Router Overview - Essential Files
(Page 13 of 15 )
In contrast to normal computers, Cisco routers have just two main files:
- ▼ The configuration file
- ▲ The Cisco IOS software
Cisco IOS software contains instructions to the router. IOS acts as the traffic cop, directing activity inside the router. IOS manages internal router operations by telling the various hardware components what to do, much like Windows XP or LINUX with a general-purpose computer. Customers cannot alter the contents of the IOS file.
The configuration file contains instructions to the router input by the customer, not Cisco. It contains information describing the network environment in which the router will run and how the network manager wants it to behave. In a phrase, the configuration file tells it whatto do; IOS tells the router howto do it.
As will be covered in the next chapter, routers also use dynamic files, which are not stored in the router’s flash memory, NVRAM, or ROM. Dynamic files instead are built from scratch when a router is booted and are strictly reactive in the sense that they only hold live information, not operational instructions.
IOS: The Internetwork Operating System We usually don’t think of an operating system as a file. After all, your PC’s operating system is made up of many thousands of files (they sit in your directory with file extensions such as SYS, EXE, DRV, and DLL).
However, IOS is indeed contained in a single file. When you ship an IOS file somewhere, it holds everything necessary to run a router. Depending on the version, an IOS software image will have a footprint from 3MB to over 10MB in size.
NOTE: Less sophisticated Cisco internetworking devices get their intelligence in the form of factory-installed software called firmware. Firmware is a subset of IOS itself. It can be field-upgraded to keep the device’s software current with the rest of the network.
IOS needs to be tightly constructed because copies of it, referred to as system images,are routinely shipped across internetworks. System images are uploaded and downloaded over routers in order to back up routers, upgrade their capabilities, and restart them after a failure. It wouldn’t be practical to send thousands of 50MB files. Being able to send a single, small, self-contained IOS file makes effective network management possible.
IOS Feature Sets Feature sets are packages that try to simplify configuring and ordering IOS software. There is no single IOS software product, per se. IOS is actually a common software platform on which a suite of IOS implementations is based, each packaged to fulfill a specific mission. Cisco calls these IOS packages feature sets (also called software images or feature packs). When you order a Cisco router, you choose an IOS feature set that contains all the capabilities your particular situation requires. Most of these requirements have to do with maintaining compatibility with the various hardware devices and network protocols in the environment in which the router will operate.
As depicted in Figure 4-11, variants of the Cisco IOS software are defined two ways: by feature set and by release. Feature sets define the job a version of IOS can do; releases are used to manage the IOS software through time.
Cisco IOS feature sets are designed to:
▼ Be compatible with certain router platforms
■ Enable interoperability between disparate networking protocols (Novell NetWare, IBM SNA, AppleTalk, and so on)
▲ Provide functional features in the form of network services and applications for such things as network management, security, and multimedia
Packaging and selling IOS software in this way simplifies things for Cisco and customers alike. For the customer, having a single part number to order simplifies figuring out what software to buy. For Cisco, it helps the company’s product engineers to figure out what goes where so that their support personnel figure out who has what. Remember, internetworking can get hopelessly complicated because the nature of the business is to enable disparate computer platforms and networking protocols to interoperate and coexist.
How Feature Sets Are Constructed Because IOS feature sets have dependencies on the router hardware on which they run, two rules of thumb apply:
- ▼ You can’t run all feature sets on all router platforms.
- ▲ Sometimes, specific features within a feature set will or will not run, depending on the router platform.
If you look at a Cisco product catalog, understanding Cisco’s feature sets can seem tough at first glance. Feature sets do nothing more than put functionality groupings into logical packages that customers can use. All feature sets, in one way or another, derive their functionality from about a dozen categories, listed in Table 4-5.
Don’t worry about the examples on the right side of this table or all the acronyms you don’t know. The important thing here is to understand that IOS software’s myriad features and functions can be grouped into about a dozen categories.
Cisco IOS feature sets try to combine features into groups most likely to match real-world customer requirements. Cisco offers dozens of point-product feature sets in the form of IOS software product numbers you can put on a purchase order. They are grouped by general characteristics into the four general feature set families shown in Table 4-6. Notice in Tables 4-5 and 4-6 that proprietary computer platforms, such as IBM, DEC, and Apple, and proprietary networking platforms, such as Novell and SNA, drive much of the need for feature sets. Each Cisco router must deal with the customer’s real-world compatibility requirements, which means being able to run with legacy hardware and software. Nearly all legacy architectures exist at the “network’s edge.” Which is to say most of the proprietary equipment with which IP must maintain compatibility sits either on LANs or on computers sitting on the LANs. This is where compatibility issues with proprietary legacy computer architectures or specialized platforms are manifested.
Tables 4-5 and 4-6 also show which software functionality groupings go into what IOS feature set products. For example, multinational enterprises are likely to be interested in IBM functionality, such as NetBEUI over PPP and Frame Relay for SNA, and would probably be interested in one of the Enterprise/APPN feature sets. By contrast, an advertising agency heavy into Apple and Windows would focus on the Desktop feature sets.
Grouping feature sets into families is Cisco’s way of bringing a semblance of order to pricing policies and upgrade paths. Ordering a single IOS part number instead of dozens helps everybody avoid mistakes. Figure 4-12 depicts the process of feature set selection.
Last, feature sets are further grouped into software product variants:
▼ Basic The basic feature set for the hardware platform
■ Plus The basic feature set and additional features, which are dependent on the hardware platform selected
▲ Encryption The addition of either a 40-bit (Plus 40) or a 56-bit (Plus 56) data encryption feature atop either the Basic or Plus feature set
The ultimate goal of feature sets is to guide you through the process of ordering, say, IOS Feature Set Enterprise 56 for a Cisco 7500/RSP running Release 11—without making a mistake that costs your network upgrade project a two-week delay.
The Anatomy of Cisco Release Numbers Cisco IOS software release numbers have four basic parts, as shown in Figure 4-13.
The first part is a major release—the “12” in Figure 4-13—which marks First Customer Shipments (FCS) of an IOS version of stable, high-quality software for customers to use in their production networks. Major releases are further defined by the following:
- ▼ Stage The “3” in Figure 4-13, which marks FCS of various major release stages (first release, general deployment release, short-lived release, and so on). Stage releases are often referred to in the future tense, when they are still planned but have not yet taken place.
- ▲ Maintenance Update The “5e” in Figure 4-13, which denotes support for additional platforms of features beyond what was available in the major release’s FCS.
The fourth part of the release number is the deployment. A general deployment (GD) release is for unconstrained use by all customers. Early deployment (ED) releases are to deliver new functionality or technologies to customers to deploy in a limited manner in their networks. Limited deployment (LD) denotes a limited lifecycle between FCS and GD.
At any given time there can be several major releases in use in the field. For example, three versions of IOS 12 are available, but 11.2 and 11.3 are still widely used. Most network managers are content to stick with a release they know works for them. Early adopters use advanced IOS releases because they need platform or feature support not available before. Most users are happy to let the early adopters help Cisco shake things out before general release.
NOTE: The terms “version” and “release” are sometimes used interchangeably in connection with editions of IOS software. In this book, version is used to specify a particular release of a particular IOS feature set.
 This chapter is from Cisco: A Beginner's Guide, by Velte and Velte (McGraw-Hill/Osborne, 2004, ISBN: 0072256354). Check it out at your favorite bookstore today.
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