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Introduction to Cisco IOS Software

Chapter Description

This sample chapter introduces the fundamentals of Cisco IOS software, the operating system that runs on Cisco routers and switches.

Objectives

  • 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.

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 bootstrap software 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

System Bootstrap, Version 11.1(20)AA2, EARLY DEPLOYMENT RELEASE SOFTWARE (fc1)
Copyright (c) 1999 by Cisco Systems, Inc.
C3600 processor with 98304 Kbytes of main memory
Main memory is configured to 64 bit mode with parity disabled

program load complete, entry point: 0x80008000, size: 0xa8d168
Self decompressing the image : #################################################
#################################################################### [OK]

       Restricted Rights Legend

Use, duplication, or disclosure by the Government is
subject to restrictions as set forth in subparagraph
(c) of the Commercial Computer Software - Restricted
Rights clause at FAR sec. 52.227-19 and subparagraph
(c) (1) (ii) of the Rights in Technical Data and Computer
Software clause at DFARS sec. 252.227-7013.

      Cisco Systems, Inc.
      170 West Tasman Drive
      San Jose, California 95134-1706

Cisco Internetwork Operating System Software
IOS (tm) 3600 Software (C3640-IS-M), Version 12.2(10), RELEASE SOFTWARE (fc2)
Copyright (c) 1986-2002 by Cisco Systems, Inc.
Compiled Mon 06-May-02 23:23 by pwade
Image text-base: 0x60008930, data-base: 0x610D2000

cisco 3640 (R4700) processor (revision 0x00) with 94208K/4096K bytes of memory.
Processor board ID 17746964
R4700 CPU at 100Mhz, Implementation 33, Rev 1.0
Bridging software.
X.25 software, Version 3.0.0.
SuperLAT software (copyright 1990 by Meridian Technology Corp).
5 Ethernet/IEEE 802.3 interface(s)
1 Serial network interface(s)
DRAM configuration is 64 bits wide with parity disabled.
125K bytes of non-volatile configuration memory.
8192K bytes of processor board System flash (Read/Write)
16384K bytes of processor board PCMCIA Slot0 flash (Read/Write)

     --- System Configuration Dialog ---

Would you like to enter the initial configuration dialog? [yes/no]:

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.

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