CCNA: Cisco IOS An Introduction

Cisco IOS (short for Internetwork Operating System) is the software used on a majority of Cisco Systems routers and switches.  IOS consists of routing, switching, internetworking and telecommunications functions in a multitasking operating system. Cisco IOS has uses command line interface (CLI), and provides a fixed set of multiple-word commands. A Cisco IOS command line interface can be accessed through either a console connection, modem connection, or a telnet session. The set of commands available at any particular level is determined by the "mode" and the privilege level of the current user. Cisco IOS follows a command hierarchy, with each level offering different set of commands  All commands are assigned a privilege level, from 0 to 15, and can only be accessed by users with the necessary privilege. Through the CLI, the commands available to each privilege level can be defined.

Some of the widely used command hierarchy levels are given below:
User EXEC level: This is the level that a connected user is allowed initially. User EXEC allows access to a limited set of basic monitoring commands. A ">" sign denotes User EXEC mode.
Privileged EXEC level: Privileged EXEC level allows access to all router commands including router configuration and management commands. This level is usually password protected for security reasons. A "#"sign denotes privileged EXEC mode.
When a user is connected to a Cisco IOS, a User EXEC prompt appears. Now, the user can enter privileged EXEC mode by typing the password shown as below:
Router> enable
Password: [enable password]
Router# configure terminal
Router(config)#
Global configuration mode: "Global configuration mode" provides commands to change the system's configuration. This is typically represented by "(config)#" sign as shown in the above example.
Interface configuration mode: "Interface configuration mode" provides commands to change the configuration of a specific interface of the router.  An interface configuration mode is denoted by "(config-in)#".
A summary of Cisco IOS router command prompt is given below:

Prompt	Explanation
Router>	User EXEC mode
Router#	Privileged EXEC mode
Router(config)#	Global configuration mode. # sign indicates this is only accessible at privileged EXEC mode.
Router(config-if)#	Interface level configuration mode.
Router(config-router)#	Routing engine level within configuration mode.
Router(config-line)#	Line level (vty, tty, async) within configuration mode.

Context Sensitive Help

Cisco IOS CLI offers context sensitive help. At any time during an EXEC session, a user can type a question mark (?) to get help.
Two types of context sensitive help are available:

• Word help and
• Command syntax help.

Word help: Word help can be used to obtain a list of commands that begin with a given character string. To use word help, type in the characters in question followed immediately by the question mark (?).  The following is an example of word help:

Router# co?
configure connect copy

Command syntax help: Command syntax help can be used to obtain a list of commands, keyword, or argument options that are available starting with the keywords that the user had already entered. To use command syntax help, enter a question mark (?) after hitting a space.  The router will then display a list of available command options with <cr> standing for carriage return. The following is an example of command syntax help:

Router# configure ?
memory Configure from NV memory
network Configure from a TFTP network host
terminal Configure from the terminal
<cr>

Cisco IOS also allows abbreviated commands support. For example, consider the following:

Router#configure terminal
Router(config)#
Router#config term
Router(config)#

Both the above commands to the same job. The IOS correctly interprets the full command words. However, if there is any ambiguity, an error message is generated as below:

Router(config)#c
% Ambiguous command: "c"

Checkout a ccna router simulator available from certexams.com.
Example Question: 
What is the command used to add a banner to a Cisco router configuration?
A. add banner
B. banner motd #
C. motd banner #
D. add banner #

Correct answer: B
Explanation:
The banner is displayed whenever anyone logs in to your Cisco router. The syntax is
"banner motd # 
MOTD stands for "Message Of The Day".
# symbol signifies the start of the banner message to the router. You will be prompted for the
message to be displayed. You need to enter "#" symbol at the end of the message, signifying
that the msg has ended.
Alternatively, you can enter the banner in the same line as below:
"banner motd # your message here#