Cisco IOS Command Line Interface Tutorial
The focus of this document is to introduce a new user of Cisco Internetworking Operating System (Cisco IOS®) to the Cisco IOS command-line interface (CLI). After reading this document, a new user will understand how to use the Cisco IOS CLI to configure andmanage a Cisco IOS router. For easier reference, Table 1 displays a collection of important terms and acronyms that are used throughout the document. Table 1 - Glossary Of Important Terms And Acronyms Used In This Tutorial Cisco IOS - Cisco Internetworking Operating System CLI EXEC Flash RAM - Command Line Interface - Command line session to the router (could be console, modem, or telnet) -Non-Volatile Memory used to store Cisco IOS software image - Random Access Memory
NVRAM - Non-Volatile RAM used to store router configuration
A Cisco IOS router command line interface can be accessed through either a console connection, modem connection, or a telnet session. Regardless of which connection method is used, access to the Cisco IOS command line interface isgenerally referred to as an EXEC session. As a security feature, Cisco IOS separates EXEC sessions into two different access levels - user EXEC level and privileged EXEC level. User EXEC level allows a person to access only a limited amount of basic monitoring commands. Privileged EXEC level allows a person to access all router commands (e.g. configuration and management) and can be password protected toallow only authorized users the ability to configure or maintain the router. For example, when an EXEC session is started, the router will display a "Router>" prompt. The right arrow (>) in the prompt indicates that the router is at the user EXEC level. The user EXEC level does not contain any commands that might control (e.g. reload or configure) the operation of the router. To list thecommands available at the user EXEC level, type a question mark (?) at the Router> prompt. (This feature is referred to as context sensitive help.) Critical commands (e.g. configuration and management) require that the user be at the privileged EXEC level. To change to the privileged EXEC level, type "enable" at the Router> prompt. If an enable password is configured, the router will then prompt forthat password. When the correct enable password is entered, the router prompt will change to "Router#" indicating that the user is now at the privileged EXEC level. To switch back to user EXEC level, type "disable" at the Router# prompt. Typing a question mark (?) at the privileged EXEC level will now reveal many more command options than those available at the user EXEC level. The text belowillustrates the process of changing EXEC levels.
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Cisco IOS CLI Tutorial
Password: [enable password] Router# disable Router>
Note: For security reasons, the router will not echo the password that is entered. Also, be advised that if configuring a router via telnet, the password is sent inclear text. Telnet does not offer a method to secure packets. Once an EXEC session is established, commands within Cisco IOS are hierarchically structured. In order to successfully configure the router, it is important to understand this hierarchy. To illustrate this hierarchy, Figure 1 provides a simple high-level schematic diagram of some Cisco IOS commands.
Figure 1 - Cisco IOS CLIhierarchy Command options and applications vary depending on position within this hierarchy. Referring to the diagram in figure 1, configuration command options will not be available until the user has navigated to the configuration branch of the Cisco IOS CLI structure. Once in the configuration branch, a user may enter system level configuration commands that apply to the entire router at the global...