Wiley the all new switch book the complete guide to LAN switching technology 2nd edition aug 2008 ISBN 0470287152 pdf

Network technologists: This includes engineers working for companies involved in the design and manufacture of computers, communications, and networking products; academics both instruct

The All-New Switch Book

The Complete Guide to LAN

Switching Technology

Second Edition

Rich Seifert Jim Edwards

Wiley Publishing, Inc.

The All-New

Switch Book

The All-New Switch Book

The Complete Guide to LAN

Switching Technology

Second Edition

Rich Seifert Jim Edwards

Wiley Publishing, Inc.

Copyright © 2008 by Rich Seifert and Jim Edwards

Published by Wiley Publishing, Inc., Indianapolis, Indiana

Published simultaneously in Canada

ISBN: 978-0-470-28715-6

Manufactured in the United States of America

10 9 8 7 6 5 4 3 2 1

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To my granddaughter, Annaliese Grace May she enjoy all of the

happiness that life has to offer I am looking forward to all of the things

that she will be teaching me for the many years to come.

— Jim Edwards

ix

Ethernet Medium Access Control 28

Bridge Performance 95

A Router by Any Other Name Would

Layer 3 Switch Operation 162

Layer 2 Versus Layer 3 Switching

Port Roles 231

BPDU — The Final Frontier er uh The New Format 234

Interconnecting the Source-Routed and

Eliminating the Link Length Restriction of

Increasing the Link Capacity 320

Maintaining Link Invariants in an

Conversation Determination Aids

He Looks Around, Around, He Sees VLANs in the

Maintaining the Filtering Database 501

Implicit Priority Determination, or

Mapping User Priority to Access Priority

Mirror, Mirror on the Switch, Which Is the Port

Management by Telnet 604

Network Analyzers (or whatever

Implementation and Performance

Switch Data Receive Path Functions 647

The invasion of Local Area Networks (LANs) into the commercial, industrial,

university, and even the home environment during the 1980s and 1990s was

nothing short of phenomenal No longer did organizations consider whether

they 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

stan-dardized 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

tech-nology) 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

decreased, they became 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

xxiii

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

regard-ing 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 for

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

net-works (in particular, Local Area Netnet-works), 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

net-work 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

archi-tecture, 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.ID 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

xxv

Chapter 4, ‘‘Principles of LAN Switches,’’ bridges the gap (pun

inten-tional) between the operation of bridges and modern switched LANs

The chapter shows how switches can be deployed in LAN

environ-ments, and then goes on to look at (1) the evolution of switch design

from its earliest days to modern, single-chip solutions, (2)

commer-cial switch configurations, and (3) switch application environments

from desktop to enterprise-wide use Equal 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

alter-native method of LAN bridging that is available on Token Ring

and FDDI networks The concepts and operation of source

rout-ing 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 Concepts

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, and 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, and 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, ‘‘Security,’’ details the security concerns that those who

manage switched environments need to be aware of and what you

can do to assist in preventing attacks, from a Layer 2 perspective

Chapter 15, ‘‘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 chapter

explains the Simple Network Management Protocol (SNMP), along with

the structure and content of the management database Special

consid-eration 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 16, ‘‘Network Troubleshooting Strategies’’ details the

top troubleshooting strategies for a Layer 2 catenet The chapter

covers the frequent issues that may arise within a switched

environment and outlines some troubleshooting strategies

Chapter 17, ‘‘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 walkthrough of the block diagram of a hypothetical switch,

an in-depth discussion of switching fabrics is provided, including

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:[BOOG76]

where BOOG is the first letters of the name of the author (for example,

John Booger-head) 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 Authors

We would like to clarify two important points:

This book discusses the abstract engineering principles behind switch

oper-ation as well as the practical applicoper-ation 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

prod-ucts 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

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 because Ethernet encompasses the vast majority of installed LANs;

it is also the most common technology for which LAN switches are designed

In some cases (for example, 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, this is appropriate considering the popularity of

Ethernet relative to alternative LAN technologies

The Laws of Networking

In Rich’s book, Gigabit Ethernet: Technology and Applications for High-Speed

LANs [SEIF98], he presented a few of ‘‘Seifert’s Laws of Networking.’’ These

are a set of general principles that underlie good network design and/or

implementation He has 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,

he 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, in the first edition of this book, Rich expanded on the presentation

of these Laws When an important discussion in the text is really a special

case of a general network design axiom, we have attempted to present the

principle succinctly as a Law of Networking In some cases, the law is from

other individuals; we 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!

In keeping with the spirit of the original edition, Jim left these Laws as they

were in the original edition He did add comments here and there (most of

which keep with a more humorous tone) This list of ‘‘It just is ism’s’’ offers

some helpful tidbits as well as just fun comments (to make the reading more

enjoyable, we hope)

A Special Bonus for People Who Read

Introductions!

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

because 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 any

time 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 Introduction in a

book — they are told to go back and do what you will have already done

A Word of Thanks from Jim

First and foremost, I want to thank Rich Seifert for the first edition of this book

It is one of the most well-written technical books that I have ever read, and it

was a tremendous pleasure to be able to work on this second edition I tried

to keep the original text untouched for the most part, removing only outdated

information and inserting updates to the technology that has been developed

since the first edition was completed

I would also like to send out a huge word of thanks for all of the individuals

that were involved in the first edition of this book Outstanding work!

Addi-tionally, there were several people that were involved in the development

of this version of the book To Carol Long, thank you for trusting me with

this revision It was a pleasure to work on and really was not a chore at all

Next, I would like to thank my development editor, Tom Dinse, for keeping

me in line It was a pleasure working with you, sir To Rachel McConlogue,

thank you for all the assistance you gave me during the production phase It is

always nice to work with people that are as friendly and helpful as everyone

that I have had the pleasure of working with at Wiley Finally, to all of the

people that work behind the scenes, thank you for your support of this project

For the photo on the front of the book, I would like to thank Richard

Bramante I have known Rich for many years As an amateur photographer,

Rich has always impressed me, and I was thrilled that he said yes when

I asked him to take some photos for me Take a look at Rich’s website

(www.richardbramante.com) Finally, I would like to send a word of thanks to

several people for what each has offered me (whether they were aware they

were helping or not): Zung Nguyen, Llaris Palomaris, Tuyet Tran, Derrick

Winston, Mike Mitz, Don Wyman, Steve Robinson, and Al Jackson Thank

you all!

Contact the Author

I welcome your feedback, both on the usefulness (or not) of this, the second

edition 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:

Jim EdwardsAllNewSwitchBook@gmail.com

P a r t

I

Foundations of LAN Switches

In This Part

Chapter 1:Laying the Foundation

Chapter 2:Transparent Bridges

Chapter 3:Bridging Between Technologies

Chapter 4:Principles of LAN Switches

Chapter 5:Loop Resolution

Chapter 6:Source Routing

C H A P T E R

1

Laying the Foundation

Before we delve into the details of Local Area Network (LAN) switch operation,

you need to consider the foundation on which LAN switches are built This

chapter examines four important building blocks that will be indispensable to

your understanding of LAN switches in later chapters:

Network architecture

Device addressing

LAN technology

LAN standards

Each is considered specifically in the context of Local Area Networks and

its relevance to LAN switching

In addition, this chapter introduces the terminology that will be used

consistently throughout the book Very often, speakers, writers, equipment

vendors, and network operations personnel use different sets of terms to

describe the elements and behavior of computer networks: Is it an Ethernet

frame or an Ethernet packet that is sent by a station?1 While a name in

itself is never inherently wrong — speakers and writers can define their own

terminology any way they want — we need to agree on the meaning of a

number of key words and phrases so that we can unambiguously describe

and understand the behavior of network protocols and devices We have tried

throughout this book to use terminology in a way that both reflects common

industry usage and is technically accurate When there is a conflict between

these points of view, we have opted for technical correctness In any case, we

have tried to be consistent and unambiguous

1 See section 1.5.2 for the answer.

3

It is not possible to provide a novice-level tutorial on every facet of

net-working that may be relevant to LAN switches This book is not intended to

be an introduction to computer networks; it is a comprehensive treatise on

the design, operation, and application of switch technology in LANs Most

of the discussions here and in later chapters presume that the reader has

some experience with networks and LAN technology While this first chapter

does provide background information, it is not intended as a primer, but as a

reminder of the technologies and concepts on which later chapters build

1.1 Network Architecture

The art of networking comprises a wide range of operations and technologies

Casual end users may think that ‘‘the network’’ is the browser or e-mail screen

interface; this is all that they know (and from their perspective, probably all

that they need to know) about networking Programmers writing application

code that must communicate among multiple machines may need to know

about the programming interfaces and network facilities provided by the local

operating system, but are generally unconcerned about the actual mechanisms

used to deliver messages Designers of high-speed optical fiber links used to

interconnect network routers and servers should not have to worry about the

data structures in the e-mail messages that may traverse a link

In addition, the applications, functions, and technologies of networking

are constantly changing Every year, new ways of increasing the data rate of

the communications channels in which our networks operate are introduced

New applications are constantly being written that use existing network

facilities to provide improved or even revolutionary new services for users

You need to make sure that advances in one area of network technology are

not constrained by limitations in other areas For example, you want to be

able to install a higher-speed communications link without having to wait for

a new application or protocol to be designed that can take advantage of that

link Similarly, you want to ensure that the new communications link does

not cause previously working applications to fail because those applications

depend on some idiosyncrasy endemic to the older technology

Padlipsky’s Rule

If you know what you’re doing, three layers is enough If you don’t, even seventeen won’t help.

The key to achieving these goals is to separate the totality

of network functions into discrete partitions called layers

Layering allows the appropriate technology to be applied

to each function and to be changed without unduly affecting

other layers The number of layers is rather arbitrary; the issue

is separation of functions Architectural layers are defined

such that each layer provides a set of distinct, related functions

Ideally, these functions are grouped such that layers can be

as independent of each other as possible; only a minimum of

information should have to pass between layer entities

Figure 1-1 depicts the Open Systems Interconnect (OSI) model of network

layering developed during the late 1970s and formally standardized in [ISO94]

It comprises seven layers of network system functions

Data Representation Independence of local data formats

Registration and access control

End-to-End Communication Error Control, Flow Control Sequenced delivery

Network-wide Communication Global Addressing, Routing, Lifetime Control Fragmentation and Reassembly

Direct Communication Framing, Link addressing, Error detection, Error and/or Flow Control (Connection-Oriented Links only )

Physical Channel Access Line Drivers/Receivers, Encoder/Decoders, Timing

Generic Application functions File transfer, Mail, Terminal emulation, etc.

Application Program Interfaces (APIs )

User-Defined Applications

Specific applications that use network services

Figure 1-1 OSI reference model for network communications

In the sections that follow, we will take a look at the functions provided by

each of these layers, with particular concern for their relevance to LANs and

LAN switches

1.1.1 Physical Layer

The Physical layer serves requests sent down from the Data Link layer

(described in the following section), and comprises those elements involved

with the actual transmission and reception of signals from the communications

medium The functions provided typically include line drivers and receivers,

signal encoders and decoders, clock synchronization circuits, and so on The

exact nature of the device(s) implementing the Physical layer is a function

of the design of the communications channel and the physical medium

itself

Examples of Physical layer interfaces are Token Ring, Ethernet, and FDDI.

The Physical layer is also concerned with the actual transmission medium,

such as network connectors, cabling types, cabling distance factors, and other

mechanical considerations

While a given networking device (for example, a LAN switch) must

obvi-ously include the circuitry needed to connect to the communications channel

on which it is to be used, the nature of that channel has little impact on

the higher-level operation of the device For example, a LAN switch performs

the same functions regardless of whether it is connected to an optical fiber

chan-nel operating at 1,000 Mb/s or a twisted pair copper wire chanchan-nel operating

at 10 Mb/s

1.1.2 Data Link Layer

The Data Link layer provides services that allow direct communication

between devices across the underlying physical channel The

communica-tion can be point-to-point in nature (exactly two communicating stacommunica-tions) or

point-to-multipoint (one-to-many), depending on the nature and configuration

of the underlying channel

In general, the Data Link layer must provide mechanisms for:

Framing: The Data Link typically must provide a way to separate

(delimit) discrete message transmissions (frames) in the Physical layer

symbol stream

Addressing: Particularly when communicating among multiple

stations on a common communications channel (as is typical of

LANs), there must be a means to identify both the sender and target

destination(s)

Error detection: It is theoretically impossible for the underlying

communications channel to be totally error free While we hope that

most transmissions will be received intact, there is always some residual

rate of data errors, regardless of the technology employed within the

Physical layer.2It is important that corrupted data not be delivered to

higher-layer clients of the Data Link At a minimum, the Data Link layer

must detect virtually all errors Depending on the design of the Data

Link, it may either discard corrupted data (leaving error recovery to

higher-layer entities) or take explicit action to correct or recover from the

2 Ultimately, quantum (thermal) noise will introduce random errors into any communications

channel, regardless of the quality of the components used or the lack of external sources of

interference.