Top down network design

Top-Down Network Design
Third Edition
Priscilla Oppenheimer
Priscilla Oppenheimer

Cisco Press
800 East 96th Street
Indianapolis, IN 46240


ii

Top-Down Network Design

Top-Down Network Design, Third Edition
Priscilla Oppenheimer
Copyright© 2011 Cisco Systems, Inc.
Published by:
Cisco Press
800 East 96th Street
Indianapolis, IN 46240 USA
All rights reserved. No part of this book may be reproduced or transmitted in any form or by any means,
electronic or mechanical, including photocopying, recording, or by any information storage and retrieval
system, without written permission from the publisher, except for the inclusion of brief quotations in a
review.
Printed in the United States of America
First Printing August 2010
Library of Congress Cataloging-in-Publication data is on file.

ISBN-13: 978-1-58720-283-4

ISBN-10: 1-58720-283-2

Warning and Disclaimer
This book is designed to provide information about top-down network design. Every effort has been
made to make this book as complete and as accurate as possible, but no warranty or fitness is implied.
The information is provided on an “as is” basis. The author, Cisco Press, and Cisco Systems, Inc. shall have
neither liability nor responsibility to any person or entity with respect to any loss or damages arising from
the information contained in this book or from the use of the discs or programs that may accompany it.
The opinions expressed in this book belong to the author and are not necessarily those of Cisco Systems, Inc.

Trademark Acknowledgments
All terms mentioned in this book that are known to be trademarks or service marks have been appropriately
capitalized. Cisco Press or Cisco Systems, Inc., cannot attest to the accuracy of this information. Use of a
term in this book should not be regarded as affecting the validity of any trademark or service mark.


iii

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iv

Top-Down Network Design

About the Author
Priscilla Oppenheimer has been developing data communications and networking systems since 1980 when she earned her master’s degree in information science from the
University of Michigan. After many years as a software developer, she became a technical
instructor and training developer and has taught more than 3000 network engineers from
most of the Fortune 500 companies. Her employment at such companies as Apple
Computer, Network General, and Cisco gave her a chance to troubleshoot real-world network design problems and the opportunity to develop a practical methodology for enterprise network design. Priscilla was one of the developers of the Cisco Internetwork
Design course and the creator of the Designing Cisco Networks course. Priscilla teaches
network design, configuration, and troubleshooting around the world and practices what
she preaches in her network consulting business.

About the Technical Reviewers
Keith Nabozny is a technology consultant with HP, an adjunct professor at Macomb
Community College, and a graduate of Oakland University in Rochester, Michigan. He
has three Cisco professional certifications and is a Certified Information Systems
Security Professional (CISSP). Keith has supported large corporate clients for the past
14 years in operations, implementation, and engineering roles. He is currently supporting
the firewalls of a major manufacturer with locations around the world. Most recently he
taught network design and troubleshooting classes at Macomb Community College.
Keith and his family live in Southeast Michigan.
Joe Wilson, MSCS, PMC, CISSP No. 100304, is a senior network design engineer for
TelcoCapital Systems, LLC. TelcoCapital is a leading provider of Cisco Unified
Communications solutions for small and medium-sized enterprises. Joe is completing his
dissertation toward a PhD in information technology at Capella University (Minneapolis,

MN), with specializations in college teaching and IT security and assurance. Joe has
worked in information technology for the past 20 years and is a retired systems engineer
from The Boeing Company in Seattle, Washington, where he designed airborne NMS
solutions for commercial aircraft. While working for AT&T Broadband Network
Solutions as a broadband systems engineer, Joe designed commercial broadband networks using advanced communications technologies such as ATM, SONET, DWDM, and
Gigabit Ethernet. Joe has been a CISSP since 2006 and has distinguished himself as a
trusted partner in providing secure communications solutions and services to public and
private organizations. Joe teaches courses in the Cisco Networking Academy program at
DeVry University in Federal Way, Washington.


v

Dedication
To my parents, Dr. Stephen T. Worland, PhD, and Mrs. Roberta Worland, MS. They gave
me an appreciation for knowledge, logic, and analysis, and taught me that “where there’s a
will, there’s a way.”

Acknowledgments
I would like to thank Mary Beth Ray, executive editor at Cisco Press, for giving me the
opportunity to update this book and for marshaling the people and resources needed to
complete the project. I would especially like to thank Christopher Cleveland, Tonya
Simpson, and Bill McManus for their hard work on the book. I am also grateful for the
work of the technical editors, Keith Nabozny and Joe Wilson. In many ways, updating a
book is even harder than writing it in the first place, and I couldn’t have done it without
the help of Chris, Tonya, Bill, Keith, and Joe.
I also wish to thank the technical editors for the first two editions, Matthew Birkner,
Blair Buchanan, Dr. Peter Welcher, Dr. Alex Cannara, David Jansson, and Hank Mauldin.
Their terrific contributions are still evident in the third edition.
I would like to thank other networking professionals who have inspired me over the

years, including Joseph Bardwell and Anita Lenk from Connect802, Laura Chappell and
her terrific Wireshark University, Howard Berkowitz, Paul Borghese, John Neiberger,
Leigh Anne Chisholm, Marty Adkins, Matthias David Moore, Tom Lisa, Scott Vermillion,
and many more.
I am grateful for my colleagues and students in Ashland, Oregon, who have inspired and
entertained me, including Dr. Lynn Ackler, Jeff McJunkin, Andrew Krug, Brandon Kester,
Stephen Perkins, Daniel DeFreeze, Christina Kaiserman, Nicole Colbert, Corey Smith,
Stefan Hutchison, Jesse Williamson, Jonathan McCoy, Jennifer Comstock, Linda
Sturgeon, Kathleen Marrs, Vinnie Moscaritolo, Louis Kowolowski, and Robert Luaders
for his ideas regarding the design scenarios.
I’d like to thank Gary Rubin, Rob Stump, and Kip Peterson from Advanced Network
Information for the many opportunities they’ve given me over the years, in particular the
terrific opportunity to work at Cisco. To my colleagues at Cisco, Patrick Stark, our manager, Lisa Bacani, Walt Sacharok, Dax Mickelson, David Daverso, and Paul Azzi; you are
terrific!
Finally, I would like to thank Alan Oppenheimer, who throughout this project acted as my
technical advisor, therapist, chef, and best friend. I’m glad he doesn’t mind that it was
finally time to remove AppleTalk.


vi

Top-Down Network Design

Contents at a Glance
Introduction

xxii

Part I

Identifying Your Customer’s Needs and Goals

Chapter 1

Analyzing Business Goals and Constraints

Chapter 2

Analyzing Technical Goals and Tradeoffs

Chapter 3

Characterizing the Existing Internetwork

Chapter 4

Characterizing Network Traffic

Part II

Logical Network Design

117

Chapter 5

Designing a Network Topology

119

Chapter 6

Designing Models for Addressing and Numbering

Chapter 7

Selecting Switching and Routing Protocols

Chapter 8

Developing Network Security Strategies

Chapter 9

Developing Network Management Strategies

Part III

Physical Network Design

Chapter 10

Selecting Technologies and Devices for Campus Networks

Chapter 11

Selecting Technologies and Devices for Enterprise Networks

Part IV

Testing, Optimizing, and Documenting Your Network
Design 351

Chapter 12

Testing Your Network Design

Chapter 13

Optimizing Your Network Design

Chapter 14

Documenting Your Network Design
Glossary
Index

407

435

1

3
25
59

87

167

199
233
263

281

353
367
393

283
319


vii

Contents
Introduction

xxii

Part I

Identifying Your Customer’s Needs and Goals

Chapter 1

Analyzing Business Goals and Constraints

3

Using a Top-Down Network Design Methodology
Using a Structured Network Design Process
Systems Development Life Cycles

1

3

5

6

Plan Design Implement Operate Optimize (PDIOO) Network Life Cycle
Analyzing Business Goals

8

Working with Your Client

8

Changes in Enterprise Networks

10

Networks Must Make Business Sense
Networks Offer a Service

10

11

The Need to Support Mobile Users

12

The Importance of Network Security and Resiliency
Typical Network Design Business Goals

13

Identifying the Scope of a Network Design Project
Identifying a Customer’s Network Applications
Analyzing Business Constraints
Politics and Policies

19

19

Budgetary and Staffing Constraints
Project Scheduling

21

Business Goals Checklist
Summary

22

23

Review Questions
Design Scenario
Chapter 2

20

23
24

Analyzing Technical Goals and Tradeoffs
Scalability

25

Planning for Expansion

26

Expanding Access to Data

26

Constraints on Scalability

27

Availability

27

Disaster Recovery

28

Specifying Availability Requirements

29

25

16

14

12

7


viii

Top-Down Network Design

Five Nines Availability

30

The Cost of Downtime

31

Mean Time Between Failure and Mean Time to Repair
Network Performance

32

Network Performance Definitions
Optimum Network Utilization
Throughput

33

34

35

Throughput of Internetworking Devices
Application Layer Throughput
Accuracy
Efficiency

37

38
39

Delay and Delay Variation
Causes of Delay

41

Delay Variation

43

Response Time
Security

40

44

44

Identifying Network Assets

45

Analyzing Security Risks

46

Reconnaissance Attacks

47

Denial-of-Service Attacks

48

Developing Security Requirements
Manageability
Usability

50
50

Affordability

51

Making Network Design Tradeoffs
Technical Goals Checklist

52

54

55

Review Questions
Design Scenario
Chapter 3

48

49

Adaptability

Summary

36

56
56

Characterizing the Existing Internetwork
Characterizing the Network Infrastructure
Developing a Network Map

59

59

60

Characterizing Large Internetworks

60

Characterizing the Logical Architecture
Developing a Modular Block Diagram

62
64

Characterizing Network Addressing and Naming

64

31


ix

Characterizing Wiring and Media

65

Checking Architectural and Environmental Constraints
Checking a Site for a Wireless Installation
Performing a Wireless Site Survey

69

70

Checking the Health of the Existing Internetwork

71

Developing a Baseline of Network Performance
Analyzing Network Availability

68

72

73

Analyzing Network Utilization

73

Measuring Bandwidth Utilization by Protocol
Analyzing Network Accuracy

75

76

Analyzing Errors on Switched Ethernet Networks
Analyzing Network Efficiency

77

79

Analyzing Delay and Response Time

80

Checking the Status of Major Routers, Switches, and Firewalls
Network Health Checklist
Summary

83

84

Review Questions

84

Hands-On Project

85

Design Scenario
Chapter 4

82

85

Characterizing Network Traffic
Characterizing Traffic Flow

87

87

Identifying Major Traffic Sources and Stores

87

Documenting Traffic Flow on the Existing Network

89

Characterizing Types of Traffic Flow for New Network Applications
Terminal/Host Traffic Flow
Client/Server Traffic Flow
Peer-to-Peer Traffic Flow

91
91

93

Server/Server Traffic Flow

94

Distributed Computing Traffic Flow

94

Traffic Flow in Voice over IP Networks

94

Documenting Traffic Flow for New and Existing Network
Applications 95
Characterizing Traffic Load

96

Calculating Theoretical Traffic Load

97

Documenting Application-Usage Patterns

99

Refining Estimates of Traffic Load Caused by Applications
Estimating Traffic Load Caused by Routing Protocols

101

99

90


x

Top-Down Network Design

Characterizing Traffic Behavior

101

Broadcast/Multicast Behavior

Network Efficiency
Frame Size

101

102

103

Windowing and Flow Control
Error-Recovery Mechanisms

103
104

Characterizing Quality of Service Requirements
ATM QoS Specifications

105

106

Constant Bit Rate Service Category

107

Real-time Variable Bit Rate Service Category

107

Non-real-time Variable Bit Rate Service Category
Unspecified Bit Rate Service Category
Available Bit Rate Service Category

107

108

108

Guaranteed Frame Rate Service Category

108

IETF Integrated Services Working Group QoS Specifications
Controlled-Load Service
Guaranteed Service

109

110

110

IETF Differentiated Services Working Group QoS Specifications
Grade of Service Requirements for Voice Applications
Documenting QoS Requirements
Network Traffic Checklist
Summary

Design Scenario

114
115

Summary for Part I

Chapter 5

114

114

Review Questions

Part II

113

115

Logical Network Design

117

Designing a Network Topology
Hierarchical Network Design

119

120

Why Use a Hierarchical Network Design Model?
Flat Versus Hierarchical Topologies
Flat WAN Topologies

122

Flat LAN Topologies

123

122

Mesh Versus Hierarchical-Mesh Topologies
Classic Three-Layer Hierarchical Model
Core Layer

127

Distribution Layer

127

125

124

121

112

111


xi

Access Layer

128

Guidelines for Hierarchical Network Design
Redundant Network Design Topologies
Backup Paths

131

Load Sharing

132

Modular Network Design

128

130

133

Cisco SAFE Security Reference Architecture

133

Designing a Campus Network Design Topology

135

Spanning Tree Protocol

135

Spanning Tree Cost Values

136

Rapid Spanning Tree Protocol

137

RSTP Convergence and Reconvergence
Selecting the Root Bridge

139

Scaling the Spanning Tree Protocol
Virtual LANs

140

141

Fundamental VLAN Designs
Wireless LANs

138

142

144

Positioning an Access Point for Maximum Coverage
WLANs and VLANs

146

Redundant Wireless Access Points

146

Redundancy and Load Sharing in Wired LANs
Server Redundancy

Workstation-to-Router Redundancy
Hot Standby Router Protocol

150

152

Gateway Load Balancing Protocol

153

Designing the Enterprise Edge Topology
Redundant WAN Segments
Circuit Diversity

153

153

154

Multihoming the Internet Connection
Virtual Private Networking
Site-to-Site VPNs

147

148

154

157

158

Remote-Access VPNs

159

Service Provider Edge

160

Secure Network Design Topologies
Planning for Physical Security

162
162

Meeting Security Goals with Firewall Topologies

162

145


xii

Top-Down Network Design

Summary

163

Review Questions
Design Scenario
Chapter 6

165
165

Designing Models for Addressing and Numbering
Guidelines for Assigning Network Layer Addresses

167

168

Using a Structured Model for Network Layer Addressing
Administering Addresses by a Central Authority
Distributing Authority for Addressing

170

Using Dynamic Addressing for End Systems
IP Dynamic Addressing

168

169

170

171

IP Version 6 Dynamic Addressing
Zero Configuration Networking

174
175

Using Private Addresses in an IP Environment
Caveats with Private Addressing
Network Address Translation

175

177

177

Using a Hierarchical Model for Assigning Addresses

178

Why Use a Hierarchical Model for Addressing and Routing?
Hierarchical Routing

179

Classless Interdomain Routing

179

Classless Routing Versus Classful Routing
Route Summarization (Aggregation)
Route Summarization Example
Route Summarization Tips

Discontiguous Subnets
Mobile Hosts

180

181

182

183

183

184

Variable-Length Subnet Masking

185

Hierarchy in IP Version 6 Addresses
Link-Local Addresses

186

187

Global Unicast Addresses

188

IPv6 Addresses with Embedded IPv4 Addresses
Designing a Model for Naming

189

Distributing Authority for Naming
Guidelines for Assigning Names

190

191

Assigning Names in a NetBIOS Environment
Assigning Names in an IP Environment
The Domain Name System

193

193

192

189

178


xiii

Dynamic DNS Names

194

IPv6 Name Resolution

195

Summary

195

Review Questions
Design Scenario
Chapter 7

196
197

Selecting Switching and Routing Protocols

199

Making Decisions as Part of the Top-Down Network Design Process
Selecting Switching Protocols

201

Switching and the OSI Layers
Transparent Bridging

202

202

Selecting Spanning Tree Protocol Enhancements
PortFast

203

204

UplinkFast and BackboneFast

204

Unidirectional Link Detection

205

LoopGuard

206

Protocols for Transporting VLAN Information
IEEE 802.1Q

207

207

Dynamic Trunk Protocol

208

VLAN Trunking Protocol

208

Selecting Routing Protocols

209

Characterizing Routing Protocols

209

Distance-Vector Routing Protocols
Link-State Routing Protocols
Routing Protocol Metrics

210

212

214

Hierarchical Versus Nonhierarchical Routing Protocols
Interior Versus Exterior Routing Protocols

214

Classful Versus Classless Routing Protocols

214

Dynamic Versus Static and Default Routing

215

On-Demand Routing

216

Scalability Constraints for Routing Protocols
Routing Protocol Convergence
IP Routing

216

217

218

Routing Information Protocol

218

Enhanced Interior Gateway Routing Protocol
Open Shortest Path First

Intermediate System-to-Intermediate System
Border Gateway Protocol

219

221
225

224

214

200


xiv

Top-Down Network Design

Using Multiple Routing Protocols in an Internetwork

225

Routing Protocols and the Hierarchical Design Model
Redistribution Between Routing Protocols
Integrated Routing and Bridging
A Summary of Routing Protocols
Summary

227

229

230

231

Review Questions
Design Scenario
Chapter 8

231
232

Developing Network Security Strategies
Network Security Design

233

233

Identifying Network Assets
Analyzing Security Risks

234
234

Analyzing Security Requirements and Tradeoffs
Developing a Security Plan

236

Components of a Security Policy
Developing Security Procedures
Maintaining Security
Security Mechanisms

Authorization

237

237

238
239

239

Accounting (Auditing)
Data Encryption

240

240

Public/Private Key Encryption
Packet Filters

237

238

Physical Security
Authentication

235

235

Developing a Security Policy

Firewalls

226

241

243

244

Intrusion Detection and Prevention Systems
Modularizing Security Design

245

Securing Internet Connections
Securing Public Servers

245

246

Securing E-Commerce Servers

247

Securing Remote-Access and VPNs

248

Securing Remote-Access Technologies
Securing VPNs

244

248

249

Securing Network Services and Network Management
Securing Server Farms

251

250


xv

Securing User Services

252

Securing Wireless Networks

253

Authentication in Wireless Networks
Data Privacy in Wireless Networks
Summary

258

261

Review Questions
Design Scenario
Chapter 9

254

261
262

Developing Network Management Strategies
Network Management Design

263

Proactive Network Management

264

Network Management Processes

264

Fault Management

263

265

Configuration Management
Accounting Management

266

266

Performance Management
Security Management

266

268

Network Management Architectures

269

In-Band Versus Out-of-Band Monitoring

270

Centralized Versus Distributed Monitoring

270

Selecting Network Management Tools and Protocols
Selecting Tools for Network Management
Simple Network Management Protocol
Management Information Bases (MIB)
Remote Monitoring (RMON)
Cisco Discovery Protocol

271

271

271
272

273

274

Cisco NetFlow Accounting

276

Estimating Network Traffic Caused by Network Management
Summary

276

277

Review Questions
Design Scenario

278
278

Summary for Part II

279

Part III

Physical Network Design

Chapter 10

Selecting Technologies and Devices for Campus Networks
LAN Cabling Plant Design
Cabling Topologies

281

284

284

Building-Cabling Topologies

285

283


xvi

Top-Down Network Design

Campus-Cabling Topologies
Types of Cables

285

LAN Technologies

289

Ethernet Basics

290

Ethernet and IEEE 802.3

285

290

Ethernet Technology Choices

291

Half-Duplex and Full-Duplex Ethernet
100-Mbps Ethernet
Gigabit Ethernet

292

292

293

10-Gbps Ethernet

295

Selecting Internetworking Devices for a Campus Network Design
Criteria for Selecting Campus Internetworking Devices

300

Optimization Features on Campus Internetworking Devices
Example of a Campus Network Design

299

302

303

Background Information for the Campus Network Design Project
Business Goals
Technical Goals

304
304

Network Applications
User Communities

305

306

Data Stores (Servers)

307

Current Network at WVCC

307

Traffic Characteristics of Network Applications

Summary of Traffic Flows

310

311

Performance Characteristics of the Current Network
Network Redesign for WVCC

312

313

Optimized IP Addressing and Routing for the Campus Backbone
Wireless Network

Design Scenario

317
317

Selecting Technologies and Devices for Enterprise Networks
Remote-Access Technologies
PPP

315

316

Review Questions

Chapter 11

313

314

Improved Performance and Security for the Edge of the Network
Summary

303

320

321

Multilink PPP and Multichassis Multilink PPP

321

Password Authentication Protocol and Challenge Handshake
Authentication Protocol 322

319


xvii

Cable Modem Remote Access

323

Challenges Associated with Cable Modem Systems
Digital Subscriber Line Remote Access
Other DSL Implementations
PPP and ADSL

324

325

326

326

Selecting Remote-Access Devices for an Enterprise
Network Design 327
Selecting Devices for Remote Users

327

Selecting Devices for the Central Site
WAN Technologies

328

328

Systems for Provisioning WAN Bandwidth
Leased Lines

330

Synchronous Optical Network
Frame Relay

329

331

332

Frame Relay Hub-and-Spoke Topologies and Subinterfaces
Frame Relay Congestion Control Mechanisms
Frame Relay Traffic Control

335

Frame Relay/ATM Interworking
ATM

335

336

337

Ethernet over ATM
Metro Ethernet

337

338

Selecting Routers for an Enterprise WAN Design
Selecting a WAN Service Provider
Example of a WAN Design

339

340

341

Background Information for the WAN Design Project
Business and Technical Goals
Network Applications
User Communities

343

Data Stores (Servers)
Current Network

342

343
344

344

Traffic Characteristics of the Existing WAN
WAN Design for Klamath Paper Products
Summary

348

Review Questions
Design Scenario

349
349

Summary for Part III

350

345
346

341

333


xviii

Top-Down Network Design

Part IV

Chapter 12

Testing, Optimizing, and Documenting Your Network Design
Testing Your Network Design
Using Industry Tests

353

354

Building and Testing a Prototype Network System

355

Determining the Scope of a Prototype System

355

Testing a Prototype on a Production Network

356

Writing and Implementing a Test Plan for Your Network Design
Developing Test Objectives and Acceptance Criteria
Determining the Types of Tests to Run
Writing Test Scripts

358

Implementing the Test Plan

361

361

Tools for Testing a Network Design

362

362

Examples of Network Testing Tools

363

CiscoWorks Internetwork Performance Monitor
WANDL Network Planning and Analysis Tools
OPNET Technologies

364

365

NetPredict’s NetPredictor

365

365

366

Review Questions
Design Scenario
Chapter 13

364

364

NetIQ Voice and Video Management Solution
Summary

359

360

Documenting the Project Timeline

Ixia Tools

357

357

Documenting Network Equipment and Other Resources

Types of Tools

351

366
366

Optimizing Your Network Design

367

Optimizing Bandwidth Usage with IP Multicast Technologies
IP Multicast Addressing

369

Internet Group Management Protocol
Multicast Routing Protocols

370

370

Distance Vector Multicast Routing Protocol
Protocol Independent Multicast
Reducing Serialization Delay

371

371

372

Link-Layer Fragmentation and Interleaving

373

Compressed Real-Time Transport Protocol

374

368


xix

Optimizing Network Performance to Meet Quality of Service
Requirements 374
IP Precedence and Type of Service
IP Differentiated Services Field
Resource Reservation Protocol

375
376

377

Common Open Policy Service Protocol
Classifying LAN Traffic

379

379

Cisco IOS Features for Optimizing Network Performance
Switching Techniques

380

Classic Methods for Layer 3 Packet Switching
NetFlow Switching

Cisco Express Forwarding
Queuing Services

382

383

First-In, First-Out Queuing
Priority Queuing

384

Custom Queuing

384

Weighted Fair Queuing

383

385

Class-Based Weighted Fair Queuing
Low-Latency Queuing

388

Weighted Random Early Detection
389

389

Review Questions
Design Scenario
Chapter 14

388

389

Committed Access Rate
Summary

386

387

Random Early Detection
Traffic Shaping

381

382

390
391

Documenting Your Network Design

393

Responding to a Customer’s Request for Proposal
Contents of a Network Design Document
Executive Summary
Project Goal

395

396

396

Project Scope

396

Design Requirements
Business Goals

397

Technical Goals

398

397

User Communities and Data Stores

399

394

380


xx

Top-Down Network Design

Network Applications

399

Current State of the Network
Logical Design

399

400

Physical Design

400

Results of Network Design Testing
Implementation Plan
Project Schedule
Project Budget

401

402
403

Return on Investment

403

Design Document Appendix
Summary

404

Review Questions
Design Scenario

Glossary
Index

407

435

405
405

404

401


xxi

Icons Used in This Book

Communication
Server

Terminal

PC

PC with
Software

File
Server

Gateway

Catalyst

Switch

Network Cloud

Sun
Workstation

Web
Server

Router

Cisco Works
Workstation

Bridge

Multilayer
Switch

Line: Ethernet

Hub

Macintosh

Access
Server

Modem

Printer

DSU/CSU
DSU/CSU

ATM
Switch

Laptop

Line: Serial

IBM
Mainframe

Command Syntax Conventions
The conventions used to present command syntax in this book are the same conventions
used in the Cisco IOS Command Reference. The Command Reference describes these
conventions as follows:
■

Boldface indicates commands and keywords that are entered literally as shown. In
actual configuration examples and output (not general command syntax), boldface
indicates commands that are manually input by the user (such as a show command).

■

Italic indicates arguments for which you supply actual values.

■

Vertical bars (|) separate alternative, mutually exclusive elements.

■

Square brackets ([ ]) indicate an optional element.

■

Braces ({ }) indicate a required choice.

■

Braces within brackets ([{ }]) indicate a required choice within an optional element.


xxii

Top-Down Network Design

Introduction
New business practices are driving changes in enterprise networks. The transition from an
industrial to an information economy has changed how employees do their jobs, and the
emergence of a global economy of unprecedented competitiveness has accelerated the
speed at which companies must adapt to technological and financial changes.
To reduce the time to develop and market products, companies are empowering employees to make strategic decisions that require access to sales, marketing, financial, and engineering data. Employees at corporate headquarters and in worldwide field offices, and
telecommuters in home offices, need immediate access to data, regardless of whether the
data is on centralized or departmental servers.
To develop, sell, and distribute products into domestic and foreign markets, businesses

are forming alliances with local and international partners. Businesses are carefully planning their network designs to meet security goals while also offering network access to
resellers, vendors, customers, prospective customers, and contract workers located all
over the world.
To accommodate increasing requirements for remote access, security, bandwidth, scalability, and reliability, vendors and standards bodies introduce new protocols and technologies at a rapid rate. Network designers are challenged to develop state-of-the-art networks
even though the state of the art is continually changing.
Whether you are a novice network designer or a seasoned network architect, you probably have concerns about how to design a network that can keep pace with the accelerating changes in the internetworking industry. The goal of this book is to teach a systematic
design methodology that can help you meet an organization’s requirements, regardless of
the newness or complexity of applications and technologies.

Objectives
The purpose of Top-Down Network Design, Third Edition, is to help you design networks that meet a customer’s business and technical goals. Whether your customer is
another department within your own company or an external client, this book provides
you with tested processes and tools to help you understand traffic flow, protocol behavior, and internetworking technologies. After completing this book, you will be equipped
to design enterprise networks that meet a customer’s requirements for functionality,
capacity, performance, availability, scalability, affordability, security, and manageability.

Audience
This book is for you if you are an internetworking professional responsible for designing
and maintaining medium- to large-sized enterprise networks. If you are a network engineer, architect, or technician who has a working knowledge of network protocols and


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technologies, this book will provide you with practical advice on applying your knowledge to internetwork design.
This book also includes useful information for consultants, systems engineers, and sales
engineers who design corporate networks for clients. In the fast-paced presales environment of many systems engineers, it often is difficult to slow down and insist on a topdown, structured systems analysis approach. Wherever possible, this book includes shortcuts and assumptions that can be made to speed up the network design process.
Finally, this book is useful for undergraduate and graduate students in computer science
and information technology disciplines. Students who have taken one or two courses in
networking theory will find Top-Down Network Design, Third Edition, an approachable
introduction to the engineering and business issues related to developing real-world networks that solve typical business problems.

Changes for the Third Edition
Networks have changed in many ways since the second edition was published. Many
legacy technologies have disappeared and are no longer covered in the book. In addition,
modern networks have become multifaceted, providing support for numerous bandwidthhungry applications and a variety of devices, ranging from smart phones to tablet PCs to
high-end servers.
Modern users expect the network to be available all the time, from any device, and to let
them securely collaborate with coworkers, friends, and family. Networks today support
voice, video, high-definition TV, desktop sharing, virtual meetings, online training, virtual
reality, and applications that we can’t even imagine that brilliant college students are busily
creating in their dorm rooms.
As applications rapidly change and put more demand on networks, the need to teach a
systematic approach to network design is even more important than ever. With that need
in mind, the third edition has been retooled to make it an ideal textbook for college students. The third edition features review questions and design scenarios at the end of each
chapter to help students learn top-down network design.
To address new demands on modern networks, the third edition of Top-Down Network
Design also has updated material on the following topics:
■

Network redundancy

■

Modularity in network designs

■

The Cisco SAFE security reference architecture

■

The Rapid Spanning Tree Protocol (RSTP)

■

Internet Protocol version 6 (IPv6)

■

Ethernet scalability options, including 10-Gbps Ethernet and Metro Ethernet

■

Network design and management tools


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Top-Down Network Design

Organization
This book is built around the steps for top-down network design. It is organized into four
parts that correspond to the major phases of network design.

Part I: Identifying Your Customer’s Needs and Goals
Part I covers the requirements-analysis phase. This phase starts with identifying business
goals and technical requirements. The task of characterizing the existing network, including the architecture and performance of major network segments and devices, follows.
The last step in this phase is to analyze network traffic, including traffic flow and load,
protocol behavior, and quality of service (QoS) requirements.

Part II: Logical Network Design
During the logical network design phase, the network designer develops a network topology. Depending on the size of the network and traffic characteristics, the topology can
range from simple to complex, requiring hierarchy and modularity. During this phase, the
network designer also devises a network layer addressing model and selects switching
and routing protocols. Logical design also includes security planning, network management design, and the initial investigation into which service providers can meet WAN and
remote-access requirements.

Part III: Physical Network Design
During the physical design phase, specific technologies and products that realize the logical design are selected. Physical network design starts with the selection of technologies
and devices for campus networks, including cabling, Ethernet switches, wireless access
points, wireless bridges, and routers. Selecting technologies and devices for remoteaccess and WAN needs follows. Also, the investigation into service providers, which
began during the logical design phase, must be completed during this phase.

Part IV: Testing, Optimizing, and Documenting Your Network Design
The final steps in top-down network design are to write and implement a test plan, build
a prototype or pilot, optimize the network design, and document your work with a network design proposal. If your test results indicate any performance problems, during this
phase you should update your design to include such optimization features as traffic
shaping and advanced router queuing and switching mechanisms. A glossary of networking terms concludes the book.

Companion Website
Top-Down Network Design, Third Edition, has a companion website at
www.topdownbook.com. The companion website includes updates to the book,
links to white papers, and supplemental information about design resources.