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
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:
Password: [enable password]
Router# configure terminal
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:
User EXEC mode
Privileged EXEC mode
Global configuration mode. # sign indicates this is only
accessible at privileged EXEC mode.
Interface level configuration mode.
Routing engine level within configuration mode.
(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:
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
Cisco IOS also allows abbreviated commands support. For example, consider the
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:
% Ambiguous command: "c"
Checkout a ccna router
simulator available from certexams.com.
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
The banner is displayed whenever anyone logs in to your Cisco router. The
"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#
note that you need to begin and end the banner with a delimiter (here #