Top Down Network Design 3rd Edition

Contents at a GlanceIntroduction xxii Part I Identifying Your Customer’s Needs and Goals 1 Chapter 1 Analyzing Business Goals and Constraints 3 Chapter 2 Analyzing Technical Goals and Tr

Top-Down Network Design, Third Edition

Printed in the United States of America

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

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Publisher: Paul Boger Manager, Global Certification: Erik Ullanderson

Associate Publisher: Dave Dusthimer Business Operation Manager, Cisco Press: Anand Sundaram Executive Editor: Mary Beth Ray Technical Editors: Keith Nabozny, Joe Wilson

Managing Editor: Sandra Schroeder Copy Editor: Bill McManus

Senior Development Editor: Christopher Cleveland Book Designer: Louisa Adair

Senior Project Editor: Tonya Simpson Proofreader: Apostrophe Editing Services

Editorial Assistant: Vanessa Evans

Composition: Mark Shirar

Indexer: Tim Wright

About the Author

Priscilla Oppenheimer has been developing data communications and networking

sys-tems since 1980 when she earned her master’s degree in information science from theUniversity of Michigan After many years as a software developer, she became a technicalinstructor and training developer and has taught more than 3000 network engineers frommost of the Fortune 500 companies Her employment at such companies as AppleComputer, Network General, and Cisco gave her a chance to troubleshoot real-world net-work design problems and the opportunity to develop a practical methodology for enter-prise network design Priscilla was one of the developers of the Cisco InternetworkDesign course and the creator of the Designing Cisco Networks course Priscilla teachesnetwork design, configuration, and troubleshooting around the world and practices whatshe 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 Hehas 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 supportingthe firewalls of a major manufacturer with locations around the world Most recently hetaught 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 hisdissertation toward a PhD in information technology at Capella University (Minneapolis,MN), with specializations in college teaching and IT security and assurance Joe hasworked in information technology for the past 20 years and is a retired systems engineerfrom The Boeing Company in Seattle, Washington, where he designed airborne NMSsolutions for commercial aircraft While working for AT&T Broadband Network

Solutions as a broadband systems engineer, Joe designed commercial broadband works using advanced communications technologies such as ATM, SONET, DWDM, andGigabit Ethernet Joe has been a CISSP since 2006 and has distinguished himself as atrusted partner in providing secure communications solutions and services to public andprivate organizations Joe teaches courses in the Cisco Networking Academy program atDeVry University in Federal Way, Washington

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

man-ager, 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

Contents at a Glance

Introduction xxii

Part I Identifying Your Customer’s Needs and Goals 1

Chapter 1 Analyzing Business Goals and Constraints 3

Chapter 2 Analyzing Technical Goals and Tradeoffs 25

Chapter 3 Characterizing the Existing Internetwork 59

Chapter 4 Characterizing Network Traffic 87

Part II Logical Network Design 117

Chapter 5 Designing a Network Topology 119

Chapter 6 Designing Models for Addressing and Numbering 167

Chapter 7 Selecting Switching and Routing Protocols 199

Chapter 8 Developing Network Security Strategies 233

Chapter 9 Developing Network Management Strategies 263

Part III Physical Network Design 281

Chapter 10 Selecting Technologies and Devices for Campus Networks 283Chapter 11 Selecting Technologies and Devices for Enterprise Networks 319

Part IV Testing, Optimizing, and Documenting Your Network

Design 351

Chapter 12 Testing Your Network Design 353

Chapter 13 Optimizing Your Network Design 367

Chapter 14 Documenting Your Network Design 393

Glossary 407Index 435

Introduction xxii

Part I Identifying Your Customer’s Needs and Goals 1

Chapter 1 Analyzing Business Goals and Constraints 3

Using a Top-Down Network Design Methodology 3

Using a Structured Network Design Process 5Systems Development Life Cycles 6

Plan Design Implement Operate Optimize (PDIOO) Network Life Cycle 7Analyzing Business Goals 8

Working with Your Client 8Changes in Enterprise Networks 10

Networks Must Make Business Sense 10 Networks Offer a Service 11

The Need to Support Mobile Users 12 The Importance of Network Security and Resiliency 12

Typical Network Design Business Goals 13Identifying the Scope of a Network Design Project 14Identifying a Customer’s Network Applications 16Analyzing Business Constraints 19

Politics and Policies 19Budgetary and Staffing Constraints 20Project Scheduling 21

Business Goals Checklist 22

Disaster Recovery 28Specifying Availability Requirements 29

Five Nines Availability 30 The Cost of Downtime 31 Mean Time Between Failure and Mean Time to Repair 31

Network Performance 32Network Performance Definitions 33Optimum Network Utilization 34Throughput 35

Throughput of Internetworking Devices 36 Application Layer Throughput 37

Accuracy 38Efficiency 39Delay and Delay Variation 40

Causes of Delay 41 Delay Variation 43

Response Time 44Security 44

Identifying Network Assets 45Analyzing Security Risks 46

Reconnaissance Attacks 47 Denial-of-Service Attacks 48

Developing Security Requirements 48Manageability 49

Usability 50Adaptability 50Affordability 51Making Network Design Tradeoffs 52Technical Goals Checklist 54

Summary 55Review Questions 56Design Scenario 56

Chapter 3 Characterizing the Existing Internetwork 59

Characterizing the Network Infrastructure 59Developing a Network Map 60

Characterizing Large Internetworks 60 Characterizing the Logical Architecture 62 Developing a Modular Block Diagram 64

Characterizing Network Addressing and Naming 64

Characterizing Wiring and Media 65Checking Architectural and Environmental Constraints 68

Checking a Site for a Wireless Installation 69 Performing a Wireless Site Survey 70

Checking the Health of the Existing Internetwork 71

Developing a Baseline of Network Performance 72Analyzing Network Availability 73

Analyzing Network Utilization 73

Measuring Bandwidth Utilization by Protocol 75

Analyzing Network Accuracy 76

Analyzing Errors on Switched Ethernet Networks 77

Analyzing Network Efficiency 79Analyzing Delay and Response Time 80Checking the Status of Major Routers, Switches, and Firewalls 82Network Health Checklist 83

Summary 84

Review Questions 84

Hands-On Project 85

Design Scenario 85

Chapter 4 Characterizing Network Traffic 87

Characterizing Traffic Flow 87

Identifying Major Traffic Sources and Stores 87Documenting Traffic Flow on the Existing Network 89Characterizing Types of Traffic Flow for New Network Applications 90

Terminal/Host Traffic Flow 91 Client/Server Traffic Flow 91 Peer-to-Peer Traffic Flow 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 97Documenting Application-Usage Patterns 99Refining Estimates of Traffic Load Caused by Applications 99Estimating Traffic Load Caused by Routing Protocols 101

Characterizing Traffic Behavior 101Broadcast/Multicast Behavior 101Network Efficiency 102

Frame Size 103 Windowing and Flow Control 103 Error-Recovery Mechanisms 104

Characterizing Quality of Service Requirements 105ATM QoS Specifications 106

Constant Bit Rate Service Category 107 Real-time Variable Bit Rate Service Category 107 Non-real-time Variable Bit Rate Service Category 107 Unspecified Bit Rate Service Category 108

Available Bit Rate Service Category 108 Guaranteed Frame Rate Service Category 108

IETF Integrated Services Working Group QoS Specifications 109

Controlled-Load Service 110 Guaranteed Service 110

IETF Differentiated Services Working Group QoS Specifications 111Grade of Service Requirements for Voice Applications 112

Documenting QoS Requirements 113Network Traffic Checklist 114

Summary 114Review Questions 114Design Scenario 115Summary for Part I 115

Part II Logical Network Design 117

Chapter 5 Designing a Network Topology 119

Hierarchical Network Design 120Why Use a Hierarchical Network Design Model? 121Flat Versus Hierarchical Topologies 122

Flat WAN Topologies 122 Flat LAN Topologies 123

Mesh Versus Hierarchical-Mesh Topologies 124Classic Three-Layer Hierarchical Model 125

Core Layer 127 Distribution Layer 127

Access Layer 128

Guidelines for Hierarchical Network Design 128

Redundant Network Design Topologies 130

Backup Paths 131

Load Sharing 132

Modular Network Design 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 138

Selecting the Root Bridge 139

Scaling the Spanning Tree Protocol 140

Virtual LANs 141

Fundamental VLAN Designs 142

Wireless LANs 144

Positioning an Access Point for Maximum Coverage 145

WLANs and VLANs 146

Redundant Wireless Access Points 146

Redundancy and Load Sharing in Wired LANs 147

Server Redundancy 148

Workstation-to-Router Redundancy 150

Hot Standby Router Protocol 152

Gateway Load Balancing Protocol 153

Designing the Enterprise Edge Topology 153

Redundant WAN Segments 153

Circuit Diversity 154

Multihoming the Internet Connection 154

Virtual Private Networking 157

Site-to-Site VPNs 158

Remote-Access VPNs 159

Service Provider Edge 160

Secure Network Design Topologies 162

Planning for Physical Security 162

Meeting Security Goals with Firewall Topologies 162

Summary 163Review Questions 165Design Scenario 165

Chapter 6 Designing Models for Addressing and Numbering 167

Guidelines for Assigning Network Layer Addresses 168Using a Structured Model for Network Layer Addressing 168Administering Addresses by a Central Authority 169Distributing Authority for Addressing 170

Using Dynamic Addressing for End Systems 170

IP Dynamic Addressing 171

IP Version 6 Dynamic Addressing 174 Zero Configuration Networking 175

Using Private Addresses in an IP Environment 175

Caveats with Private Addressing 177 Network Address Translation 177

Using a Hierarchical Model for Assigning Addresses 178Why Use a Hierarchical Model for Addressing and Routing? 178Hierarchical Routing 179

Classless Interdomain Routing 179Classless Routing Versus Classful Routing 180Route Summarization (Aggregation) 181

Route Summarization Example 182 Route Summarization Tips 183

Discontiguous Subnets 183Mobile Hosts 184

Variable-Length Subnet Masking 185Hierarchy in IP Version 6 Addresses 186

Link-Local Addresses 187 Global Unicast Addresses 188 IPv6 Addresses with Embedded IPv4 Addresses 189

Designing a Model for Naming 189Distributing Authority for Naming 190Guidelines for Assigning Names 191Assigning Names in a NetBIOS Environment 192Assigning Names in an IP Environment 193

The Domain Name System 193

Dynamic DNS Names 194 IPv6 Name Resolution 195

Summary 195

Review Questions 196

Design Scenario 197

Chapter 7 Selecting Switching and Routing Protocols 199

Making Decisions as Part of the Top-Down Network Design Process 200

Selecting Switching Protocols 201

Switching and the OSI Layers 202Transparent Bridging 202

Selecting Spanning Tree Protocol Enhancements 203

PortFast 204 UplinkFast and BackboneFast 204 Unidirectional Link Detection 205

LoopGuard 206Protocols for Transporting VLAN Information 207

IEEE 802.1Q 207 Dynamic Trunk Protocol 208 VLAN Trunking Protocol 208

Selecting Routing Protocols 209

Characterizing Routing Protocols 209

Distance-Vector Routing Protocols 210 Link-State Routing Protocols 212 Routing Protocol Metrics 214 Hierarchical Versus Nonhierarchical Routing Protocols 214 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 216 Routing Protocol Convergence 217

IP Routing 218

Routing Information Protocol 218 Enhanced Interior Gateway Routing Protocol 219 Open Shortest Path First 221

Intermediate System-to-Intermediate System 224 Border Gateway Protocol 225

Using Multiple Routing Protocols in an Internetwork 225

Routing Protocols and the Hierarchical Design Model 226 Redistribution Between Routing Protocols 227

Integrated Routing and Bridging 229

A Summary of Routing Protocols 230Summary 231

Review Questions 231Design Scenario 232

Chapter 8 Developing Network Security Strategies 233

Network Security Design 233Identifying Network Assets 234Analyzing Security Risks 234Analyzing Security Requirements and Tradeoffs 235Developing a Security Plan 235

Developing a Security Policy 236

Components of a Security Policy 237

Developing Security Procedures 237Maintaining Security 237

Security Mechanisms 238Physical Security 238Authentication 239Authorization 239Accounting (Auditing) 240Data Encryption 240

Public/Private Key Encryption 241

Packet Filters 243Firewalls 244Intrusion Detection and Prevention Systems 244Modularizing Security Design 245

Securing Internet Connections 245

Securing Public Servers 246 Securing E-Commerce Servers 247

Securing Remote-Access and VPNs 248

Securing Remote-Access Technologies 248 Securing VPNs 249

Securing Network Services and Network Management 250Securing Server Farms 251

Securing User Services 252Securing Wireless Networks 253

Authentication in Wireless Networks 254 Data Privacy in Wireless Networks 258

Summary 261

Review Questions 261

Design Scenario 262

Chapter 9 Developing Network Management Strategies 263

Network Management Design 263

Proactive Network Management 264Network Management Processes 264

Fault Management 265 Configuration Management 266 Accounting Management 266 Performance Management 266 Security Management 268

Network Management Architectures 269

In-Band Versus Out-of-Band Monitoring 270Centralized Versus Distributed Monitoring 270Selecting Network Management Tools and Protocols 271

Selecting Tools for Network Management 271Simple Network Management Protocol 271

Management Information Bases (MIB) 272 Remote Monitoring (RMON) 273

Cisco Discovery Protocol 274Cisco NetFlow Accounting 276Estimating Network Traffic Caused by Network Management 276Summary 277

Review Questions 278

Design Scenario 278

Summary for Part II 279

Part III Physical Network Design 281

Chapter 10 Selecting Technologies and Devices for Campus Networks 283

LAN Cabling Plant Design 284

Cabling Topologies 284

Building-Cabling Topologies 285

Campus-Cabling Topologies 285

Types of Cables 285LAN Technologies 289Ethernet Basics 290

Ethernet and IEEE 802.3 290

Ethernet Technology Choices 291

Half-Duplex and Full-Duplex Ethernet 292 100-Mbps Ethernet 292

Gigabit Ethernet 293 10-Gbps Ethernet 295

Selecting Internetworking Devices for a Campus Network Design 299Criteria for Selecting Campus Internetworking Devices 300Optimization Features on Campus Internetworking Devices 302Example of a Campus Network Design 303

Background Information for the Campus Network Design Project 303Business Goals 304

Technical Goals 304Network Applications 305User Communities 306Data Stores (Servers) 307Current Network at WVCC 307

Traffic Characteristics of Network Applications 310 Summary of Traffic Flows 311

Performance Characteristics of the Current Network 312

Network Redesign for WVCC 313

Optimized IP Addressing and Routing for the Campus Backbone 313 Wireless Network 314

Improved Performance and Security for the Edge of the Network 315

Summary 316Review Questions 317Design Scenario 317

Chapter 11 Selecting Technologies and Devices for Enterprise Networks 319

Remote-Access Technologies 320PPP 321

Multilink PPP and Multichassis Multilink PPP 321 Password Authentication Protocol and Challenge Handshake Authentication Protocol 322

Cable Modem Remote Access 323

Challenges Associated with Cable Modem Systems 324

Digital Subscriber Line Remote Access 325

Other DSL Implementations 326

PPP and ADSL 326

Selecting Remote-Access Devices for an Enterprise

Network Design 327

Selecting Devices for Remote Users 327

Selecting Devices for the Central Site 328

Frame Relay Hub-and-Spoke Topologies and Subinterfaces 333

Frame Relay Congestion Control Mechanisms 335

Frame Relay Traffic Control 335

Frame Relay/ATM Interworking 336

ATM 337

Ethernet over ATM 337

Metro Ethernet 338

Selecting Routers for an Enterprise WAN Design 339

Selecting a WAN Service Provider 340

Example of a WAN Design 341

Background Information for the WAN Design Project 341

Business and Technical Goals 342

Network Applications 343

User Communities 343

Data Stores (Servers) 344

Current Network 344

Traffic Characteristics of the Existing WAN 345

WAN Design for Klamath Paper Products 346

Part IV Testing, Optimizing, and Documenting Your Network Design 351

Chapter 12 Testing Your Network Design 353

Using Industry Tests 354Building and Testing a Prototype Network System 355Determining the Scope of a Prototype System 355Testing a Prototype on a Production Network 356Writing and Implementing a Test Plan for Your Network Design 357Developing Test Objectives and Acceptance Criteria 357

Determining the Types of Tests to Run 358Documenting Network Equipment and Other Resources 359Writing Test Scripts 360

Documenting the Project Timeline 361Implementing the Test Plan 361Tools for Testing a Network Design 362Types of Tools 362

Examples of Network Testing Tools 363

CiscoWorks Internetwork Performance Monitor 364 WANDL Network Planning and Analysis Tools 364 OPNET Technologies 364

Ixia Tools 365 NetIQ Voice and Video Management Solution 365 NetPredict’s NetPredictor 365

Summary 366Review Questions 366Design Scenario 366

Chapter 13 Optimizing Your Network Design 367

Optimizing Bandwidth Usage with IP Multicast Technologies 368

IP Multicast Addressing 369Internet Group Management Protocol 370Multicast Routing Protocols 370

Distance Vector Multicast Routing Protocol 371 Protocol Independent Multicast 371

Reducing Serialization Delay 372Link-Layer Fragmentation and Interleaving 373Compressed Real-Time Transport Protocol 374

Optimizing Network Performance to Meet Quality of Service

Requirements 374

IP Precedence and Type of Service 375

IP Differentiated Services Field 376

Resource Reservation Protocol 377Common Open Policy Service Protocol 379Classifying LAN Traffic 379

Cisco IOS Features for Optimizing Network Performance 380

Random Early Detection 388

Weighted Random Early Detection 388

Traffic Shaping 389Committed Access Rate 389Summary 389

Review Questions 390

Design Scenario 391

Chapter 14 Documenting Your Network Design 393

Responding to a Customer’s Request for Proposal 394

Contents of a Network Design Document 395

Executive Summary 396Project Goal 396Project Scope 396Design Requirements 397

Business Goals 397 Technical Goals 398 User Communities and Data Stores 399

Network Applications 399

Current State of the Network 399Logical Design 400

Physical Design 400Results of Network Design Testing 401Implementation Plan 401

Glossary 407

Index 435

Icons Used in This Book

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

Terminal File

Server

WebServer

Cisco WorksWorkstation

Laptop

IBMMainframe

MacintoshCommunication

Server

AccessServer

Catalyst

Switch

MultilayerSwitch

ATMSwitch

Network Cloud Line: Ethernet Line: Serial

New business practices are driving changes in enterprise networks The transition from anindustrial to an information economy has changed how employees do their jobs, and theemergence of a global economy of unprecedented competitiveness has accelerated thespeed at which companies must adapt to technological and financial changes

To reduce the time to develop and market products, companies are empowering ees to make strategic decisions that require access to sales, marketing, financial, and engi-neering data Employees at corporate headquarters and in worldwide field offices, andtelecommuters in home offices, need immediate access to data, regardless of whether thedata is on centralized or departmental servers

employ-To develop, sell, and distribute products into domestic and foreign markets, businessesare forming alliances with local and international partners Businesses are carefully plan-ning their network designs to meet security goals while also offering network access toresellers, vendors, customers, prospective customers, and contract workers located allover the world

To accommodate increasing requirements for remote access, security, bandwidth, ity, and reliability, vendors and standards bodies introduce new protocols and technolo-gies at a rapid rate Network designers are challenged to develop state-of-the-art networkseven though the state of the art is continually changing

scalabil-Whether you are a novice network designer or a seasoned network architect, you bly have concerns about how to design a network that can keep pace with the accelerat-ing changes in the internetworking industry The goal of this book is to teach a systematicdesign methodology that can help you meet an organization’s requirements, regardless ofthe newness or complexity of applications and technologies

proba-Objectives

The purpose of Top-Down Network Design, Third Edition, is to help you design

net-works that meet a customer’s business and technical goals Whether your customer isanother department within your own company or an external client, this book providesyou with tested processes and tools to help you understand traffic flow, protocol behav-ior, 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 designingand maintaining medium- to large-sized enterprise networks If you are a network engi-neer, architect, or technician who has a working knowledge of network protocols and

technologies, this book will provide you with practical advice on applying your

knowl-edge 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

environ-ment of many systems engineers, it often is difficult to slow down and insist on a

top-down, structured systems analysis approach Wherever possible, this book includes

short-cuts 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

net-works 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

bandwidth-hungry 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

stu-dents 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

This book is built around the steps for top-down network design It is organized into fourparts 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 businessgoals and technical requirements The task of characterizing the existing network, includ-ing 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 ogy Depending on the size of the network and traffic characteristics, the topology canrange from simple to complex, requiring hierarchy and modularity During this phase, thenetwork designer also devises a network layer addressing model and selects switchingand routing protocols Logical design also includes security planning, network manage-ment design, and the initial investigation into which service providers can meet WAN andremote-access requirements

topol-Part III: Physical Network Design

During the physical design phase, specific technologies and products that realize the ical design are selected Physical network design starts with the selection of technologiesand devices for campus networks, including cabling, Ethernet switches, wireless accesspoints, wireless bridges, and routers Selecting technologies and devices for remote-access and WAN needs follows Also, the investigation into service providers, whichbegan during the logical design phase, must be completed during this phase

log-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 work design proposal If your test results indicate any performance problems, during thisphase you should update your design to include such optimization features as trafficshaping and advanced router queuing and switching mechanisms A glossary of network-ing terms concludes the book

net-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

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

Analyzing Business

Goals and Constraints

This chapter serves as an introduction to the rest of the book by describing top-down

network design The first section explains how to use a systematic, top-down process

when designing computer networks for your customers Depending on your job, your

customers might consist of other departments within your company, those to whom you

are trying to sell products, or clients of your consulting business

After describing the methodology, this chapter focuses on the first step in top-down

net-work design: analyzing your customer’s business goals Business goals include the

capa-bility to run network applications to meet corporate business objectives, and the need to

work within business constraints, such as budgets, limited networking personnel, and

tight timeframes

This chapter also covers an important business constraint that some people call the

eighth layer of the Open System Interconnection (OSI) reference model: workplace

poli-tics To ensure the success of your network design project, you should gain an

under-standing of any corporate politics and policies at your customer’s site that could affect

your project

The chapter concludes with a checklist to help you determine if you have addressed the

business issues in a network design project

Using a Top-Down Network Design Methodology

According to Albert Einstein:

“The world we’ve made as a result of the level of thinking we have done thus far

cre-ates problems that we cannot solve at the same level at which we created them.”

To paraphrase Einstein, networking professionals have the ability to create networks that

are so complex that when problems arise they can’t be solved using the same sort of

thinking that was used to create the networks Add to this the fact that each upgrade,

patch, and modification to a network can also be created using complex and sometimes

convoluted thinking, and you soon realize that the result is a network that is hard tounderstand and troubleshoot A network created with this complexity often doesn’t per-form as well as expected, doesn’t scale as the need for growth arises (as it almost alwaysdoes), and doesn’t match a customer’s requirements A solution to this problem is to use astreamlined, systematic methodology in which the network or upgrade is designed in atop-down fashion.

Many network design tools and methodologies in use today resemble the dots” game that some of us played as children These tools let you place internetworkingdevices on a palette and connect them with LAN or WAN media The problem with thismethodology is that it skips the steps of analyzing a customer’s requirements and select-ing devices and media based on those requirements

“connect-the-Good network design must recognize that a customer’s requirements embody many ness and technical goals, including requirements for availability, scalability, affordability,security, and manageability Many customers also want to specify a required level of net-

busi-work performance, often called a service level To meet these needs, difficult netbusi-work

design choices and tradeoffs must be made when designing the logical network beforeany physical devices or media are selected

When a customer expects a quick response to a network design request, a bottom-up(connect-the-dots) network design methodology can be used, if the customer’s applica-tions and goals are well known However, network designers often think they understand

a customer’s applications and requirements only to discover, after a network is installed,that they did not capture the customer’s most important needs Unexpected scalabilityand performance problems appear as the number of network users increases These prob-lems can be avoided if the network designer uses top-down methods that performrequirements analysis before technology selection

Top-down network design is a methodology for designing networks that begins at theupper layers of the OSI reference model before moving to the lower layers The top-downmethodology focuses on applications, sessions, and data transport before the selection

of routers, switches, and media that operate at the lower layers

The top-down network design process includes exploring organizational and group tures to find the people for whom the network will provide services and from whom thedesigner should get valuable information to make the design succeed

struc-Top-down network design is also iterative To avoid getting bogged down in details tooquickly, it is important to first get an overall view of a customer’s requirements Later,more detail can be gathered on protocol behavior, scalability requirements, technologypreferences, and so on Top-down network design recognizes that the logical model andthe physical design can change as more information is gathered

Because top-down methodology is iterative, some topics are covered more than once inthis book For example, this chapter discusses network applications Chapter 4,

“Characterizing Network Traffic,” covers network applications in detail, with emphasis onnetwork traffic caused by application- and protocol-usage patterns A top-down

approach enables a network designer to get “the big picture” first before spiraling

down-ward into detailed technical requirements and specifications

Using a Structured Network Design Process

Top-down network design is a discipline that grew out of the success of structured

soft-ware programming and structured systems analysis The main goal of structured systems

analysis is to more accurately represent users’ needs, which unfortunately often are

ignored or misrepresented Another goal is to make the project manageable by dividing it

into modules that can be more easily maintained and changed

Structured systems analysis has the following characteristics:

■ The system is designed in a top-down sequence

■ During the design project, several techniques and models can be used to characterize

the existing system, determine new user requirements, and propose a structure for

the future system

■ A focus is placed on data flow, data types, and processes that access or change

the data

■ A focus is placed on understanding the location and needs of user communities that

access or change data and processes

■ A logical model is developed before the physical model The logical model represents

the basic building blocks, divided by function, and the structure of the system The

physical model represents devices and specific technologies and implementations

■ Specifications are derived from the requirements gathered at the beginning of the

top-down sequence

With large network design projects, modularity is essential The design should be split

functionally to make the project more manageable For example, the functions carried

out in campus LANs can be analyzed separately from the functions carried out in

remote-access networks, virtual private networks (VPN), and WANs

Cisco recommends a modular approach with its three-layer hierarchical model This

model divides networks into core, distribution, and access layers The Cisco SAFE

archi-tecture, which is discussed in Part II of this book, “Logical Network Design,” is another

modular approach to network design

With a structured approach to network design, each module is designed separately, yet in

relation to other modules All the modules are designed using a top-down approach that

focuses on requirements, applications, and a logical structure before the selection of

physical devices and products to implement the design

Systems Development Life Cycles

Systems analysis students are familiar with the concept that typical systems are developedand continue to exist over a period of time, often called a systems development life cycle.Many systems analysis books use the acronym SDLC to refer to the system’s life cycle,which might sound strange to older networking students who know SDLC as

Synchronous Data Link Control, a bit-oriented, full-duplex protocol used on synchronousserial links, often found in a legacy Systems Network Architecture (SNA) environment.Nevertheless, it’s important to realize that most systems, including network systems, fol-low a cyclical set of phases, where the system is planned, created, tested, and optimized.Feedback from the users of the system causes the system to then be redesigned or modi-fied, tested, and optimized again New requirements arise as the network opens the door

to new uses As people get used to the new network and take advantage of the services itoffers, they soon take it for granted and expect it to do more

In this book, network design is divided into four major phases that are carried out in acyclical fashion:

■ Analyze requirements: In this phase, the network analyst interviews users and

tech-nical personnel to gain an understanding of the business and techtech-nical goals for a new

or enhanced system The task of characterizing the existing network, including thelogical and physical topology and network performance, follows The last step in thisphase is to analyze current and future network traffic, including traffic flow and load,protocol behavior, and quality of service (QoS) requirements

■ Develop the logical design: This phase deals with a logical topology for the new or

enhanced network, network layer addressing, naming, and switching and routing tocols Logical design also includes security planning, network management design,and the initial investigation into which service providers can meet WAN and remoteaccess requirements

pro-■ Develop the physical design: During the physical design phase, specific

technolo-gies and products that realize the logical design are selected Also, the investigationinto service providers, which began during the logical design phase, must be com-pleted during this phase

■ Test, optimize, and document the design: The final steps in top-down network

de-sign are to write and implement a test plan, build a prototype or pilot, optimize thenetwork design, and document your work with a network design proposal

These major phases of network design repeat themselves as user feedback and networkmonitoring suggest enhancements or the need for new applications Figure 1-1 shows thenetwork design and implementation cycle

DevelopLogicalDesign

DevelopPhysicalDesign

Figure 1-1 Network Design and Implementation Cycle

Plan Design Implement Operate Optimize (PDIOO) Network Life Cycle

Cisco documentation refers to the Plan Design Implement Operate Optimize (PDIOO)

set of phases for the life cycle of a network It doesn’t matter which life cycle you use, as

long as you realize that network design should be accomplished in a structured, planned,

modular fashion, and that feedback from the users of the operational network should be

fed back into new network projects to enhance or redesign the network The PDIOO life

cycle includes the following steps:

■ Plan: Network requirements are identified in this phase This phase also includes an

analysis of areas where the network will be installed and an identification of users

who will require network services

■ Design: In this phase, the network designers accomplish the bulk of the logical and

physical design, according to requirements gathered during the plan phase

■ Implement: After the design has been approved, implementation begins The

net-work is built according to the design specifications Implementation also serves to

verify the design

■ Operate: Operation is the final test of the effectiveness of the design The network

is monitored during this phase for performance problems and any faults to provide

input into the optimize phase of the network life cycle

■ Optimize: The optimize phase is based on proactive network management that

iden-tifies and resolves problems before network disruptions arise The optimize phase

may lead to a network redesign if too many problems arise because of design errors

or as network performance degrades over time as actual use and capabilities diverge

Redesign can also be required when requirements change significantly

O

P

R D

Figure 1-2 PDIOO Network Life Cycle

■ Retire: When the network, or a part of the network, is out-of-date, it might be taken

out of production Although Retire is not incorporated into the name of the life cycle(PDIOO), it is nonetheless an important phase The retire phase wraps around to theplan phase The PDIOO life cycle repeats as network requirements evolve

Figure 1-2 shows a graphical representation of the Cisco PDIOO network life cycle

Analyzing Business Goals

Understanding your customer’s business goals and constraints is a critical aspect of work design Armed with a thorough analysis of your customer’s business objectives, youcan propose a network design that will meet with your customer’s approval

net-It is tempting to overlook the step of analyzing business goals, because analyzing suchtechnical goals as capacity, performance, security, and so on is more interesting to manynetwork engineers Chapter 2, “Analyzing Technical Goals and Tradeoffs,” covers analyz-

ing technical goals In this chapter, you learn the importance of analyzing business goals,

and you pick up some techniques for matching a network design proposal to a customer’sbusiness objectives

Working with Your Client

Before meeting with your customer to discuss business goals for the network design ect, it is a good idea to research your client’s business Find out what industry the client

proj-is in Learn something about the client’s market, suppliers, products, services, and petitive advantages With the knowledge of your customer’s business and its externalrelations, you can position technologies and products to help strengthen the customer’sstatus in the customer’s own industry

com-In your first meeting with your customers, ask them to explain the organizational ture of the company Your final internetwork design will probably reflect the corporatestructure, so it is a good idea to gain an understanding of how the company is structured

struc-in departments, lstruc-ines of busstruc-iness, vendors, partners, and field or remote offices

Understanding the corporate structure can help you locate major user communities and

characterize traffic flow Chapter 4 covers traffic flow in more detail Understanding the

corporate structure can also help you understand the corporate culture, which can affect

the network design For example, a company with a centralized management structure

might require that products and vendors be chosen by headquarters management A

decentralized company might let branch offices have more say

Note Understanding the corporate structure can also help you recognize the

manage-ment hierarchy One of your primary goals in the early stages of a network design project

should be to determine who the decision makers are Who will have the authority to accept

or reject your network design proposal? Sometimes, this can be a rather complicated issue,

as discussed in the section “Politics and Policies,” later in this chapter

Ask your customer to state an overall goal of the network design project Explain that

you want a short, business-oriented statement that highlights the business purpose of the

new network Why is the customer embarking on this new network design project? For

what will the new network be used? How will the new network help the customer be

more successful in the customer’s business?

After discussing the overall business goals of the network design project, ask your

cus-tomer to help you understand the cuscus-tomer’s criteria for success What goals must be met

for the customer to be satisfied? Sometimes success is based on operational savings

because the new network allows employees to be more productive Sometimes success is

based on the ability to increase revenue or build partnerships with other companies

Make sure you know upfront how “success” is defined by executives, managers, end

users, network engineers, and any other stakeholders Also, determine whether the

cus-tomer’s definition of success will change as yearly fiscal goals change

In addition to determining the criteria for success, you should ascertain the consequences

of failure:

■ What will happen if the network design project fails or if the network, when

in-stalled, does not perform to specification?

■ How visible is the project to upper-level management?

■ Will the success (or possible failure) of the project be visible to executives?

■ To what extent could unforeseen behavior of the new network disrupt business

oper-ations?

In general, gather enough information to feel comfortable that you understand the extent

and visibility of the network design project

You should try to get an overall view of whether the new network is critical to the

busi-ness’s mission Investigate the ramifications of the network failing or experiencing

prob-lems Chapter 2 discusses the details of performance and reliability analysis, but at this

point in the design process, you should start addressing these issues (Remember that

top-down network design is iterative Many network design requirements are addressedmore than once.)

Changes in Enterprise Networks

Enterprise networks at many corporations have been undergoing major changes Thevalue of making vast amounts of data available to employees, customers, and businesspartners has been recognized Corporate employees, field employees, contract employ-ees, and telecommuters need access to sales, marketing, engineering, and financial data,regardless of whether the data is stored on centralized or distributed servers or main-frames Suppliers, vendors, and customers also need access to many types of data

A network that is used by only internal users is no longer the norm at many companies.Companies are seeking ways to build networks that more closely resemble modern organ-izations Many modern organizations are based on an open, collaborative environmentthat provides access to information and services for many different constituents, includ-ing customers, prospective customers, vendors, suppliers, and employees

To remain competitive, companies need ways to reduce product development time andtake advantage of just-in-time manufacturing principles A lot of companies achieve thesegoals by partnering with suppliers and by fostering an online, interactive relationshipwith their suppliers An example is automobile manufacturing Instead of producing everyautomobile component in-house, many manufacturers contract with partners who spe-cialize in specific components and technologies For example, one partner might producethe engine while another produces the body If all the partners can access data and servic-

es on the manufacturer’s network, production costs are reduced, just-in-time ing can be accomplished, and it is easier to plan around component shortages The ability

manufactur-to share information saves time and money for the aumanufactur-tomobile manufacturer and for itspartners

A network designer must carefully consider requirements for extending the network tooutside users For security reasons, external access should not mean full network access.Using a modular approach to network design is important here so that a clear boundaryexists between the enterprise’s private networks and the portions of the internetwork thatpartners can access

Networks Must Make Business Sense

Although in the past many companies made “technology for technology’s sake” choices,this is no longer the case Business leaders are more involved in Information Technology(IT) decisions than they once were, and IT managers rely on business managers to helpthem prioritize and fund IT projects Network upgrades are made not because some newtechnology sounds interesting to the engineers, but because it will help an enterpriseincrease profits, productivity, market share, and cash flow Network designers mustchoose solutions that address the business dilemmas faced by business managers.Network applications have become mission critical Despite this trend, large budgets fornetworking and telecommunications operations have been reduced at some companies

Many companies have gone through difficult reengineering projects to reduce

opera-tional costs and are still looking for ways to manage networks with fewer resources and

to reduce the recurring costs of WAN circuits

Companies are researching ways to make their data centers more efficient in their usage

of power, cabling, racks, storage, and WAN circuits Companies seek to reduce data

cen-ter costs and to make data cencen-ters more “green” (whereby energy usage is reduced) Data

center managers have discovered that many of their servers’ CPUs are underutilized A

major trend in enterprise network design is server virtualization, where one hardware

platform supports multiple virtual servers Instead of many underutilized hardware boxes,

there are now just a few hardware boxes, each of which supports multiple virtual servers

Each virtual server looks and acts just like a physical server, including a fully functional

operating system and one or more applications

Streamlining processes and protocols has also led to an increased use of IP telephony and

to the continued convergence of voice and data networks To save money and to reduce

the need for specialized data or voice engineers, companies continue to adopt IP

telepho-ny technologies In previous network designs, telecommunications and voice networks

were separate Telecommunications engineers knew little about data networks, and data

communications engineers didn’t know the difference between a time-division

multiplex-er (TDM) and a tandem switching system (TSS) In today’s environment, voice, data, and

video networks are merged

Networks Offer a Service

Modern IT departments are more service-oriented than they used to be To meet the

needs of their customers, IT departments are spending more time analyzing and

docu-menting their processes for delivering services A focus on processes helps to ensure

effective service delivery and to avoid wasted expenditures on technology that doesn’t

provide a needed service

As a network designer, you might find yourself working with IT architects who adhere to

the IT Service Management (ITSM) discipline ITSM defines frameworks and processes

that can help an organization match the delivery of IT services with the business needs of

the organization ITSM focuses on processes rather than technology and helps an IT

organization think of its users as valued customers rather than problem-generating

adver-saries A version of ITSM is documented in the Information Technology Infrastructure

Library (ITIL), a series of books published by the United Kingdom Office of Government

Commerce (OGC), each of which covers an IT management topic The details of ITSM

and ITIL are outside the scope of this book, but it is worth noting that both ITSM and

top-down network design address the need to align the delivery of IT services to the

business needs of an organization This book will help you design networks that comply

with ITSM practices

Other trends in IT management that affect network design are related to governance and

compliance Governance refers to a focus on consistent, cohesive decisions, policies, and

processes that protect an organization from mismanagement and illegal activities of users

of IT services Compliance refers to adherence to regulations that protect against fraud

and inadvertent disclosure of private customer data For example, in the United States,retail organizations must comply with the Payment Card Industry Data Security Standard(PCI DSS) and healthcare organizations must comply with the Health Insurance

Portability and Accountability Act (HIPAA)

The Need to Support Mobile Users

Notebook computers have finally become small enough to carry around, and workersnow expect to get work done at home, on the train, in hotels, in meeting rooms, at cus-tomer sites, and even while having their morning latte at the local coffee shop Notebookcomputers ship with wireless networking built in to facilitate users getting work doneoutside the office

It shouldn’t matter (to the user anyway) where data is and in what format Network usersexpect network performance to be uniform, regardless of where the user or data resides

A user should be able to read email on a cell phone, for example, and read voice mailfrom a web browser while sipping coffee in an Internet cafe Users should have secureand reliable access to tools and data wherever they are The challenge for network design-ers is to build networks that allow data to travel in and out of the enterprise networkfrom various wired and wireless portals without picking up any viruses and without beingread by parties for whom it was not intended

One of the biggest trends in network design is virtual private networking (VPN), where

private networks make use of the Internet to reach remote locations or possibly otherorganizations Customers getting involved in VPN projects have concerns about security,reliable and predictable performance, and data throughput requirements Chapter 5,

“Designing a Network Topology,” covers VPNs in greater detail

Network architectures are taking on a virtual and ubiquitous form for users, while ing highly structured and managed from the network engineers’ point of view Thedesigner is challenged to develop secure, resilient, and manageable solutions that enableusers to work efficiently and securely wherever they are physically located

remain-The Importance of Network Security and Resiliency

Network security has filtered to the top of the list of business goals at many companies.Although security was always important, it has become even more important as networksbecome indispensable and as tools for breaking into networks become ubiquitous.Enterprises must protect their networks from both the unsophisticated “script kiddies”and from more advanced attacks launched by criminals or political enemies There is also

a continued requirement to protect networks from Trojan horses and viruses

Many enterprise managers now report that the network must be available 99.999 percent

of the time Although this goal might not be achievable without expensive redundancy instaff and equipment, it might be a reasonable goal for companies that would experience asevere loss of revenue or credibility if the network were down for even short periods oftime This goal is linked to goals for security, as the network can’t be available if security

breaches and viruses are disabling network devices and applications When security and

operational problems occur, networks must recover quickly Networks must be resilient

More than ever, IT and business managers require high-availability and resiliency features

for their network equipment and protocols, as they realize the extent to which network

downtime can jeopardize business success

In addition to security, another goal that has filtered to the top of the list of business

goals is the need for business continuity during and after a disaster Companies that have

survived hurricanes, earthquakes, fires, and terrorist attacks have learned the importance

of a disaster recovery plan that promotes business continuity despite the loss of critical

network devices and services Many companies have not had the misfortune of learning

these lessons the hard way but are nonetheless embarking on network design projects

with the goal of developing a network that will recover quickly if a natural or unnatural

disaster occurs

One aspect of analyzing a customer’s business goals is the process of analyzing

vulnera-bilities related to disasters and the impact on business operations Help your customer

determine which network capabilities are critical and which facilities provide them

Consider how much of the network could be damaged without completely disrupting the

company’s mission Determine whether other locations in the company are prepared to

take on mission-critical functions

In the past few years, networks have become more interconnected and complex, which

can make meeting goals for business continuity and network resiliency more difficult

Many enterprise networks are linked to telecommuter home networks, branch-office

net-works, extranets that offer access to business partners and customers, and the Internet

The diversity and quantity of portals into the enterprise network pose many security and

stability risks On the other hand, geographical diversity of mission-critical capabilities

has turned out to be a lifesaver for some companies hit with disaster One reason that

The Wall Street Journal was able to publish its newspaper the day after the 9/11 attacks

was because it had learned from 1990s power outages about the need to disperse critical

functions across many different sites

In the current business environment, security and disaster recovery should be considered

with every network design choice, and the network designer must propose solutions that

provide resiliency and stability A systematic and modular design process, as taught in

this book, is even more important than it once was, as networks become increasingly

more complex and vital to an organization’s success

Typical Network Design Business Goals

After considering the changes in business strategies and enterprise networking discussed

in the previous sections, it is possible to list some typical network design business goals:

■ Increase revenue and profit

■ Increase market share

■ Expand into new markets

■ Increase competitive advantages over companies in the same market

■ Reduce costs

■ Increase employee productivity

■ Shorten product-development cycles

■ Use just-in-time manufacturing

■ Plan around component shortages

■ Offer new customer services

■ Offer better customer support

■ Open the network to key constituents (prospects, investors, customers, businesspartners, suppliers, and employees)

■ Avoid business disruption caused by network security problems

■ Avoid business disruption caused by natural and unnatural disasters

■ Modernize outdated technologies

■ Reduce telecommunications and network costs, including overhead associated withseparate networks for voice, data, and video

■ Make data centers more efficient in their usage of power, cabling, racks, storage, andWAN circuits

■ Comply with IT architecture design and governance goals

Identifying the Scope of a Network Design Project

One of the first steps in starting a network design project is to determine its scope Some

of the most common network design projects these days are small in scope—for ple, projects to allow a few people in a sales office to access the enterprise network via aVPN On the other hand, some design projects are large in scope Ask your customer tohelp you understand if the design is for a single network segment, a set of LANs, a set ofWANs or remote-access networks, or the entire enterprise network Also ask your cus-tomer if the design is for a new network or a modification to an existing one

exam-Explain to your customer any concerns you have about the scope of the project, ing technical and business concerns Subsequent sections in this chapter discuss politicsand scheduling, which are tightly linked to the scope of a network design project (Manynetwork designers have learned the hard way what happens when you don’t help yourcustomers match the schedules of their projects to the scope.)

includ-Make sure your customers tell you everything they can about the network and the designproject You might want to poke around outside the stated scope of the project, just tomake sure nothing essential has been omitted Double-check that you have gathered all

ApplicationPresentationSessionTransportNetworkData LinkPhysical

Figure 1-3 Open System Interconnection (OSI) Reference Model

the requirements and that you have accurate information about sites, links, and devices If

the project addresses network security, make sure you know about all external links,

including any legacy dial-in access

Note Designers rarely get a chance to design a network from scratch Usually a network

design project involves an upgrade to an existing network However, this is not always the

case Some senior network designers have developed completely new next-generation

net-works to replace old netnet-works Other designers have designed netnet-works for a new building

or new campus Even in these cases, however, the new network usually has to fit into an

existing infrastructure—for example, a new campus network that has to communicate with

an existing WAN Where there is an existing network, the design project must include

plans for migrating to the new design with minimal disruption and risk

When analyzing the scope of a network design, you can refer to the seven layers of the

OSI reference model to specify the types of functionality the new network design must

address For example, you might decide that the design project is concerned only with

network layer matters such as routing and IP addressing Or you might decide that the

design also concerns the application layer because the focus is on voice applications,

such as Interactive Voice Response (IVR), which directs customers to the correct location

in a call center, or unified messaging, where email can be retrieved via voice mail and text

messages can be converted into speech Figure 1-3 shows the OSI reference model

In addition to using the OSI reference model, this book also uses the following terms to

define the scope of a network and the scope of a network design project:

■ Segment: A single network bounded by a switch or router and based on a particular

Layer 1 and Layer 2 protocol such as Fast Ethernet

■ LAN: A set of switched segments based on a particular Layer 2 protocol such as Fast

Ethernet and an interswitch trunking protocol such as the IEEE 802.1Q standard