Eventually you will want to upgrade your router’s software. 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 is to use a TFTP (Trivial File Transfer Protocol) server or an RCP (Remote Copy Protocol) server somewhere on your network. 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 (for example, the Windows registry, or the files in /etc on a Unix system) that tell the kernel how you want it to operate in a particular situation.