Cisco IOS Operating System Overview

IOS (Internetwork Operating System) is a multitasking operating system used on most Cisco Systems routers and switches. IOS has a command-line interface with a predetermined number of multiple-word commands. The IOS operating system is used to configure routing, switching, internetworking and other features supported by Cisco IOS devices.

Previous versions of Cisco switches ran CatOS, a legacy version of a CLI-based operating system.


Below you can see how IOS looks like when a Cisco device is started for the first time, using a Cisco 3745 router as an example:

Cisco 3745 (R7000) processor (revision 2.0) with 249856K/12288K bytes of memory.
Processor board ID FTX0945W0MY
R7000 CPU at 350MHz, Implementation 39, Rev 2.1, 256KB L2, 512KB L3 Cache
5 FastEthernet interfaces
DRAM configuration is 64 bits wide with parity enabled.
151K bytes of NVRAM.

Press RETURN to get started!


Accessing the Cisco IOS

There are three most common ways to access the Cisco IOS:

1. Console Access – this type of access is usually used to configure newly acquired devices. These devices usually don’t have an IP address configured, and therefore can not be accessed through the network. Most of the Cisco devices have a physical console port. This port can be connected to a computer using a rollover cable, a special type of cable with pins on one end and reversed on the other end of the cable. The rollover cable is a serial cable, which means that you can’t just plug it into an Ethernet port on your computer. You will need an adapter that converts an interface on your computer (usually a 9-pin serial interface) into RJ-45.


Newer Cisco devices usually include a USB console port, since serial ports are rare on modern PCs.


2. Telnet Access – this type of access used to be a common way to access network devices. Telnet is a terminal emulation program that enables you to access IOS through the network and configure the device remotely. The device that is being configured needs to have a Telnet server installed and an IP address configured.

Telnet uses a well-known TCP port 23. One of the biggest disadvantages of this protocol is that it sends all data as clear text, which includes the passwords! This is the reason why this type of access is usually not used anymore. Instead, SSH is usually used.


3. SSH Access – like Telnet, this access type enables you to configure devices remotely, but it adds an extra layer of security by encrypting all communications using public-key cryptography. SSH uses the well-known TCP port 22. 


IOS Modes

IOS has many different modes. There are three main modes and many sub-modes. We will describe the three main modes and one sub-mode.

  • User EXEC Mode – the default mode for the IOS CLI. This is the mode that a user is placed in after accessing the IOS. Only basic commands (like ping or telnet) are available in this mode.
  • Privileged EXEC Mode – is accessed by typing the enable command from the user EXEC mode. This mode can be password protected. In this mode, a user can view and change a device’s configuration.
  • Global Configuration Mode – this mode can be accessed by typing the configure terminal command from the privileged EXEC mode. It is used to change the device’s configuration.


A global configuration mode can have many sub-modes. For example, when a user wants to configure an interface, he will have to enter the interface sub-mode by entering the interface INTERFACE_TYPE INTERFACE_NUMBER command (e.g. interface FastEthernet 0/1 ) from the global configuration mode. This sub-mode can have many commands that are specific to the interface.

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