The sections below will cover the following:

Cisco designed the IOS using command containers called modes that provide hierarchical separation based on the system operations the commands can perform (Cisco, 2002).

When this router is at it's defaults, a setup dialog will can be used to automate initial setup. Note: the first step illustrates the use of the embedded terminal. All subsequnet steps are performed on a live router and you can use the embedded terminal to follow along.

Continue with configuration dialog? [yes/no]:n

Cisco's help is a question mark "?".

Router>?

• Press the q key to exit
• Press the Enter key to display a line at a time
• Press the spacebar to display a page at a time

The commands under User EXEC mode are mostly used for basic troubleshooting. One command option is the enable command, which is used to go from User EXEC mode to Privilege EXEC mode (a.k.a. Enable Mode).

Router>enable

Privilege EXEC mode contains the same commands as User EXEC, but with some addition commands as well. The commands under both of these modes are known as EXEC commands. The additional commands within Privilege EXEC mode are still mostly used for troubleshooting, but these commands can be more taxing on the system (ex. debug, reload, etc.).

Router#?

The configure terminal command is used to go from Privilege EXEC mode to Global Configuration mode.

Router#configure terminal

The commands under Global configuration mode are used to make changes that will effect how the system looks or behaves holistically.

Router(config)#?

The IOS contains more specific configuration modes (interface configuration, routing configuration, dhcp configuration, etc.).

Router(config)#interface FastEthernet 0/0

The end command, or CTRL^z orCTRL^c key combinations can be used to go back to Privilege EXEC mode from almost any position within the mode hierarchy.

The exit command can be used to go back one mode.

Router#exit

The Cisco IOS, like most operating systems, was designed with access control in mind. The first mode (EXEC mode) is split into two levels of access that are assigned different privilege levels. Cisco uses the concept of privilege levels to authorize access to objects on the system. The Cisco IOS has two levels set up by default (1 and 15) and relates privilege 1 with User EXEC mode, and privilege level 15 to access all other modes (Cisco, n.d.).

By default User EXEC mode has privilege level 1 assigned to it, and can be thought of as regular user privileges.

Router>show privilege

All other modes are assigned a privilege level 15 by default. This level is often compared to a super user in Linux or a Mac OS X.

Router>enable
Router#show privilege

Below lists the commands used to navigate EXEC modes.

disable   Turn off privileged commands
exit      Exit from the EXEC
enable    Turn on privileged commands

Router#disable

When working within the IOS, you must become familiar with the error messages that are presented within the CLI.

The Cisco IOS uses a CLI-parser for interpreting commands and command syntax. If an admin attempts to execute the short form of a command, but there is more than one command that contains the same characters, then an error will be generated.

Router>e

The error message let's us know that the interpreter cannot differentiate between commands.

Router>e?

Router>en

Command syntax means command structure. Think of this as meaning correct spelling with spaces for a sentence if we are talking about the complete command syntax. An example of this is to execute the first level-command show, which requires, at a minimum, a second-level command.

Router#clock

Router#clock ?

The Tab key can be used to autocomplete commands so long as they are not ambiguous.

Router#clock r

A carriage return (cr) is another way to say press Enter (Cisco, 2014).

Router#clock read-calendar ?

Router#clock read-calendar

Router#clock read-calendars

This same error can be generated if the command is not understood under a mode.

Router#conf t
Router(config)#clock read-calendar

The do command can be used to execute EXEC commands in other non-EXEC modes.

Router(config)#do clock read-calendar

Your Cisco IOS image file provides commands for enabling features on your device. This section will review some common key options that can be used when dealing with commands inside a CLI or Terminal. However for simlicity sake, below will only make reference to the CLI.

When it comes to Cisco CLI basics, the question mark is the most important key option. But in order to use it properly, one has to understand the system. Below we will take a lower-level look at how it works. Let's begin by looking at the command heirarchy again and describe the IOS modes in terms of the command interpreters. At a basic-level, there are two modes: EXEC and Configuration, where each contains many sub-modes that are accessed from them.

A command interpreter enables a user to execute commands that are available within it's command list. Most times, when accessing a different IOS mode, you are interacting with a different command interpreter.

Router#show parser dump all | include Mode

The result.

The command interpreter provides access to various commands and the command parser validates their syntax (command structure). Think of a command interpreter as a gateway to a specific command list, and a command parser as a chunk of code that is used to analyze and validate syntax for each command in the list. The interpreter accepts input (commands) from the user and instructs the parser to use a particular command list when analyzing and validating the user's command against that list. Note: the below image does not display EXEC sub-modes because they are not often accessed during CCNA training.

Router#show parser dump exec

The result.

To better utilize the question mark, think of each command in the command list as a string in an array that is compared each time a command is executed. If the message "% Ambiguous command:" is displayed, this means that there is more than one match in the array. The question mark can be used up against the command to display all matches. The only other way to use the question would be to put space between it and the command, which will return different results.

The command parser also validates and analyzes command arguments and what you get back from Cisco's help is dependent on it's placement within the command structure. Commands may have one or more arguments to them. If a question mark is used where an argument may exist, then the array will only consist of level-x argument strings.

The other key shortcuts listed in the table are typical command shortcuts also seen in other terminals and CLIs. When comfort in the CLI is attained, these key shortcuts can ease administration, similar to how they do in most applications. There are many more key shortcuts, but below will demonstrate the ones which are listed in the above table.

Router#clock read-calendar

Click the start button to review the questions that pertain to this webpage.