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 and other features supported by a Cisco device.

Cisco Ios Software
Cisco Ios/xr
Cisco Ios

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

The running IOS images you can use on GNS3 are the c3640, c3660, c3725, c3745, and c7200 versions. Here are a few additional Cisco Router images. For example, if you use a c1700 series Router, it will be insufficient because it does not support some features when you create a more advanced network topology. IOS XE is a train of Cisco Systems ' widely deployed Internetworking Operating System (IOS), introduced with the ASR 1000 series. It is built on Linux and provides a distributed software architecture that moves many operating system responsibilities out of the IOS process and has a copy of IOS running as a separate process.

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

Accessing the IOS

There are three most common ways to access the 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 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 need an adapter that converts an interface on your computer (usually a 9-pin serial interface) into RJ-45.

NOTE
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 and configure the device remotely. The device that is being configured 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 is 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 well known TCP port 22.

IOS modes

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

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 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 submodes. For example, when a user wants to configure an interface, he will 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’ll describe each of the modes mentioned above in more detail in the following lectures.

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Objectives

Cisco Ios Software

Understand Cisco IOS system architecture components.
Work with the Cisco IOS Command Line Interface (CLI) and common commands.
Learn about Cisco IOS troubleshooting techniques.
Understand upgrading and release information.

The Cisco IOS software is network system software that runs on Cisco routers and switches. It is used to configure, monitor, and troubleshoot the system.

System Architecture

Like a computer, a router has a CPU that varies in performance and capabilities depending on the router platform. Two examples of processors that Cisco uses are the Motorola 68030 and the Orion/R4600. The Cisco IOS software running in the router requires the CPU or processor to make routing and bridging decisions, maintain routing tables, and other system management functions. The CPU must have access to data in memory to make decisions or to get instructions.

Cisco Ios/xr

There are usually four types of memory on a Cisco router:

ROM—ROM is generally the memory on a chip or multiple chips. It is available on a router's processor board. It is read-only, which means that data cannot be written to it. The initial software that runs on a Cisco router is called the bootstrapsoftware and is usually stored in ROM. The bootstrap software is invoked when the router boots up.
Flash—Flash memory is located on a processor board SIMM but can be expanded using PCMCIA (removable) cards. Flash memory is most commonly used to store one or more Cisco IOS software images. Configuration files or system information can also be copied to Flash. On some high-end systems, Flash memory is also used to hold bootstrap software.
RAM—RAM is very fast memory that loses its information when the system is restarted. It is used in PCs to store running applications and data. On a router, RAM is used to hold IOS system tables and buffers. RAM memory is basically used for all system operational storage requirements.
NVRAM—On the router, NVRAM is used to store the startup configuration. This is the configuration file that IOS reads when the router boots up. It is extremely fast memory and is persistent across reboots.

Although CPU and memory are required components to run IOS, a router must also have various interfaces to allow packet forwarding. Interfaces are input and output connections to the router that carries data that needs to be routed or switched. The most common types of interfaces are Ethernet and serial. Similar to the driver software on a computer with parallel ports and USB ports, IOS has device drivers to support these various interface types.

All Cisco routers have a console port that provides an EIA/TIA-232 asynchronous serial connection. The console port can be connected to a computer's serial connection to gain terminal access to the router. Most routers also have an auxiliary port that is very similar to the console port, but is typically used for modem connection for remote router management.

Example 4-1 shows the console output of a new Cisco 3640 router that has just been started. Notice the processor, interface, and memory information that is listed.

Example 4-1 Cisco 3640 Router Console Output at Startup

Cisco Ios

When a new router is first started, IOS runs an autoinstall process wherein the user is prompted to answer a few questions. IOS then configures the system based on the input provided. After initial setup, the configuration is most commonly modified using the command-line interface (CLI). Other ways of configuring the router include HTTP and network management applications.