copy

To list a command's associated keywords or arguments, enter a question mark (?) in place of a keyword or argument on the command line. This form of help is called command syntax help, because it lists the keywords or arguments that apply based on the command, keywords, and arguments you have already entered. e.g.

The following example shows how to use command syntax help to display the next argument of a partially complete access-list command. One option is to add a wildcard mask. The <cr> symbol indicates that the other option is to press Return to execute the command.
Router(config)# access-list 99 deny 10.5.8.24 ?
A.B.C.D Mask of bits to ignore
<cr>

Note: If you enter an incorrect command, the caret symbol (^) and help response indicate the error. Notice that the caret symbol character is displayed at the point in the command string where the IOS detected that you entered an incorrect command, keyword, or argument.
This error location facility together with the interactive help system allows you to find and correct syntax errors easily.
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Keyboard Editing & Hot Keys

The following table offers a comprehensive list of the hot keys and some other keyboard editing functions.

Delete	- Removes one character to the right of the cursor.
Backspace	- Removes one character to the left of the cursor.
TAB	- Finishes a partial command.
Ctrl-A	- Moves the cursor to the beginning of the current line.
Ctrl-N	- Forwards the history buffer.
Ctrl-R	- Creates new command prompt, followed by all the characters typed at the last one. This is useful for syslog messages.
Ctrl-U	- Erases a line from the command prompt and also from memory buffer.
Ctrl-W	- Erases a word.
Ctrl-Z	- Ends configuration mode and returns to the EXEC mode.
Up Arrow	- Allows user to scroll forward through previous commands.
Down Arrow	- Allows user to scroll backward through previous commands.
Esc-B	- Moves the cursor to the beginning of previous word.
Esc-F	- Moves the cursor to the beginning of next word.

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Cisco® IOS Basics

Cisco® IOS

Context Sensitive Help

Keyboard Editing & Hot Keys