Cisco IOS in a Nutshell, 2nd Edition

Chapter 2. IOS Images and Configuration Files

Eventually you will want to upgrade your router’s software. The IOS (Internetwork Operating System) is the software that resides inside the Cisco device. Upgrading IOS involves transferring a new IOS image to your router from some kind of server. As we’ll see in this chapter, there are several methods for uploading a new IOS image: the most common include using Trivial File Transfer Protocol (TFTP), Remote Copy Protocol (RCP), or Secure Copy Protocol (SCP). Before we discuss how to transfer a new image, let’s define exactly what is meant by an “image file” and how it differs from a configuration file.

IOS image files contain the system code that your router uses to function, that is, the image contains the IOS itself, plus various feature sets (optional features or router-specific features). However, the features are not configured in any way. The router’s actual configuration—which features are enabled and how they are used in your particular network environment—is stored in a configuration file written in IOS’s configuration language. The commands in this file describe everything from the router’s name and the IP address of each interface to the protocols that you’re using, address translation, security, and more. The router is useless without a concrete configuration—just like an operating system kernel is useless without the configuration files (such as the Windows registry or the files in /etc on a Unix system) that tell the kernel ...