Cisco IOS and CLI

Last Updated on July 15, 2015 by Admin


For the Cisco CCNA you are required to know how to configure Cisco routers and Cisco switches using the command line interface or CLI. A command line interface is a command driven user shell that allows the user to interface with the operating system. The command line interface or CLI is operated with just a keyboard. In contrast a graphical user interface or GUI is an icon and menu driven user shell characterized by the use of a mouse in addition to a keyboard. The Cisco operating system, used with Catalyst switches and integrated services routers is known as the Cisco IOS, or Internetwork Operating System.

  • RAM (temporary memory) – The IOS and the config file are loaded and run in RAM when the router boots up, but they are typically saved or stored in FLASH (IOS) and NVRAM (startup-config). The routing table is run from RAM. Routers and switches execute everything in RAM which is why they are so fast. Configuration changes are immediately executed in RAM (running-config) but can be saved to NVRAM (startup-config) to be made permanent.
  • FLASH (permanent memory) – This is where the IOS is saved
  • NVRAM (permanent memory) – This is where the startup-config file is saved
  • ROM (permanent and unchangeable) – This is where the BIOS, POST, and ROMMON are stored.

The IOS and CLI

The Cisco IOS is the Cisco operating system. The IOS is specific to the Cisco device it was designed for, having different capabilities and tools included in it.

In this way, the Cisco IOS comes in many different sizes, capabilities, specifications and revisions.

As part of the Cisco IOS, the CLI or command line interface is included on every Cisco device including, Cisco routers, switches, and wireless access points and bridges. Most Cisco devices also have a GUI or graphical user interface. The focus of the Cisco CCNA is learning the CLI, command line interface. The command line interface is an administrative interface used to configure the Cisco device. There are three ways to access the CLI:

Console – the console port is a direct serial connection using a console/rollover cable attached from the Cisco device’s console port to a computer serial port. Usually the initial method of configuring a router or switch, because it does not rely on networking being enabled. A console connection is also how you would recover a router with a deleted configuration file, IOS file, or forgotten password.

Telnet or SSH – the ability to telnet or SSH into a Cisco device is a remote administrative connection that can also be done from the local network. In order to telnet or SSH into a Cisco router or switch you will first need to bring up a network interface by configuring it with an IP address, subnet mask, and issuing the “no shutdown” command .

Aux – The auxiliary port is designed to connect to a modem. It is used for a dial in connection to the router or switch. This remote administrative connection can also be done locally.

The CLI has different command modes, with specific commands available in each mode. The different command modes are:

  • User exec mode – Only a few commands are available in this mode. Commands like “ping”, a few of the “show” commands
  • Privileged exec mode – All of the User exec commands plus all of the “show” and “debug” commands
  • Global config mode – Access to all of configuration commands and addition configuration modes
  • Global sub configuration modes – interface configuration mode, router configuration mode, etc.


  • Router>enable
  • Router#configure terminal
  • Router#show running-config
  • Router#show startup-config
  • Router#show version
  • Router#show flash
  • Router#copy running-config startup config

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