The Switch Book: The Complete Guide to LAN Switching Technology

The Switch Book: The Complete Guide to LAN Switching Technology

by Rich Seifert

Hardcover(New Edition)

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Product Details

ISBN-13: 9780471345862
Publisher: Wiley
Publication date: 06/28/2000
Edition description: New Edition
Pages: 720
Product dimensions: 7.52(w) x 9.54(h) x 1.73(d)

About the Author

RICH SEIFERT is President of Networks & Communications Consulting. He was the chair and editor for numerous LAN and switch-related standards and is the author of the bestselling book Gigabit Ethernet. During more than thirty years in the computer and communications industry, he has contributed to the design of a wide range of products including LAN switches, consulted to hundreds of companies on network architecture and implementation, and taught thousands of students in professional seminars and at the undergraduate and graduate levels at the University of California.

Read an Excerpt

The Switch Book

The Complete Guide to
LAN Switching Technology


The invasion of Local Area Networks (LANs) into the commercial, industrial, university, and even the home environment during the 1980s and 1990s has been nothing short of phenomenal. No longer do organizations consider whetherthey need a network, but only what type of network should be employed and what devices should be used to build the network infrastructure.

Most early LANs were designed around the use of a shared communications channel--for example, a coaxial cable bus. During the late 1980s and early 1990s, two phenomena occurred that would fundamentally change the way that end user LANs were designed:

  • LAN topology migrated from the use of a shared medium to standardized structured wiring systems, implemented primarily using unshielded twisted pair (UTP) cable and central wiring hubs.

  • End user computing equipment and application requirements advanced to the point where the capacity of a shared LAN could actually limit overall system performance.

These two factors (together with commensurate advances in silicon technology) fostered the development and deployment of LAN switches. While traditional, shared-bandwidth wiring hubs are still in use today, they are generally considered acceptable only at the edge of the network or when application demands do not seriously tax LAN performance. Switches have become almost ubiquitous for backbone interconnections. As switch prices decrease, they have become popular even for desktop use, as they can provide performance advantages and growth capability for only a very small premium over their non-switched counterparts.

Along with the power and performance advantages offered by switches comes an increase in features, options, and complexity. This book will guide both network users and product developers through the murky sea of issues surrounding the capabilities, use, and design of LAN switches and switched internetworks.

Who Should Read This Book

This book is aimed at the needs of:

Network users. This includes network planners, designers, installers, and administrators; MIS management; value-added resellers (VARs); and operations staff in any organization that selects, installs, or uses LAN switches and related network products. This book will help these people to understand and become more comfortable with switching technology and to make informed decisions regarding the selection, purchase, and deployment of LAN switches. In many cases today, these people depend primarily on equipment suppliers as their main source of information. Such information is always suspect, as suppliers have a strong motivation to sell their particular technology regardless of whether it is appropriate or not.

Network technologists. This includes engineers working in companies involved in the design and manufacture of computers, communications, and networking products; academics (both instructors and students); network product marketing and sales personnel; independent consultants; and anyone interested in understanding LAN switch operation beyond the level of detail typically available in product data sheets, trade journals, and general networking books.

The reader is assumed to be at least casually familiar with computer networks (in particular, Local Area Networks), network protocols, and common network application environments. No attempt is made to provide a complete, from-the-ground-up tutorial suitable for novices. Indeed, such a work would require an encyclopedia and would make it impossible to focus on the critical issues of LAN switching. Network technologists and users grounded in network fundamentals will learn everything they need to completely understand the workings of LAN switches; in the process they will gain enormous insight into the reasons why things are done the way they are, rather than just getting a presentation of cold facts.

Organization of the Book

The book is divided into two main sections.

Part I: Foundations of LAN Switches

The first part of the book teaches the essentials of LAN switches. It comprises six chapters:

Chapter 1, Laying the Foundation, provides a review of the core technologies underlying LAN switch design, including network architecture, addressing, LAN technology, and LAN standards. In addition, it introduces some key terms that are used throughout the book. While not intended as a primer for first-time networkers, this chapter sets the framework for the rest of the book and can serve as a refresher for readers who may not have looked at these subjects for a while. Some important insights are provided into the relationship between network architecture and implementation, along with a lot of the history behind the development of modern LAN technology and the relevant standards.

Chapter 2, Transparent Bridges, explains the details of how bridges operate to allow communication among stations on multiple LANs. In addition to explaining the functional behavior of bridges, the chapter explores bridge implementation and performance issues and provides a guide to the IEEE 802.1D bridge standard.

Chapter 3, Bridging between Technologies, looks at the problems that arise when bridges are used between dissimilar LANs and between LAN and Wide Area Network (WAN) technologies. The important issues of frame translation, encapsulation, checksum protection, bit-ordering, and so on, are all examined in detail, along with the solutions offered both by the standards and the commercial products. The chapter also explains the limitations inherent in such mixed technology bridged LANs.

Chapter 4, Principles of LAN Switches, bridges the gap (pun intentional) between the operation of bridges and modern switched LANs. The chapter shows how switches can be deployed in LAN environments, then goes on to look at (1) the evolution of switch design from its earliest days to modern, single-chip solutions, (2) commercial switch configurations, and (3) switch application environments from desktop to enterprise-wide use. Special treatment is given to Layer 3 switches and their implementation and application, as well as the issue of cut-through versus store-and-forward switch operation.

Chapter 5, Loop Resolution, explains how the Spanning Tree protocol provides for automatic detection and resolution of loops in bridged/ switched LANs. A detailed explanation of the operation of the protocol is provided, along with some important implementation issues and performance implications resulting from its use. The chapter also provides a guide to the relevant IEEE standard, along with a discussion of loop resolution across Wide Area Networks.

Chapter 6, Source Routing, explains the operation of this alternative method of LAN bridging that is available on Token Ring and FDDI networks. The concepts and operation of source routing are presented, followed by a detailed discussion of the source routing algorithms implemented in both end stations and bridges. The chapter provides an extensive discussion of the problems (and some solutions) associated with the use of source routing and transparent bridges in a mixed, heterogeneous LAN environment.

Part II: Advanced LAN Switch

The second part of the book builds on the fundamentals discussed earlier to explore many advanced features and capabilities being offered in modern switches.

Chapter 7, Full Duplex Operation, explains how dedicated media and dedicated switched bandwidth can be used to eliminate the access control algorithms common in shared LANs. Following an explanation of the operation of both full duplex Ethernet and Token Ring, the chapter goes on to consider the implications of full duplex operation both for end-user network design and for switch implementations, along with a discussion of the application environments that can best leverage full duplex technology.

Chapter 8, LAN and Switch Flow Control, looks at the problems of link congestion arising from the deployment of LAN switches. It first describes the various methods of backpressure that can be implemented in shared LANs, then explains the operation of the explicit flow control protocol used on full duplex Ethernets. Special consideration is given to some of the implementation details of this protocol, as well as to the IEEE 802.3x standard itself.

Chapter 9, Link Aggregation, explains how switches and end stations can use a group of independent LAN segments as if they were a single link (often called trunking). The chapter looks at the uses for aggregated links and some of the important effects on system and higher-layer protocol behavior arising from their use. A detailed discussion of the IEEE 802.3ad Link Aggregation standard is provided.

Chapter 10, Multicast Pruning, looks at the capability of switches to restrict the propagation of multicast traffic to exactly those links and stations that need to see it. It explains in detail how switches use the GARP Multicast Registration Protocol (GMRP) to distribute traffic along subsets of the spanning tree.

Chapter 11, Virtual LANs: Applications and Concepts, is the first chapter in a two-part miniseries. It introduces the ideas and principles underlying the design of Virtual LANs (VLANs) by first exploring the various uses for VLANs, then explaining the key concepts employed by devices that implement VLAN capability, including VLAN tagging, VLAN-awareness, and the application of VLAN association rules.

Chapter 12, Virtual LANs: The IEEE Standard, shows how the concepts presented in the previous chapter are applied in industry-standard VLANs. The chapter provides a guide to the IEEE 802.1Q standard, along with detailed explanations of VLAN tag and frame formats and the internal operation of standards-compliant switches. The design and use of the GARP VLAN Registration Protocol (GVRP) is explained, and there is also a discussion on the interrelationship between VLANs and the spanning tree.

Chapter 13, Priority Operation, explores the mechanisms employed by switches to provide preferred service to specific applications, users, and/ or stations in the network. After a discussion of the native priority mechanisms available in some LAN technologies, the chapter explains how VLAN mechanisms can be used for explicit priority indication. Following this, a detailed examination of the internal operation of a priority-enabled switch is provided, including priority determination, class-of-service mapping, and output scheduling.

Chapter 14, Switch Management, considers the extra functionality required that both allows a switch to be configured and managed and also allows switches to monitor traffic in the catenet. The Simple Network Management Protocol (SNMP) is explained, along with the structure and content of the management database. Special consideration is given to network monitoring tools, including port and switch mirrors, and the implementation of Remote Monitor (RMON) capability within the switch. Alternatives to the use of SNMP are also presented.

Chapter 15, Make the Switch!, ties together all of the basic and advanced features discussed earlier and shows how these functions are combined and arranged in practical switch implementations. In addition to a walk-through of the block diagram of a hypothetical switch, an in-depth discussion of switching fabrics is provided, included shared memory, shared bus, and crosspoint matrix architectures.

References are sprinkled liberally throughout the book, both to indicate the sources of specific information or statements and to provide pointers to documents where the reader can research the area under discussion in ever greater detail. References are shown in the form:


where BOOG is the first letters of the name of the author (e. g., John Boogerhead) and 76 is the last two digits of the year Mr. Boogerhead's document was published. The full citation for all such references is provided in a separate section at the end of the book.

A glossary is also provided that gives definitions for all of the specialized terms and expressions used in the book as well as an expansion of all abbreviations and acronyms.

A Few Words from the Author

I would like to clarify two important points:

1. This book discusses the abstract engineering principles behind switch operation as well as the practical application of those principles in commercial products. Many of the examples and discussions of practical implementations are derived directly from real commercial products where the author was either involved in or personally responsible for the design. However, the book does not discuss the specific features, performance, or comparative value of products that may be available in the marketplace today; this work is not a selection or buyer's guide to LAN switches. The landscape of network products changes on an almost daily basis, and any attempt to present feature, performance, or price comparisons would be hopelessly obsolete in a very short time. Trade journals and trade shows are a good way to stay informed on the latest array of vendors, products, and features; this book should provide you with the tools to see through the marketing hype that often characterizes those information channels.

2. As discussed and reviewed in Chapter 1, LAN switches are most commonly deployed on some mix of Ethernet, Token Ring, and FDDI technologies. When it is necessary to distinguish among switch behaviors on these different LANs, I have provided separate, LAN-specific discussions and explanations. However, for many features and functions, the operation and behavior of a switch are independent of the underlying technology. In these cases, most explanations are provided in the context of Ethernet as the example LAN technology, since Ethernet comprises the vast majority of installed LANs; it is also the most common technology for which LAN switches are designed.

In some cases (e. g., full duplex flow control and link aggregation, as discussed in Chapters 8 and 9, respectively) certain switch features are applicable only to Ethernet LANs. Thus, some sections of the book may appear to be Ether-centric. However, I believe this to be appropriate considering the popularity of Ethernet relative to alternative LAN technologies.

The Laws of Networking

In my last book, Gigabit Ethernet: Technology and Applications for High-Speed LANs [SEIF98], I presented a few of "Seifert's Laws of Networking." These are a set of general principles that underlie good network design and/ or implementation. I have been collecting and refining these principles over many years of teaching and working in the network industry. When the text discussed some specific situation that exemplified a basic design rule, I would often present the principle as one of these Laws of Networking. Feedback from that book indicated that many readers enjoyed these Laws and found that they could more easily appreciate the rationale behind certain design decisions when they understood the underlying principle.

As a result, I have expanded on the presentation of these Laws in this book. When an important discussion in the text is really a special case of a general network design axiom, I have attempted to present the principle succinctly as a Law of Networking. In some cases, the law is not mine; I have tried to give credit to others who have expounded the same principle in earlier works or public forums (including bars in hotels where network standards meeting are held). In addition, each Law is now packaged in a neat graphic at no extra cost to you--a significant improvement over the earlier book!

A Special Bonus for People
Who Read Prefaces!

Following the introductory Chapter 1, Chapters 2 and 3 immediately delve into the technical details of transparent bridges as used between similar and dissimilar technologies. For over 140 pages, virtually no mention is made of a device called a switch. This may seem odd for a book that purports to be a treatise on LAN switches.

The answer to this puzzle is provided in Chapter 4, where we reveal (lo and behold!) that a LAN switch is, in fact, nothing more than a bridge. If you understand how a bridge works, you understand how a switch works, since they are simply different names for the same device. As explained in Chapter 4, bridge was the term popularized in the 1980s when the technology first emerged, and switch was the term popularized by network product marketers when they wanted to revive the technology in newer, speedier versions.

When reading Chapters 2 and 3, feel free to substitute the word switch anytime you see bridge. The text reads correctly either way. You will also save yourself from the tedium of the "homework assignment" that will be given in Chapter 4 for all readers who routinely skip over the preface in a book--they are told to go back and do what you will have already done.

My Thanks

It's a huge task to write a book of this depth and magnitude. Fortunately, I had the help of experts and colleagues who reviewed material, corrected my errors, and gave excellent advice on the content and organization of this work. Thanks to Scott Bradner, Norm Finn, Michael Gilbert, Kristin Hansen, Tony Jeffree, Mick Seaman, and Craig Wiesner for their time, reviews, and advice; to my students at the University of California at Berkeley who used the early manuscript chapters and pointed out where the book needed more clarity and detail; to Cisco Systems and Nortel Networks for permission to use the photographs in Chapter 4; to Addison-Wesley Longman for permission to use material from my earlier book, Gigabit Ethernet, as the basis of some of the text in this work; to Christina Berry, Micheline Frederick, and Kathryn Malm for their editorial assistance, and to Carol Long for managing the development process at John Wiley&Sons and picking up the tab at Salamander.

I also would like to thank all of my consulting clients, whose interesting projects give me the opportunity both to stay on top of current technology and to bring many of my ideas on switch architecture and design to commercial reality, along with my colleagues in the IEEE 802.1 and 802.3 Working Groups, who consistently develop the highest quality and most widely-adopted standards in the LAN industry.

Of course, any errors contained in this work are my responsibility alone.

Technology Updates

Network and switch technology changes quickly, especially in relation to the time required to write and publish a book such as this one. New products and features are constantly being introduced by equipment manufacturers. At the same time, the standards bodies (composed for the most part by those same equipment manufacturers) continually formalize and publish new technology standards relating to LANs and switching.

This book was written contemporaneously with a number of important standards developments, including the publication of the IEEE 802.3ad Link Aggregation and IEEE 802.3ac VLAN Frame Extension standards (both of which I edited), and the IEEE 802.5r Dedicated/ Full Duplex Token Ring standard. At the time of this writing, there are active projects considering modifications to the Spanning Tree protocol, LAN security and access control, enhanced VLAN characteristics, and 10 Gigabit Ethernet, among others. Outside of the standards community, amazing developments are occurring on a daily basis in switch chip development, network processors, and high-speed, hardware-based Layer 3 routing.

It is impossible to revise a printed book as fast as this industry moves. Therefore, to keep you informed with the most up-to-date information, including any corrections to this book, I will be maintaining a World Wide Web site for this purpose at:

I will try to keep you pleasantly surprised with updates on technology and standards as they become available.

Contact the Author

I welcome your feedback, both on the usefulness (or not) of this book, as well as any additions or corrections that should be made in future editions. Good network-related stories, jokes, and puns are always welcome. Please feel free to contact me:

Rich Seifert

Table of Contents


Laying the Foundation.

Transparent Bridges.

Bridging between Technologies.

Principles of LAN Switches.

Loop Resolution.

Source Routing.


Full Duplex Operation.

LAN and Switch Flow Control.

Link Aggregation.

Multicast Pruning.

Virtual LANs: Applications and Concepts.

Virtual LANs: The IEEE Standard.

Priority Operation.

Switch Management.

Make the Switch!





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