CCNA®

Cisco Certified Network Associate

Review Guide

Todd Lammle

Senior Acquisitions Editor: Jeff KellumDevelopment Editor: Kathi DugganTechnical Editors: Troy McMillan and Dax MichelsonProduction Editor: Christine O’ConnorCopy Editor: Kathy Grider-CarlyleEditorial Manager: Pete GaughanProduction Manager: Tim TateVice President and Executive Group Publisher: Richard SwadleyVice President and Publisher: Neil EddeMedia Project Manager 1: Laura Moss-HollisterMedia Associate Producer: Marilyn HummelMedia Quality Assurance: Doug KuhnBook Designer: Happenstance Type-O-RamaCompositor: Craig Woods, Happenstance Type-O-RamaProofreader: Louise Watson, Word One New YorkIndexer: Robert SwansonProject Coordinator, Cover: Katherine CrockerCover Designer: Ryan Sneed

Copyright © 2011 by Wiley Publishing, Inc., Indianapolis, Indiana

Published simultaneously in Canada

ISBN: 978-1-118-06346-0ISBN: 978-1-118-11361-5 (ebk.)ISBN: 978-1-118-11360-8 (ebk.)ISBN: 978-1-118-11362-2 (ebk.)

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10 9 8 7 6 5 4 3 2 1

Dear Reader,

Thank you for choosing CCNA: Cisco Certified Network Associate Review Guide. This book is part of a family of premium-quality Sybex books, all of which are written by out-standing authors who combine practical experience with a gift for teaching.

Sybex was founded in 1976. More than 30 years later, we’re still committed to producing consistently exceptional books. With each of our titles, we’re working hard to set a new standard for the industry. From the paper we print on, to the authors we work with, our goal is to bring you the best books available.

I hope you see all that reflected in these pages. I’d be very interested to hear your com-ments and get your feedback on how we’re doing. Feel free to let me know what you think about this or any other Sybex book by sending me an email at nedde@wiley.com. If you think you’ve found a technical error in this book, please visit http://sybex.custhelp.com. Customer feedback is critical to our efforts at Sybex.

Best regards,

Neil Edde Vice President and Publisher Sybex, an Imprint of Wiley

AcknowledgmentsTo my best editor, Kathi Duggan: You came in and helped me knock this out with great enthusiasm. Thank you!

Thanks to Jeff Kellum, who always keeps me working hard and makes sure I am headed in the right direction. This is no easy task for Jeff!

And thanks to Christine O’Connor, who somehow made sense of my work and helped me put it together in a great, easy-to-study format.

And as always, Troy McMillian was instrumental in helping me put this book together, as well as tech editing it.

Thank you all!

About the AuthorTodd Lammle CCSI, CCNA/CCNP/CCSP, MCSE, CEH/CHFI, FCC RF Licensed, is the authority on Cisco Certification internetworking. He is a world-renowned author, speaker, trainer, and consultant. Todd has over 30 years of experience working with LANs, WANs, and large licensed and unlicensed wireless networks. He is president of GlobalNet Training and Consulting, Inc., a network integration and training firm based in Dallas, Texas. You can reach him through GlobalNet Training Solutions, Inc. at www.globalnettc.com or through his online forum at www.lammle.com.

Contents at a Glance

Introduction xiii

Chapter 1 Describe How a Network Works 1

Chapter 2 Configure, Verify, and Troubleshoot a Switch with VLANs and Interswitch Communications 57

Chapter 3 Implement an IP Addressing Scheme and IP Services to Meet Network Requirements in a Medium-Sized Enterprise Branch Office Network 119

Chapter 4 Configure, Verify, and Troubleshoot Basic Router Operation and Routing on Cisco Devices 153

Chapter 5 Explain and Select the Appropriate Administrative Tasks Required for a WLAN 235

Chapter 6 Identify Security Threats to a Network and Describe General Methods to Mitigate Those Threats 255

Chapter 7 Implement, Verify, and Troubleshoot NAT and ACLs in a Medium-Sized Enterprise Branch Office Network 269

Chapter 8 Implement and Verify WAN Links 299

Appendix About the Companion CD 325

Index 329

ContentsIntroduction xiii

Chapter 1 Describe How a Network Works 1

Describe the Purpose and Functions of Various Network Devices 2

Use the OSI and TCP/IP Models and Their Associated Protocols to Explain How Data Flows in a Network 8

Describe Common Networked Applications, Including Web Applications 10

Describe the Purpose and Basic Operation of the Protocols in the OSI and TCP Models 15

Describe the Impact of Applications (Voice over IP and Video over IP) on a Network 19

Interpret Network Diagrams 23Determine the Path between Two Hosts across a Network 31Describe the Components Required for Network and

Internet Communications 35Identify and Correct Common Network Problems at

Layers 1, 2, 3, and 7 Using a Layered Model Approach 39Differentiate between LAN/WAN Operation and Features 42Review Questions 54Answers to Review Questions 56

Chapter 2 Configure, Verify, and Troubleshoot a Switch with VLANs and Interswitch Communications 57

Select the Appropriate Media, Cables, Ports, and Connectors to Connect Switches to Other Network Devices and Hosts 59

Explain the Technology and Media Access Control Method for Ethernet Networks 62

Explain Network Segmentation and Basic Traffic Management Concepts 67

Explain Basic Switching Concepts and the Operation of Cisco Switches 69

Perform and Verify Initial Switch Configuration Tasks, Including Remote Access Management 74

Verify Network Status and Switch Operation Using Basic Utilities (Including ping, traceroute, Telnet, SSH, arp, and ipconfig) and SHOW and DEBUG Commands 77

Identify, Prescribe, and Resolve Common Switched Network Media Issues, Configuration Issues, Auto Negotiation, and Switch Hardware Failures 83

Describe Enhanced Switching Technologies (Including VTP, RSTP, VLAN, PVSTP, and 802.1q) 84

Contents

Introduction

and TCP Models

Network

x Contents

Describe How VLANs Create Logically Separate Networks and the Need for Routing between Them 88

Configure, Verify, and Troubleshoot VLANs 92Configure, Verify, and Troubleshoot Trunking on Cisco Switches 95Configure, Verify, and Troubleshoot InterVLAN Routing 99Configure, Verify, and Troubleshoot VTP 103Configure, Verify, and Troubleshoot RSTP Operation 109Interpret the Output of Various SHOW and DEBUG

Commands to Verify the Operational Status of a Cisco Switched Network 111

Implement Basic Switch Security (Including Port Security, Trunk Access, Management VLAN Other Than VLAN 1, Etc.) 111

Review Questions 114Answers to Review Questions 118

Chapter 3 Implement an IP Addressing Scheme and IP Services to Meet Network Requirements in a Medium-Sized Enterprise Branch Office Network 119

Describe the Operation and Benefits of Using Private and Public IP Addressing 121

Explain the Operation and Benefits of Using DHCP and DNS 124Configure, Verify, and Troubleshoot DHCP and DNS

Operation on a Router (Including CLI/SDM) 124Implement Static and Dynamic Addressing Services for Hosts

in a LAN Environment 127Calculate and Apply an Addressing Scheme, Including

VLSM IP Addressing Design, to a Network 132Determine the Appropriate Classless Addressing Scheme

Using VLSM and Summarization to Satisfy Addressing Requirements in a LAN/WAN Environment 134

Describe the Technological Requirements for Running IPv6 in Conjunction with IPv4 (Including Protocols, Dual Stack, Tunneling, Etc.) 142

Describe IPv6 Addresses 145Identify and Correct Common Problems Associated

with IP Addressing and Host Configurations 149Review Questions 150Answer to Review Questions 152

Chapter 4 Configure, Verify, and Troubleshoot Basic Router Operation and Routing on Cisco Devices 153

Describe Basic Routing Concepts (Including Packet Forwarding and the Router Lookup Process) 155

Contents xi

Describe the Operation of Cisco Routers (Including the Router Bootup Process, POST, and Router Components) 158

Select the Appropriate Media, Cables, Ports, and Connectors to Connect Routers to Other Network Devices and Hosts 160

Configure, Verify, and Troubleshoot RIPv2 162Access and Utilize the Router to Set Basic Parameters

(Including CLI/SDM) 165Connect, Configure, and Verify the Operational Status

of a Device Interface 175Verify Device Configuration and Network Connectivity

Using ping, traceroute, Telnet, SSH, or Other Utilities 189Perform and Verify Routing Configuration Tasks for a Static

or Default Route Given Specific Routing Requirements 189Manage IOS Configuration Files (Including Save, Edit,

Upgrade, and Restore) 192Manage Cisco IOS 196Compare and Contrast Methods of Routing and

Routing Protocols 200Configure, Verify, and Troubleshoot OSPF 202Configure, Verify, and Troubleshoot EIGRP 211Verify Network Connectivity (Including Using ping,

traceroute, and Telnet or SSH) 216Troubleshoot Routing Issues 216Verify Router Hardware and Software Operation Using

the show and debug Commands 221Implement Basic Router Security 227Review Questions 231Answers to Review Questions 233

Chapter 5 Explain and Select the Appropriate Administrative Tasks Required for a WLAN 235

Describe Standards Associated with Wireless Media (Including IEEE, Wi-Fi Alliance, and ITU/FCC) 236

Identify and Describe the Purpose of the Components in a Small Wireless Network (Including SSID, BSS, and ESS) 243

Identify the Basic Parameters to Configure on a Wireless Network to Ensure That Devices Connect to the Correct Access Point 246

Compare and Contrast Wireless Security Features and Capabilities of WPA Security (Including Open, WEP, and WPA-1/2) 248

Identify Common Issues with Implementing Wireless Networks (Including Interfaces and Misconfigurations) 251

Review Questions 252Answers to Review Questions 254

xii Contents

Chapter 6 Identify Security Threats to a Network and Describe General Methods to Mitigate Those Threats 255

Describe Today’s Increasing Network Security Threats and Explain the Need to Implement a Comprehensive Security Policy to Mitigate the Threats 256

Explain General Methods to Mitigate Common Security Threats to Network Devices, Hosts, and Applications 260

Describe the Functions of Common Security Appliances and Applications 261

Describe Security Recommended Practices, Including Initial Steps to Secure Network Devices 264

Review Questions 266Answers to Review Questions 268

Chapter 7 Implement, Verify, and Troubleshoot NAT and ACLs in a Medium-Sized Enterprise Branch Office Network 269

Describe the Purpose and Types of ACLs 270Configure and Apply ACLs Based on Network Filtering

Requirements (Including CLI/SDM) 270Configure and Apply ACLs to Limit Telnet and SSH Access

to the Router Using (Including SDM/CLI) 283Verify and Monitor ACLs in a Network Environment 285Troubleshoot ACL Issues 288Explain the Basic Operation of NAT 288Configure NAT for Given Network Requirements

Using (Including CLI/SDM) 290Troubleshoot NAT Issues 293Review Questions 295Answers to Review Questions 297

Chapter 8 Implement and Verify WAN Links 299

Describe Different Methods for Connecting to a WAN 300Configure and Verify a Basic WAN Serial Connection 301Configure and Verify Frame Relay on Cisco Routers 304Troubleshoot WAN Implementation Issues 310Describe VPN Technology (Including Importance,

Benefits, Role, Impact, and Components) 314Configure and Verify a PPP Connection between

Cisco Routers 315Review Questions 319Answers to Review Questions 323

Appendix About the Companion CD 325

Index 329

IntroductionWelcome to the exciting world of Cisco certifi cation! You picked up this book because you want something better—namely, a better job with more satisfaction. Rest assured that you made a good decision. Cisco certifi cation can help you get your fi rst networking job, or more money and a promotion if you are already in the fi eld.

Cisco certifi cation can also improve your understanding of the internetworking of more than just Cisco products. By studying the material in this book, you will develop a complete understanding of networking and how different network topologies work together to form a network. This is benefi cial to every networking job and is the reason Cisco certifi cation is in such high demand, even at companies with few Cisco devices.

Cisco is the king of routing and switching, the Microsoft of the internetworking world. The Cisco certifi cations reach beyond the popular certifi cations, such as the MCSE, to provide you with an indispensable factor in understanding today’s network—insight into the Cisco world of internetworking. By deciding that you want to become Cisco certifi ed, you are saying that you want to be the best—the best at routing and the best at switching. This book will lead you in that direction.

What Is CCNA Certification?The CCNA certifi cation was the fi rst in the new line of Cisco certifi cations and was the precursor to all current Cisco certifi cations. Now you can become a Cisco Certifi ed Network Associate for the meager cost of this book and either one exam or two exams However, the CCNA exams are extremely hard and cover a lot of material, so you have to really know your stuff. Taking a Cisco class or spending months getting hands-on experience is not out of the norm.

Once you have your CCNA certifi cation, you don’t have to stop there. You can continue your studies and achieve a higher certifi cation, called the Cisco Certifi ed Network Professional (CCNP). Someone with a CCNP certifi cation has all the skills and knowledge he or she needs to attempt the Routing and Switching CCIE lab. Just becoming a CCNA can land you the job you’ve dreamed about.

This book covers everything CCNA related. For up-to-date information on Todd Lammle Cisco Authorized CCNA, CCNP, CCSP, CCVP, and CCIE bootcamps, please see www.lammle.com and/or www.globalnettc.com.

xiv Introduction

Is This Book For You?CCNA: Cisco Certified Network Associate Review Guide is designed to be a succinct, porta-ble exam review guide that can be used in conjunction with a more complete study guide such as the CCNA: Cisco Certified Network Associate Study Guide, 7th Edition or CCNA: Cisco Certified Network Associate Deluxe Study Guide, 6th Edition (Sybex, 2011), Todd Lammle Video Series, and a CBT courseware, and a classroom/lab environment. It can also be used as an exam review for those who don’t feel the need for more extensive test preparation. It isn’t my goal to give away the answers, but rather to identify those topics on which you can expect to be tested and to provide sufficient coverage of those topics.

Perhaps you’ve been working with information technologies for years. The thought of paying lots of money for a specialized IT exam-preparation course probably doesn’t sound appealing. What can they teach you that you don’t already know, right? Be careful, though—many experienced network administrators have walked confidently into the test center only to sheepishly walk out of it after failing an IT exam. After you’ve finished read-ing this book, you should have a clear idea of how your understanding of the technologies involved matches up with the expectations of the CCNA test makers.

Perhaps you’re relatively new to the world of IT, drawn to it by the promise of challeng-ing work and higher salaries. You’ve just waded through an 800-page study guide or taken a week-long class at a local training center. That is a lot of information to keep track of, isn’t it? Well, by organizing this book according to Cisco’s exam objectives, and by breaking up the information into concise, manageable pieces, we’ve created what we think is the handiest exam review guide available. Throw it in your briefcase and carry it to work with you. As you read the book, you’ll be able to quickly identify those areas you know best and those that require a more in-depth review.

How Is This Book Organized?This book is organized according to the official objectives list prepared by Cisco for the CCNA exam. The chapters correspond with the eight broad objective categories:

Chapter 1: Describe How a Network Works

Chapter 2: Configure, Verify, and Troubleshoot a Switch with VLANs and Interswitch Communications

Chapter 3: Implement an IP Addressing Scheme and IP Services to Meet Network Requirements in a Medium-Size Enterprise Branch Office Network

Chapter 4: Configure, Verify, and Troubleshoot Basic Router Operation and Routing on Cisco Devices

Chapter 5: Explain and Select the Appropriate Administrative Tasks Required for a WLAN

Chapter 6: Identify Security Threats to a Network and Describe General Methods to Mitigate Those Threats

Introduction xv

Chapter 7: Implement, Verify, and Troubleshoot NAT and ACLs in a Medium-Sized Enterprise Branch Office Network

Chapter 8: Implement and Verify WAN Links

Within each chapter, the individual exam objectives are addressed. Each section of a chapter covers one exam objective. For each objective, the critical information is first presented, and then there are several Exam Essentials for each exam objective. Additionally, each chapter ends with a section of Review Questions. Here is a closer look at each of these components:

Exam Objectives The individual exam objective sections present the greatest level of detail on information that is relevant to the CCNA exam. This is the place to start if you’re unfamiliar with or uncertain about the technical issues related to the objective.

Exam Essentials Here you are given a short list of topics that you should explore fully before taking the exam. Included in the Exam Essentials areas are notations of the key information you should take out of the exam objective section.

Review Questions This section ends every chapter and provides 10 questions to help you gauge your mastery of the chapter.

On the CDWe’ve included several testing features on the CD that accompanies this book. These tools will help you retain vital exam content as well as prepare you to sit for the actual exams:

Two CCNA Practice Exams The CD contains the Sybex Test Engine, with two exclusive CCNA bonus practice exams. Using this custom test engine, you can identify weak areas up front and then develop a solid studying strategy using each of these robust testing features. Our thorough readme file will walk you through the quick, easy installation process.

Electronic Flashcards You’ll find flashcard questions on the CD for on-the-go review. These are short questions and answers, just like the flashcards you probably used to study in school.

Glossary of Terms in PDF The CD contains a very useful Glossary of Terms in PDF (Adobe Acrobat) format, so you can easily read it on any computer. If you have to travel and brush up on any key terms, and you have a laptop with a CD drive, you can do so with this useful resource.

Tips for Taking Your CCNA ExamThe CCNA exam contains about 55 questions or more, to be completed in about 90 minutes or less. This can change per exam. You must get a score of about 85 percent to pass this exam, but again, each exam can be different.

Many questions on the exam have answer choices that at first glance look identical—especially the syntax questions! Remember to read through the choices carefully, because

xvi Introduction

close doesn’t cut it. If you get commands in the wrong order or forget just one character, you’ll get the question wrong. So, to practice, do the hands-on exercises at the end of the chapters in this book over and over again until they feel natural to you.

Also, never forget that the right answer is the Cisco answer. In many cases, more than one appropriate answer is presented, but the correct answer is the one that Cisco recommends. The exam always tells you to pick one, two, or three answers (never “choose all that apply”).

The CCNA 640-802 exam includes the following test formats:

Multiple-choice single answerNN

Multiple-choice multiple answerNN

Drag-and-dropNN

Fill-in-the-blankNN

Router simulationsNN

Here are some general tips for exam success:

Arrive early at the exam center, so you can relax and review your study materials.NN

Read the questions NN carefully. Don’t jump to conclusions. Make sure you’re clear about exactly what each question asks.

When answering multiple-choice questions that you’re not sure about, use the process NN

of elimination to get rid of the obviously incorrect answers first. Doing this greatly improves your odds if you need to make an educated guess.

You can no longer move forward and backward through the Cisco exams—so double-NN

check your answer before clicking Next, because you can’t change your mind.

After you complete an exam, you’ll get immediate, online notifi cation of your pass or fail status, a printed Examination Score Report that indicates your pass or fail status, and your exam results by section. (The test administrator will give you the printed score report.) Test scores are automatically forwarded to Cisco within fi ve working days after you take the test, so you don’t need to send your score to them. If you pass the exam, you’ll receive confi rma-tion from Cisco, typically within two to four weeks.

The CCNA Exam ObjectivesCisco has posted eight categories with specifi c objectives within each category. As men-tioned previously, these exam objectives form the outline for this book. Following are Cisco’s objectives for the CCNA exam:

Exam objectives are subject to change at any time without prior notice and at Cisco’s sole discretion. Please visit the CCNA page of Cisco’s website (www.cisco.com) for the most current listing of exam objectives.

Introduction xvii

Describe how a network works

Describe the purpose and functions of various network devicesNN

Select the components required to meet a network specificationNN

Use the OSI and TCP/IP models and their associated protocols to explain how data NN

flows in a network

Describe common networked applications, including web applicationsNN

Describe the purpose and basic operation of the protocols in the OSI and TCP modelsNN

Describe the impact of applications (Voice over IP and Video over IP) on a networkNN

Interpret network diagramsNN

Determine the path between two hosts across a networkNN

Describe the components required for network and Internet communicationsNN

Identify and correct common network problems at layers 1, 2, 3, and 7 using a layered NN

model approach

Differentiate between LAN/WAN operation and featuresNN

Configure, verify, and troubleshoot a switch with VLANs and interswitch communications

Select the appropriate media, cables, ports, and connectors to connect switches to NN

other network devices and hosts

Explain the technology and media access control method for Ethernet networksNN

Explain network segmentation and basic traffic management conceptsNN

Explain basic switching concepts and the operation of Cisco switchesNN

Perform and verify initial switch configuration tasks, including remote access NN

management

Verify network status and switch operation using basic utilities (including ping, NN

traceroute, Telnet, SSH, arp, and ipconfig) and SHOW and DEBUG commands

Identify, prescribe, and resolve common switched network media issues, configuration NN

issues, auto negotiation, and switch hardware failures

Describe enhanced switching technologies (including VTP, RSTP, VLAN, PVSTP, NN

and 802.1q)

Describe how VLANs create logically separate networks and the need for routing NN

between them

Configure, verify, and troubleshoot VLANsNN

Configure, verify, and troubleshoot trunking on Cisco switchesNN

Configure, verify, and troubleshoot interVLAN routingNN

Configure, verify, and troubleshoot VTPNN

xviii Introduction

Configure, verify, and troubleshoot RSTP operationNN

Interpret the output of various Show and Debug commands to verify the operational NN

status of a Cisco switched network

Implement basic switch security (including port security, trunk access, management NN

VLAN other than VLAN 1, etc.)

Implement an IP addressing scheme and IP Services to meet network requirements in a medium-size Enterprise branch office network

Describe the operation and benefits of using private and public IP addressingNN

Explain the operation and benefits of using DHCP and DNSNN

Configure, verify, and troubleshoot DHCP and DNS operation on a router (including NN

CLI/SDM)

Implement static and dynamic addressing services for hosts in a LAN environmentNN

Calculate and apply an addressing scheme, including VLSM IP addressing design, to NN

a network

Determine the appropriate classless addressing scheme using VLSM and summarization NN

to satisfy addressing requirements in a LAN/WAN environment

Describe the technological requirements for running IPv6 in conjunction with IPv4 NN

(including protocols, dual stack, tunneling, etc.)

Describe IPv6 addressesNN

Identify and correct common problems associated with IP addressing and host NN

configurations

Configure, verify, and troubleshoot basic router operation and routing on Cisco devices

Describe basic routing concepts (including packet forwarding and the router NN

lookup process)

Describe the operation of Cisco routers (including the router bootup process, POST, NN

and router components)

Select the appropriate media, cables, ports, and connectors to connect routers to other NN

network devices and hosts

Configure, verify, and troubleshoot RIPv2NN

Access and utilize the router to set basic parameters (including CLI/SDM)NN

Connect, configure, and verify the operational status of a device interfaceNN

Verify device configuration and network connectivity using ping, traceroute, Telnet, NN

SSH, or other utilities

Perform and verify routing configuration tasks for a static or default route given NN

specific routing requirements

Introduction xix

Manage IOS configuration files. (including save, edit, upgrade, and restore)NN

Manage Cisco IOSNN

Compare and contrast methods of routing and routing protocolsNN

Configure, verify, and troubleshoot OSPFNN

Configure, verify, and troubleshoot EIGRPNN

Verify network connectivity (including using ping, traceroute, and telnet or SSH)NN

Troubleshoot routing issuesNN

Verify router hardware and software operation using the NN SHOW and DEBUG commands

Implement basic router securityNN

Explain and select the appropriate administrative tasks required for a WLAN

Describe standards associated with wireless media (including IEEE WI-FI Alliance NN

and ITU/FCC)

Identify and describe the purpose of the components in a small wireless network NN

(including SSID, BSS, and ESS)

Identify the basic parameters to configure on a wireless network to ensure that devices NN

connect to the correct access point

Compare and contrast wireless security features and capabilities of WPA security NN

(including open, WEP, and WPA-1/2)

Identify common issues with implementing wireless networks (including interfaces and NN

misconfigurations)

Identify security threats to a network and describe general methods to mitigate those threats

Describe today’s increasing network security threats and explain the need to implement NN

a comprehensive security policy to mitigate the threats

Explain general methods to mitigate common security threats to network devices, NN

hosts, and applications

Describe the functions of common security appliances and applicationsNN

Describe security recommended practices, including initial steps to secure network devicesNN

Implement, verify, and troubleshoot NAT and ACLs in a medium-size Enterprise branch office network

Describe the purpose and types of ACLsNN

Configure and apply ACLs based on network filtering requirements (including CLI/SDM)NN

Configure and apply ACLs to limit Telnet and SSH access to the router using NN

(including SDM/CLI)

Verify and monitor ACLs in a network environmentNN

xx Introduction

Troubleshoot ACL issuesNN

Explain the basic operation of NATNN

Configure NAT for given network requirements using (including CLI/SDM)NN

Troubleshoot NAT issuesNN

Implement and verify WAN links

Describe different methods for connecting to a WANNN

Configure and verify a basic WAN serial connectionNN

Configure and verify Frame Relay on Cisco routersNN

Troubleshoot WAN implementation issuesNN

Describe VPN technology (including importance, benefits, role, impact, components)NN

Configure and verify a PPP connection between Cisco routersNN

Chapter

1Describe How a Network Works

THE CISCO CCNA ExAM OBjECTIVES COVERED IN THIS CHAPTER INCLuDE THE fOLLOWING:

Describe the purpose and functions of various NÛnetwork devices.

Select the components required to meet a network NÛspecification.

Use the OSI and TCP/IP models and their associated NÛprotocols to explain how data flows in a network.

Describe common networked applications, including NÛweb applications.

Describe the purpose and basic operation of the protocols NÛin the OSI and TCP models.

Describe the impact of applications (Voice over IP and NÛVideo over IP) on a network.

Interpret network diagrams.NÛ

Determine the path between two hosts across a network.NÛ

Describe the components required for network and NÛInternet communications.

Identify and correct common network problems at NÛlayers 1, 2, 3, and 7 using a layered model approach.

Differentiate between LAN/WAN operation and features.NÛ

In this chapter, we will review the basics of internetworking. First, we will examine what an internetwork is; next, we will look at the Open Systems Interconnection (OSI) model and

review each part. We will also review the difference between TCP and UDP with regard to VoIP and video streaming. Our review will also include PDU encapsulation, TCP/IP trouble-shooting, Ethernet, and WAN connections.

Describe the Purpose and Functions of Various Network DevicesIt is likely that at some point you’ll have to break up one large network into a bunch of smaller ones because user response will have dwindled to a slow crawl as the network grows and grows. With all that growth, your LAN’s traffic congestion will have reached epic proportions. The best way to handle this will be to break up your large network into several smaller networks—something called network segmentation.

You can do this by using devices like routers, switches, and bridges. Figure 1.1 displays a network that’s been segmented with a switch so each network segment connected to the switch is a separate collision domain. But make note of the fact that this network is still one broadcast domain.

Keep in mind that the hub used in Figure 1.1 just extended the one collision domain from the switch port. Here’s a list of some of the things that commonly cause LAN traffic congestion:

Too many hosts in a broadcast domainNN

Broadcast stormsNN

MulticastingNN

Low bandwidthNN

Adding hubs for connectivity to the networkNN

Routers are used to connect networks together and route packets of data from one network to another. Cisco became the de facto standard of routers because of its high-quality router products, great selection, and fantastic service. Routers, by default, break up a broadcast domain—the set of all devices on a network segment that hear all the broadcasts sent on that segment. Figure 1.2 shows a router in a little network that creates an internetwork and breaks up broadcast domains.

Describe the Purpose and Functions of Various Network Devices 3

f I Gu R E 1.1 A switch can replace the hub, breaking up collision domains.

(Hub)

Switch

f I Gu R E 1. 2 Routers create an internetwork.

A router creates an internetwork and provides connections to WAN services.

Switch

Switch

Serial 0

The network in Figure 1.2 shows that each host is connected to its own collision domain, and the router has created two broadcast domains—and don’t forget that the router provides connections to WAN services as well. The router uses something called a serial interface for WAN connections—specifically, a V.35 physical interface on a Cisco router.

4 Chapter 1 N Describe How a Network Works

Breaking up a broadcast domain is important because when a host or server sends a net-work broadcast, every device on the network must read and process that broadcast—unless you have a router. When the router’s interface receives this broadcast, it can respond by discarding the broadcast without forwarding it to other networks. Even though routers are known for breaking up broadcast domains by default, it’s important to remember that they break up collision domains as well.

Here are two advantages of using routers in your network:

They don’t forward broadcasts by default.NN

They can filter the network based on layer 3 (Network layer) information (such as NN

an IP address).

Four router functions in your network can be listed as follows:

Packet switchingNN

Packet filteringNN

Internetwork communicationNN

Path selectionNN

Routers are actually layer 3 switches (we’ll talk about layers later in this chapter). Unlike layer 2 switches, which forward or fi lter frames, routers (layer 3 switches) use logical address-ing and provide what is called packet switching. When routers connect two or more networks together and use logical addressing (IP or IPv6), this is called an internetwork. Last, routers use a routing table (map of the internetwork) to make path selections and to forward packets to remote networks.

Conversely, switches don’t break up broadcast domains to create internetworks. The main purpose of a switch is to optimize a LAN’s performance and provide more bandwidth for its users. And switches don’t forward packets to other networks as routers do. Instead, they only “switch” frames from one port to another within the switched network.

By default, switches break up collision domains. This is an Ethernet term used to describe a network scenario wherein one particular device sends a packet on a network segment, forcing every other device on that same segment to pay attention to it. At the same time, a different device tries to transmit, leading to a collision, after which both devices must retransmit, one at a time, which is not very effi cient. This situation is typi-cally found in a hub environment where each host segment connects to a hub that repre-sents only one collision domain and only one broadcast domain. By contrast, each and every port on a switch represents its own collision domain.

Switches create separate collision domains but a single broadcast domain. Routers provide a separate broadcast domain for each interface.

The term bridging was introduced before routers, switches, and hubs were implemented, so it’s pretty common to hear people referring to bridges as switches. That’s because bridges and switches basically do the same thing: they break up collision domains on a LAN. (In reality, you cannot buy a physical bridge these days; you can buy only LAN switches, but they use bridging technologies, so Cisco still calls them multiport bridges.)

Select the Components Required to Meet a Network Specification 5

A switch is basically a multiple-port bridge with greatly enhanced management capabilities and features. Also, while bridges may have up to 16 ports, switches can have hundreds!

You would use a switch/bridge in a network to reduce collisions within broadcast domains and to increase the number of collision domains in your network. Doing this provides more bandwidth for users. Keep in mind that using hubs in your network can contribute to congestion on your Ethernet network. As always, plan your network design carefully!

Exam Essentials

Understand the difference between a hub, a bridge, a switch, and a router. Hubs create one collision domain and one broadcast domain. Bridges break up collision domains but create one large broadcast domain. They use hardware addresses to fi lter the network. Switches are really just multiple-port bridges with more intelligence. They break up collision domains but create one large broadcast domain by default. Switches use hardware addresses to fi lter the network. Routers break up broadcast domains (and collision domains) and use logical addressing to fi lter the network.

Identify the functions and advantages of routers. Routers perform packet switching, fi ltering, and path selection; and they facilitate internetwork communication. One advantage of routers is that they reduce broadcast traffi c.

Remember the possible causes of LAN traffic congestion. Too many hosts in a broadcast domain, broadcast storms, multicasting, and low bandwidth are all possible causes of LAN traffi c congestion.

Understand the difference between a collision domain and a broadcast domain. Collision domain is an Ethernet term used to describe a network collection of devices in which one par-ticular device sends a packet on a network segment, forcing every other device on that same segment to pay attention to it. On a broadcast domain, a set of all devices on a network seg-ment hears all broadcasts sent on that segment.

Select the Components Required to Meet a Network SpecificationAs mentioned in the previous objective, routers, bridges, and switches are used in an internetwork.

Figure 1.3 shows how a network would look with all these internetwork devices in place. Remember that the router will not only break up broadcast domains for every LAN interface, but it will also break up collision domains.

6 Chapter 1 N Describe How a Network Works

f I Gu R E 1. 3 Internetworking devices

Router

Bridge

Switch

When you looked at Figure 1.3, you may have noticed that the router is at center stage and that it connects each physical network together. This type of layout is necessary because of the older technologies involved—bridges and hubs.

On the top internetwork in Figure 1.3, you’ll notice that a bridge was used to connect the hubs to a router. The bridge breaks up collision domains, but all the hosts connected to both hubs are still crammed into the same broadcast domain. Also, the bridge created only two collision domains, so each device connected to a hub is in the same collision domain as every other device connected to that same hub. This is still better than having one collision domain for all hosts.

Also, notice that the three hubs at the bottom are connected to the router, creating one col-lision domain and one broadcast domain. This makes the bridged network work much better.

Although bridges/switches are used to segment networks, they will not isolate broadcast or multicast packets.

Select the Components Required to Meet a Network Specification 7

In Figure 1.3, the best network connected to the router is the LAN switch network on the left. This is because each port on that switch breaks up collision domains. But it’s not all good—all devices are still in the same broadcast domain. Do you remember why this can be a really bad thing? Because all devices must listen to all broadcasts transmitted, that’s why. If your broadcast domains are too large, users have less bandwidth and are required to process more broadcasts, and network response time will slow to an unacceptable level.

Once you have only switches in your network, things change a lot! Figure 1.4 shows the network that is typically found today.

f I Gu R E 1. 4 Switched networks creating an internetwork

Router

Here, the LAN switches are placed at the center of the network world so that the routers are connecting only logical networks together. If this kind of setup is implemented, virtual LANs (VLANs) are created. But it is really important to understand that even though you have a switched network, you still need a router to provide your inter-VLAN communication, or internetworking.

Obviously, the best network is one that’s correctly configured to meet the business requirements of the company it serves. LAN switches with routers, correctly placed in the network, are the best network design. This book will help you understand the basics of routers and switches, so you can make tight, informed decisions on a case-by-case basis.

Let’s go back to Figure 1.3. Looking at the figure, how many collision domains and broad-cast domains are in this internetwork? Hopefully, you answered nine collision domains and three broadcast domains. The broadcast domains are definitely the easiest to see because only routers break up broadcast domains by default. Because there are three connections, that gives you three broadcast domains. The all-hub network is one collision domain; the bridge network equals three collision domains. Add in the switch network of five collision domains—one for each switch port—and you’ve got a total of nine collision domains.

So now that you’ve reviewed internetworking and the various devices in an internetwork, it’s time to head into internetworking models.

8 Chapter 1 N Describe How a Network Works

Exam Essentials

Understand which devices create a LAN and which separate and connect LANs. Switches and bridges are used to create LANs. While they do separate collision domains, they do not create separate LANs (collision domain and LAN are not the same concept). Routers are used to separate LANs and connect LANs (broadcast domains).

Understand the difference between a hub, a bridge, a switch, and a router. Hubs create one collision domain and one broadcast domain. Bridges break up collision domains but create one large broadcast domain. They use hardware addresses to fi lter the network. Switches are really just multiple-port bridges with more intelligence. They break up collision domains but create one large broadcast domain by default. Switches use hardware addresses to fi lter the network. Routers break up broadcast domains (and collision domains) and use logical addressing to fi lter the network.

Use the OSI and TCP/IP Models and Their Associated Protocols to Explain How Data Flows in a NetworkThe Department of Defense (DoD) model is basically a condensed version of the OSI model—it’s composed of the following four (instead of seven) layers:

Process/Application layerN

Host-to-Host layerNN

Internet layerN

Network Access layerNN

Figure 1.5 shows a comparison of the DoD model and the OSI reference model. As you can see, the two are similar in concept, but each has a different number of layers with different names.

When the different protocols in the IP stack are discussed, the layers of the OSI and DoD models are interchangeable. In other words, the Internet layer and the Network layer describe the same thing, as do the Host-to-Host layer and the Transport layer.

A vast array of protocols combine at the DoD model’s Process/Application layer to inte-grate the various activities and duties spanning the focus of the OSI’s corresponding top three layers (Application, Presentation, and Session). We’ll be looking closely at those protocols in the next part of this chapter. The Process/Application layer defi nes protocols for node-to-node application communication and controls user-interface specifi cations.