04-02-2013, 11:30 AM
Cisco IOS in a Nutshell
1Cisco IOS.pdf (Size: 4.86 MB / Downloads: 80)
Introduction
The modern world is networked in a way that could barely be imagined a few
decades ago. Today, the Internet reaches into virtually every business and almost
every home. Our children and even our grandparents speak of dot-coms, email,
and web sites. The Internet is now part of our culture.
Routers are the glue that holds the Internet together. And Cisco is the most
prominent router manufacturer, holding the largest share of the market. Their
routers come in all sizes, from inexpensive units for homes and small offices to
equipment costing well over $100,000 and capable of routing at gigabit speeds.
One of the most impressive facts about their product line is its unified operating
system. Almost all of their routers, as well as half of their switches—from the
smallest to the largest—run the Internetwork Operating System (IOS). Therefore,
they share the same command set, the same user interface, and the same
configuration techniques. While an 800-series home router doesn't have the
features or the capacity of a 7500-series router that might be used to connect an
ISP to an Internet backbone, you configure them the same way. Both routers use
access lists, have similar security mechanisms, support the same set of protocols
in the same way, and so on. A home router probably wouldn't have a Frame Relay
interface, but if it did, it would be configured just like a Frame Relay interface on a
mid-sized corporate router.
IOS User Modes
There are two primary modes of operation within the IOS: user mode and
privileged mode. When you first connect to the router, you are placed in the user
mode. The Cisco documentation refers to this as the user exec mode; I am going
to omit "exec" throughout this book.
Pausing Output
Using the terminal command, you can set an important feature of the user
interface: the pausing of lengthy output. For example, if you run a command that
has more than one page of output, the router will pause after 24 lines with a
"—More—" prompt. The value 24 is the default terminal length. Depending on the
size of your terminal window, this might not be adequate.
IOS Images
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.
Feature set
The feature set describes the options that are included in the IOS image. Table 2-2 lists the most popular
feature sets. Keep in mind that features are often tied to platforms—any given platform will support only
some feature sets. Also, note that the feature set indicators may be combined. For example, the IOS image
C3640-JS-M has the feature set JS, which combines the Enterprise Subset feature set (J) with the Source
Route Switch feature set (S).
Loading Image Files Through the Network
From time to time you will need to load a new system image, either to upgrade the image that was shipped
with your router, to install bug fixes, or to add new feature sets that you've purchased. The most common
way to upload an image is to copy it to the router from some other system on your network—after all, the
router is a network device, and it's easiest to use the router's networking capabilities. You can also use the
console port or an AUX port to upload an image using X-MODEM. In this book, I'll focus on loading image
files over the network.
For example, say that you have just purchased a 2501 router and want to upgrade to the latest version of
IOS. Assume that you have downloaded the new image from Cisco's web site or that you have it on a
floppy disk or some other medium. If you haven't already set the router up, you'll need a serial connection
from a terminal (or a PC with a terminal-emulation program) to do some simple configuration before you
can install the image: at a minimum, you need to set up an IP address. You'll also need a connection to the
network, so the router can access your TFTP server. Once you've set things up, your primitive network will
look like Figure 2-1.