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Basics of the Cisco IOS

Posted On 2007-05-02 by FortyPoundHead
Keywords: 640-607:Cisco Certified Network Associate (CCNA) Exam
Tags: Cisco Networking 
Views: 1702


Cisco's Internetwork Operating System (IOS) software platform is implemented on the varied hardware used in this course. It is the embedded software architecture in all of the Cisco routers and is also the operating system of the Catalyst 1900 series switches.

Command Line Interface (CLI)
The Cisco IOS command line interface can be accessed through a console connection, modem connection, or a Telnet session. Regardless of which connection method is used, access to the IOS command line interface is generally referred to as an EXEC session.

Network Device Configuration
Configuration Features

Configuration sets up the device with:

  • Network policy of the functions required
  • Protocol addressing and parameter settings
  • Options for administration and management.
Initializing Cisco Devices
Catalyst Switch When you start the Catalyst switch for the first time, the switch uses an initial configuration with default settings.
Cisco Router When you start the Cisco router for the first time, the router does not have an initial configuration. The router software will prompt you for a minimum of details using an optional dialogue called "setup."

Overview of Cisco Device Startup
When the Catalyst switch or Cisco router starts up, there are three main operations performed on the networking device:
  1. The device performs hardware checking routines . A term often used to describe this initial set of routines is power on self test (POST).
  2. Once the hardware has been shown to be in good working order, the devices perform system startup routines . These initiate the switch or router operating software.
  3. Once the operating system is loaded, the devices try to find and apply software configuration settings that establish the details needed for network operation.
There is typically a sequence of fallback routines that provide software startup alternatives if needed.

External Configuration Sources
The switch and the router can be configured from many locations:

Upon initial installation, the network administrator typically configures the networking devices from the console terminal, which is connected via the console port.
If the administrator is supporting a remote device, a modem connection to the device''s auxiliary port permits the administrator to configure it.
For selected routers and switches, a CD-ROM (such as Cisco Fast Step) can provide a rapid configuration application, to make the most simple configuration tasks easier to accomplish.

Additional Access Upon Initial Configuration
After initial start up, there are additional ways to access and configure the device. All of these require TCP/IP to be configured on the device. The methods are:
  • Establish a terminal session using Telnet
  • Download a configuration file from a trivial file transfer protocol (TFTP ) server on the network
  • Download a configuration file using a WWW browser
IOS User Interface Functions
Cisco IOS software uses a Command Line Interface (CLI) as its traditional console environment. While Cisco IOS software is a core technology that extends across many products, Cisco IOS operation details vary on different internetworking devices. Cisco IOS software uses a hierarchy of commands in its command-mode structure. Each command mode supports specific Cisco IOS commands related to a type of operation on the device.

Entering Commands
To enter commands into the user interface, you type or paste your entries within one of the several console command modes. Each command mode is indicated with a distinctive prompt. The Enter Key instructs the device to parse and execute the command

Security and Access Levels
As a security feature, Cisco IOS separates the 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 number of basic monitoring commands.
Privileged EXEC level allows a person to access all router commands (for example, configuration and management) and can be password protected to allow only authorized individuals to access the router.

User Mode
When an EXEC session is started, the router or switch will display a hostname> prompt. The right arrow (>) in the prompt indicates that the router or switch is at the user EXEC level. The user EXEC level does not contain any commands that might control (for example, reloading or configuring) the operation of the router or switch.

Privileged Mode
Critical commands (for example, configuration and management) require that the user be at the privileged EXEC level. The Privileged Mode is the most commonly used and allows a person to access all router commands for management and configuration. Its characteristics include:
  • Detailed examination of switch or router
  • Enables configuration and debugging
  • Prerequisite for other configuration modes


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