The majority of Cisco routers run Cisco IOS Software with the command-line interface (CLI). The CLI is
used to interface with the device and send commands to the device. This is achieved through the use of a
terminal, a terminal emulator, or a Telnet connection. Some routing cards, such as the Multilayer Switch
Feature Card (MSFC) daughter card for the Catalyst 6000 series LAN switches, also run Cisco IOS
Software. Understanding the Cisco IOS Software CLI is as fundamental to supporting routers.
There are three ways in which you can access the CLI: you access the router through the console; through a
dialup device through a modem attached to the auxiliary port; or by using a Telnet connection. Which ever
method you use, you enter user exec mode first. User exec mode is one of three command exec modes in
the IOS user interface. Enable mode, also known as privileged mode or Privileged exec mode, and
command mode are the others. Enable mode is so named because the enable command is used to reach this
mode. User mode allows commands that are not disruptive to be issued, with some information being
displayed to the user. Privileged mode supports a superset of commands compared to user mode. However,
none of the commands in user mode or privileged mode changes the configuration of the router.
Passwords are required for Telnet and auxiliary access as of Cisco IOS Release 12.x and later. However,
there are no preconfigured passwords; therefore, you must configure passwords for Telnet and auxiliary
access from the console first.
All Cisco routers have a console port, and most have an auxiliary port. The console port is intended for local
administrative access from an ASCII terminal or a computer using a terminal emulator. The auxiliary port is
intended for asynchronous dial access from an ASCII terminal or terminal emulator; the auxiliary port is
often used for dial backup.