Cisco IOS (Internetwork Operating System) overview

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

Previous versions of Cisco switches ran CatOS, a discounted version of a CLI-based operating sytem.

Below you can see how IOS looks like when a Cisco device starts for the first time:

Cisco IOS overview: Internetwork Operating System

Accessing the IOS (Internetwork Operating System)

There are three most common ways to access the IOS (Internetwork Operating System) :

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. Therefore these are not accessible through the network. Most of the Cisco devices have a physical console port. This port can also be connected to a computer using a rollover cable, a special type of cable with pins on one end reversed on the other end of the cable. The rollover cable is a serial cable, which means that you can’t just plug it in an Ethernet port on your computer. You will also 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 an terminal emulation program that enables you to access IOS through the network. This also helps to configure the device remotely. Also, the device that we want to configure needs to have an 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 also sends all data as clear-text, which includes the passwords. This is the reason why this type of access is usually not in use anymore. Instead, SSH is widely in use now.

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

IOS modes

IOS has many different modes. There are three main modes and many submodes. We will describe the three main modes and one submode here.

  • 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 – this mode is accessible by typing the enable command from the user EXEC mode. This mode can also have a password protection. In this mode a user can view and change a device’s configuration.
  • global configuration mode – this mode is accessible by typing the configure terminal command from the privileged EXEC mode. It is useful in changing the device’s configuration.

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

We will now describe each of the modes mentioned above in more detail in upcoming articles.

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