Introduction

You can think of a Cisco router as a special-purpose computer. It has its own operating system, which is called the Internetwork Operating System (IOS), as well as files and filesystems. So we'll start with a discussion of the basic system administration functions that a router engineer must perform. This includes managing your router's filesystems, upgrading the operating system, doing backups, and restoring the system configuration.

Cisco routers use flash memory, rather than disks, for storing information. Flash storage media is significantly more expensive and slower than disk storage, but the amount of storage needed to run a router is relatively small compared to the amount needed to run a general-purpose computer. Flash also has the important benefit that it tends to be more reliable than disk storage.

Flash storage is similar to Random Access Memory (RAM), but it doesn't need power to retain information, so it is called non-volatile. And, unlike Read Only Memory (ROM), you can erase and rewrite flash easily. There are other types of non-volatile solid state storage, such as Erasable Programmable Read Only Memory (EPROM) and Electronically Erasable Programmable Read Only Memory (EEPROM). EPROM is not suitable for routers because it generally requires an external device such as an ultraviolet light shone through a window on the chip to erase it. EEPROM, on the other hand, can be erased by simply sending an erase signal to the chip. But there is a key difference between EEPROM and flash memory: when you erase something from an EEPROM device, you must erase the entire device, while flash devices allow selective deletion of parts of the medium.

This is an important feature for routers, because you don't always want to erase the entire storage medium in order to erase a single file. In Recipe 1.11 and Recipe 1.12, we discuss ways to erase single files on some types of routers, depending on the type of filesystem used.

There are at least two main pieces of non-volatile storage in a Cisco router. The router's configuration information is stored in a device called the Non-Volatile RAM (NVRAM), and the IOS images are stored in a device called the flash (lowercase). It's important to keep these names straight because, of course, all Flash memory is non-volatile RAM. And, in fact, most routers use Flash technology for their NVRAM. So it's easy to get confused by the terms.

On most Cisco routers, the NVRAM area is somewhere between 16 and 256Kb, depending on the size and function of the router. Larger routers are expected to have larger configuration files, so they need more NVRAM. The flash device, on the other hand, is usually upgradeable, and can be anywhere from a few megabytes to hundreds of megabytes.

We often talk about a router's configuration file, but there are actually two important configuration files on any router. There is the configuration file that describes the current running state of the router, which is called the running-config. Then, there is the configuration file that the router uses to boot, which is called the startup-config. Only the startup-config is stored in NVRAM, so it is important to periodically check that the version of the configuration in the NVRAM is synchronized with the version that the router is currently running. Otherwise you could get a surprise from ancient history the next time the router reboots. You can synchronize the two configuration files by simply copying the running-config onto the startup-config file:

Router1#copy running-config startup-config

Many Cisco engineers, including the authors, still use the old-fashioned version of this command out of force of habit:

Router1#write memory

However, this command is not only deprecated, it's also less descriptive of what the router is doing.

The router uses the larger flash storage device for holding the operating system, or IOS. Unlike the operating systems on most computers, the IOS is a single file containing all of the features and functions available on the router. You can obtain the IOS image files from Cisco on CD or, if you have an account on their system, you can download IOS files from the Cisco web site using FTP.

Most of the examples throughout this book assume that you have IOS Version 12. However, many of the features we discuss are also available in earlier versions. Although there may be slight syntax changes, we expect that Cisco will continue to support all of the features we describe well into the future. It is important to be flexible because if you work with Cisco routers a lot, you will encounter a large variety of different IOS versions, with various subtle differences. Unfortunately, some of these subtle differences are actually bugs. Cisco offers a detailed bug tracking system on their web site for registered users.

There are several important things to consider when you go to change the IOS version on a router. First is the feature set. For each IOS release, Cisco produces several different versions. They usually offer an Enterprise Feature Set, which includes all of the different feature options available at a given time. Because the IOS is a monolithic file containing all features and all commands, the Enterprise IOS files are usually quite large. The Enterprise version is generally much more expensive than the various stripped-down versions.

The simplest IOS version is usually the IP Only Feature Set. As the name suggests, this includes only TCP/IP based functionality. In most networks, you will find that the IP Only Feature Set is more than sufficient. In fact, almost all of the recipes in this book will work with the IP Only version of IOS.

If you require other protocols such as IPX or AppleTalk, Cisco produces an IOS Feature Set called Desktop that contains these protocols. They also offer several other important variations such as IP Plus, IP Plus IPSec 56, IP Plus IPSec 3DES, and so forth. The contents of these different versions (and even their names to some extent) vary from release to release. We encourage you to consult Cisco's feature matrixes to ensure that the features you need are in the IOS version that you have.

One of the most important considerations with any IOS release is whether you have sufficient RAM and Flash memory to support the new version. You can see how much storage your router has by looking at the output of the show version command.

The other important thing to remember about IOS images on Cisco routers is that every router has a fallback image located in the router's ROM. This IOS image cannot be changed or upgraded without physically replacing the ROM chips in the router.

The router's ROM contains three items: the power on self test (POST), the bootstrap program, and a limited version of the router's operating system. The router uses the bootstrap program while booting. The IOS image in ROM is usually an extremely stripped-down version that doesn't support many common features (routing protocols, for example). In the normal boot cycle, the router will first load the POST, then the bootstrap program followed by the appropriate IOS image. Please refer to Recipe 1.7 for more information about booting from different IOS files.

Recipe 1.7 also shows how to adjust the configuration register values. These values set a variety of boot options, and even allow you to force the router to stop its boot process before loading the IOS. This can be useful if the IOS image is corrupted, or if you need to do password recovery.