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OSPF Network Design SolutionsForeword

Part 1: Contemporary Intranets

Foundations of Networking

Networking Routing Fundamentals

Understanding & Selecting Networking Protocols

Part 2: OSPF Routing & Network Design

Introduction to OSPF

The Fundamentals of OSPF Routing & Design

Advanced OSPF Design Concepts

Part 3: OSPF Implementation, Troubleshooting & Management

Monitoring and Troubleshooting and OSPF Network

Managing Your OSPF Network

Part 4: Network Security & Future Expansion

Securing Your OSPF Network

The Continuing Evolution of OSPF

Future Network Considerations

Appendix A: Cisco Keyboard Commands

Bibliography

Designing & Implementing an OSPF Network

Copyright 1989-2000 Cisco Systems Inc.

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Internet Routing ArchitecturesForeword, Trademark, Acknowledgments, and Introduction

Evolution of the Internet

ISP Services and Characteristics

Handling IP Address Depletion

Interdomain Routing Basics

Tuning BGP Capabilities

Redundancy, Symmetry, and Load Balancing

Controlling Routing Inside the Autonomous System

Designing Stable Internets

Configuring Basic BGP Functions and Attributes

Configuring Effective Internet Routing Policies

RIPE-181


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Internetworking Terms andAcronyms

Introduction

Numerics

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Table of Contents

Foreword

ForewordNetwork routing protocols have emerged as key enabling technologies in a computing world nowdominated by connectivity. From a very high-level perspective, these routing protocols can be split intointerior gateway protocols (IGPs) and exterior gateway protocols (EGPs). In general, the routingtechniques used by IGPs are based on either distance-vector or link-state algorithms.

The Open Shortest Path First (OSPF) routing protocol has evolved into the link-state protocol of choicefor many IP networks. This has come about for a variety of converging reasons. Most importantly, OSPFhas proved to be both reliable and scalable. In addition, its underlying protocol assumptions encourage astructured network design approach, while these same characteristics promote rapid route convergenceduring operation. The basic features and capabilities of OSPF are described together as a set ofspecifications under the Requests for Comment (RFCs) regulated by the OSPF Working Group of theInternet Engineering Task Force (IETF).

From OSPF's earliest days, Cisco has been closely involved with the evolution of related IETF standards.Throughout this process, Cisco's development engineering staff has worked carefully to ensure that theimplementation of OSPF in Cisco routers is both robust and comprehensive. However, as with anycomplex network topology, uncontrolled growth without careful network design can lead to performanceand convergence problems--even with OSPF. At its core, one of the key objectives of Tom Thomas'book, OSPF Network Design Solutions, is to help network engineers and architects avoid the pitfalls ofunstructured network deployment.

This book aims to provide specific Cisco solutions for network engineers deploying OSPF in large-scaleIP networks. In doing so, we hope that it contributes to your information toolkit in a substantive way andfacilitates the creation of robust and reliable network infrastructures. While the emphasis here is onOSPF and Cisco's implementation, we also hope that the ideas presented will help anyone deployinglarge networks using link-state routing protocols--regardless of the specific underlying protocols orequipment.

Cisco's OSPF implementation was initially released in early 1992 with IOS software release 9.0 (1).

Since that time it has logged many operational years on large-scale production networks, incorporatedcountless improvements to add robustness, and added optimizations that allow Cisco's largest customersto succeed with globally dispersed networks. As always, Cisco continues to make enhancements basedon lessons learned from customers and their implementations. This process of continuous improvement isat the very heart of Cisco's approach to supporting IP networks worldwide.

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We at Cisco believe it is vitally important for information to be shared among networking professionalsand we view this book as an important step in the process of disseminating practical hands-onknowledge--knowledge that is often locked up in the busy lives of networking gurus. Cisco will continueto support in its products many protocols for routing, transporting, protecting, and labeling networkeddata. As a mature, modern routing protocol, OSPF is an important member of that suite. We stronglysupport books like OSPF Network Design Solutions as being an important next step beyond basicproduct documentation for the people who actually plan and implement real IP networks.

Dave Rossetti

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IP Internet Services Unit

Cisco Systems, Inc.

Posted: Wed Aug 2 16:27:47 PDT 2000Copyright 1989-2000Cisco Systems Inc.Copyright 1997 Macmillan Publishing USA, a Simon & Schuster Company

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Table of Contents

PART 1:

Contemporary Intranets

The complexity of networks has been increasing steadily since the 1970s into the contemporary intranetswe see in use today. In order to fully understand the information presented in this book, a firm foundationin network technologies must first be built.

Chapter 1, "Foundations of Networking," provides an essential perspective on the historicalfoundations and issues facing networks and intranets.

Chapter 2, "Networking Routing Fundamentals," discusses the fundamentals of routing within anetworked environment.

Chapter 3, "Understanding & Selecting Network Protocols," discusses one of the most importantsubjects facing anyone involved in today's growing networks.


Part 1: Contemporary Intranets

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Table of Contents


Intranets--The Latest Stage in the Evolution of Networking

Mainframe/Host Network ModelClient/Server Model

Typical Corporate Intranets

Reacting to Accelerated Network GrowthManaging Accelerated Network GrowthScaling PerformanceExtending Network ReachControlling Your Intranet

Understanding the OSI Reference Model

What Is the OSI Reference Model?Why Was the OSI Reference Model Needed?Characteristics of the OSI Layers

Understanding the Seven Layers of the OSI Reference Model

Upper Layers (Layers 5, 6, 7--Handle Application Issues)

Layer 7--ApplicationLayer 6--PresentationLayer 5--Session

Lower Layers (Layers 1, 2, 3, 4--Handle Data Transport Issues)

Layer 4--TransportLayer 3--NetworkLayer 2--Data LinkLayer 1--Physical

OSI Reference Model Layers and Information Exchange

Headers and DataHow Does the OSI Reference Model Process Work?Open Systems Interconnection (OSI) Protocols

Intranet Topologies

Local Area Networks (LANs)


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EthernetToken RingFiber Distributed Data Internetworking (FDDI)

Wide Area Networks (WANs)

Frame RelaySwitched Virtual Circuits (SVCs)Point-to-Point Protocol (PPP)Asynchronous Transfer Mode (ATM)Integrated Systems Digital Network (ISDN)

Summary

Foundations of Networking"Priorities: A hundred years from now it will not matter what my bank account was, the sort of car Idrove . . . but the world may be a different because I was important in the life of a child."--Successories

The physical and logical structures of networks have become varied and diverse as the technologies theyuse have evolved. The "legacy networks" of years past have evolved into the complex architecturesknown as Enterprise Networks. In many cases, these intranets of today have also generated newnetworking challenges.

To understand the value of intranets and the challenges they create, it helps to remember how peopletraditionally have connected to corporate information. This chapter covers the following important topicsand objectives:

Intranets--The Latest Stage in the Evolution of Networking. What is an intranet? A briefhistory on network evolution and an overview of the issues facing today's corporate intranets.

Open Systems Interconnection (OSI) Reference Model. An overview of the OSI referencemodel and description of the various layers to include how and where routers operate within themodel.

Intranet Topologies. Description, brief discussion, and examples of the most common Local AreaNetwork (LAN) and Wide Area Network (WAN) topologies.

Intranets--The Latest Stage in the Evolution ofNetworkingOne of the most important questions that must be answered is: "What is an intranet?" Although there aremany definitions possible, for the purposes of this book, an intranet is an Internet Protocol (IP)-basednetwork that can span various geographical regions or just connect several buildings in a campusenvironment. This is a somewhat simplistic definition, but you can ask 10 network engineers to define anintranet and get 10 different responses. The characteristics and their relationship to networking are shownin Table 1-1.



Table 1-1: Internet, intranet, and network characteristics.

Internet Intranet Network

Underlying Protocol TCP/IP TCP/IP MultipleProprietaryProtocols

Capabilities ofNetwork Management

Limited Management Varied ManagementCapabilities

Closely Managed

Level of Security Unsecured Varied Levels ofSecurity

Varied of Security

Network Routing Dynamic Dynamic Static andDynamic

Overall NetworkArchitecture

Web-based Similar to the Internet Legacy

As demonstrated in Table 1-1, the various networking archetypes have become very complex. How didthey get way? The evolution of networking archetypes has generally moved towards shorter applicationdevelopment times, faster deployment of new technology, lower cost per user, greater scalability, andhigher performance. As they have made this movement throughout the evolution of networking, vastimprovements have been made. This evolution is discussed in the following sections.

Gordon Moore of Intel made an interesting observation in 1965, just six years after he invented the firstplanar transistor. His observation was that the "doubling of transistor density on a manufactured dieevery year" would occur. Now some 30 years later his statement has become known as "Moore's Law,"and it has continued to hold true. According to Intel, "There are no theoretical or practical challenges thatwill prevent Moore's Law being true for another 20 years at least, this is another five generations ofprocessors." Using Moore's Law to predict into the year 2012, Intel should have the capability tointegrate one billion transistors on a production die that will be operating at 10GHz. This could result in aperformance of 100,000 MIPS. This is the same increase over the Pentium II processor as the Pentium IIprocessor was to the 386."

Mainframe/Host Network Model

The first "networks" can be traced back to the standard mainframe/host model, which was pioneered byIBM in the early 1960s. This centralized computing was the topology of choice during this era ofnetworking. The protocol running in this environment was known as System Network Architecture



(SNA). It is a time-sensitive broadcast intensive protocol that is based hierarchically. SNA requiredlarge, powerful mainframes to properly operate within its standards.

The mainframe/host type of topology provided mission-critical applications that stored data on themainframe. Terminals, known as logical units or hosts, provided a common interface to the user forrunning the applications and accessing the data.

The terminals in this model were considered "dumb" in the sense that they had no capability to processdata. Equipment known as the cluster controllers formatted the screens and collected data for theterminals. They were known as cluster controllers because each one had a "cluster" of terminalsconnected to it. These controllers were in turn connected to communication controllers that handled theinput and output processing needed by the terminals. Then the communication controllers in turn wereconnected to the mainframe computer that housed the company's applications and processors. Figure 1-1illustrates typical mainframe architecture.

Figure 1-1: Mainframe-centered network with remote terminals.

On a logical level, the mainframe model has many drawbacks when compared to the networks andapplications of today. Its application development was a slow and ponderous process and the cost ofcomputing power was very high; however, the mainframe model did have some benefits as well:

Mainframe components were networked together with a single protocol, typically SNA

The largely text-based traffic consumed little bandwidth

Tight security with a single point of control

Hierarchical design had highly predictable traffic flows



Client/Server Model

During the 1980s, the computing world was rocked by the introduction of the personal computer (PC).This intelligent terminal or workstation drove an industry-wide move towards intelligent workstations.This move had wide ramifications that continue to be felt to this day.

The introduction of the PC propelled the evolution of the mainframe model toward LANs. There werealready quite a few token-ring networks deployed in support of mainframes, but they did not yet have thelarge number of PCs attached to them as they have today. It was during this time that mainframes andclient/servers melded together as the PC slowly replaced mainframe systems. The PC's capability to beboth a terminal emulator and an intelligent workstation--client--blurred the lines between host-basedsystems and client servers, because applications and data were stored on a dedicated workstation thatbecame known as a server. This melding also resulted in early routers known as gateways that providedthe connectivity between various types of clusters and the evolving LANs back to the mainframes. Figure1-2 shows a typical client/server-mainframe hybrid network.

Figure 1-2: Client/server- mainframe hybrid network.

The importance of digital-based WANs became more prevalent at this time. This was also assisted by thePC's capability to perform protocol-based calculations as required for different physical media types.

In the client/server model, computing power is less expensive and the application development cycles areshorter; however, this architecture results in multi-protocol traffic and unpredictable traffic flows. This isa drawback of the decentralized control of the client/server model with its dispersed architecture.



Although the traffic can be uneven and bursty, it is still somewhat predictable due to the hierarchicalstructure that still exists, in which clients communicate primarily with the server.

As this model developed and evolved through the 1980s, it drove the development of technology in boththe LAN and WAN arenas. This resulting evolution of networking models has resulted in the corporateintranets of today.

Typical Corporate IntranetsThe typical intranet model of today has toppled traditional hierarchies of previous network models. Therapid changes in networking during the 1990s are astounding and far-reaching, as indicated by thefollowing factors:

Distributed processing enables many different intelligent devices to work together so that theymeet and, in many cases, exceed the computing power of mainframes.

Corporate legacy systems are downsized as movement continues away from mainframe-basedcomputing.

Increased demands for more bandwidth have created many emerging technologies that havepushed networks to the limit.

Intelligent routing protocols and equipment intelligently and dynamically build routingdatabases, reducing design and maintenance work.

Internetworking topologies have evolved as routers and bridges are used to network more andmore mini and personal computers.

Protocol interoperability connecting different LAN and WAN architectures together hasincreased standards between protocols. Through the increasingly prevalent melding of the twonetwork types, the applicable protocols become more and more intertwined.

The Telecommunications Act of 1996, known as Public Law 104-104, provided opportunities fortelecommunications suppliers to increase bandwidth and competition.

All of these factors have resulted in and raised many issues that must be considered by everyoneinvolved in networking. Foremost is the issue of accelerated network growth. As sweeping changes havebecome standard, everyone must learn how to react and manage this growth.

Reacting to Accelerated Network Growth

In recent years, the growth of networks everywhere has accelerated as many organizations move into theinternational business arena and join the Internet. This expansion has continued to drive the development,refinement, and complexity of network equipment and software, consequently resulting in some uniqueissues and exciting advances.

Can you imagine modern business or life these days without computers, fax machines and services,e-mail, Internet commerce and access, automatic teller machines, remote banking, check cards, or videoconferencing? Even more importantly, today's children will think that these tools are commonplace andthat business cannot be done without them.

Nevertheless, many of these tools are used to track, process, and perform the day-to-day business of



today's organizations in a variety of different ways. The need for the newest and best technology appearsto be the solution to growing organizational requirements. As this newer technology becomes available,it must be implemented, immediately.

Perhaps the most frustrating issue in dealing with unrestrained network growth is the reactivemanagement required as opposed to the more effective proactive style. This issue is further exasperatedby the melding of many different technologies.

Managing Accelerated Network Growth

To properly manage the current and future needs of a rapidly growing network, you must have thenecessary tools and techniques at your disposal.

Part IV, "Network Security & Future Expansion," discusses some of the emerging tools and technologyin further detail. Some of the things you need to consider doing now in order to effectively manage yournetwork's growth, include the following:

Reliability. This can have a major impact on how expensive it is, in both time and money, tomaintain your network. An unreliable network will consume vast amounts of technically skilledlabor made up of people who must constantly configure and react to network problems. "SecuringYour OSPF Network," discusses the actual ways you go about increasing the reliability of yourOSPF network.

Base Line Measurement. This is an essential part of planning the expansion of andtroubleshooting of your network. How can you accurately understand the impact of growth,changes, modifications, or possible future network changes? You can easily do so if youunderstand the utilization, error rates, and characteristics of your network from a point in time."Future Network Considerations," discusses some of the more useful tools and techniques used toaddress these types of issues.

Capacity Planning. An integral part of determining when you should expand network capacity."The Continuing Evolution of OSPF," discusses the overhead OSPF demands of a network,enabling you to plan your network's needs accordingly.

Network Monitoring. This is a fundamental toolset that should be used for managing thenetwork's growth. Tracking and monitoring expansion and changes within the network isimportant to ensure that these changes are fulfilling their purpose. Of course, this also works inreverse; you can monitor your existing network to ensure legacy equipment is also performing."Managing Your OSPF Network," discusses managing your OSPF network in much greater detail.

Scaling Performance

Network growth can impose heavy new loads on your infrastructure. Financial data or inventory reports,for example, can be extremely popular when initially released, resulting in an increased network load.Within a week, network performance is back to "normal." It is during these surges that your planning isextremely important. "Advanced OSPF Design Concepts," discusses methods to partition and loadbalance your network.



Extending Network Reach

Extending network reach seems like an issue that is never ending in intranets the world over. There arealways new sites to add or another feature that needs to be implemented. Many networks have kept pacewith the growth on their backbone but have sites located away from the backbone that must also beconsidered. Intranets require that users at one site have transparent access to resources located at anyother site. In the yet to be discovered "perfect" network, local and remote connectivity and performancemust be considered equally.

You can strive for equality in the network's performance, response time, and reliability, between localand remote users by following a few steps:

Optimize your WAN bandwidth and its use throughout your network to keep bandwidth costs at aminimum. "The Fundamentals of OSPF Routing & Design," covers the methods OSPF provides toassist you in optimizing network bandwidth.

Properly secure the network in such a way that it does not exact performance penalties or placeunneeded barriers. "Securing Your OSPF Network," covers the security features of OSPF in detail.

Make your Enterprise network accessible throughout to provide yourself with a dynamic"end-to-end" infrastructure. Making the Enterprise network accessible provides you with severaladvantages, such as low bandwidth usage, scalability, and a widely supported underlying protocol.OSPF is an obvious choice for implementing an Enterprise network because OSPF is a supportedprotocol found in many Enterprise networks.

An intranet can intensify the bandwidth crunch that rules most planning and strategy due to itsdistributed architecture and unpredictable traffic flows; however, effective allocation ofbandwidth, security, and proper routing protocol implementation can provide the performance,security, and flexibility needed to extend intranet reach.

Controlling Your Intranet

It is essential that you keep control of your intranet. Without control, the dangers and issues can result ina loss of connectivity to a full network crash. This book covers some of the more common problems andissues relating to accelerated growth. The proposed solutions have been tested in network after network,and through the use of OSPF, you will be able to address the many different problems. First, you buildthe structure of a network.

To monitor and evaluate reliability, baseline measurements, capacity planning, and network monitoringensure controlled network growth. This is much more desirable than allowing uncontrolled networkgrowth to be the norm for your intranet.

Understanding the OSI Reference ModelIt is important for you to understand the basic concepts of the OSI reference model because it is theunderpinning of every intranet and network. This section will introduce the reader to the OSI referencemodel's history, purpose, basic terminology, as well as concepts associated with the OSI reference model.A thorough discussion of the OSI reference model is outside the scope of this book. For complete and



exhaustive coverage of the OSI reference model, the following important ISO standards andspecifications for the OSI protocol are recommended:

Physical layerCCITT X.21. 15-pin physical connection specification CCITT X.21 BIS-25 pin connection similar to EIA RS-232-C

Data Link layer

ISO 4335/7809. High-level data link control specification (HDLC) ISO 8802.2. Local area logical link control (LLC) ISO 8802.3. (IEEE 802.3) Ethernet standard ISO 8802.4. (IEEE 802.4) Token Bus standard ISO 8802.5. (IEEE 802.5) Token Ring standard ISO 802.3u. Fast Ethernet standard ISO 802.3z. Gigabit Ethernet standard ISO 802.10. VLAN standard

Network layerISO 8473. Network layer protocol and addressing specification for connectionless networkservice

ISO 8208. Network layer protocol specification for connection-oriented service based onCCITT X.25

CCITT X.25. Specifications for connecting data terminal equipment to packet-switchednetworks

CCITT X.21. Specifications for accessing circuit-switched networks

Transport layerISO 8072. OSI Transport layer service definitions ISO 8073. OSI Transport layer protocol specifications

Session layerISO 8326. OSI Session layer service definitions, including transport classes 0, 1, 2, 3, and 4 ISO 8327. OSI Session layer protocol specifications

Presentation layerISO 8822/23/24. Presentation layer specification ISO 8649/8650. Common application and service elements (CASE) specifications andprotocols

Application layerX.400. OSI Application layer specification for electronic message handling (electronic mail) FTAM. OSI Application layer specification for file transfer and access method VTP. OSI Application layer specification for virtual terminal protocol, specifying commoncharacteristics for terminals

JTM. Job transfer and manipulation standard



Some other good references include the ISO Web page (http://www.iso.ch/ cate/35.html)and the Institute of Electrical and Electronics Engineers (IEEE) Web page (http://www.ieee.com)

Note The Consultative Committee for International Telephone and Telegraph (CCITT) is responsible forwide-area aspects of national and international communications and publishing recommendations.

In addition, because the OSI reference model has become the standard upon which protocols andapplications are based throughout the networking community, knowledge about its features andfunctionality will always be of use to you. The sections that follow will answer a few basic questionsconcerning the OSI reference model.

What Is the OSI Reference Model?

OSI stands for Open Systems Interconnection, where "open systems" refers to the specificationssurrounding its structure as well as its non-proprietary public availability. Anyone can build the softwareand hardware needed to communicate within the OSI structure.

The work on OSI reference model was initiated in the late 1970s, and came to maturity in the late 1980sand early 1990s. The International Organization of Standardization (ISO) was the primary architect ofthe model in place today.

Why Was the OSI Reference Model Needed?

Before the development of the OSI reference model, the rapid growth of applications and hardwareresulted in a multitude of vendor-specific models. In terms of future network growth and design, thisrapid growth caused a great deal of concern among network engineers because they had to ensure thesystems under their control could to interact with every standard. This concern encouraged theInternational Organization of Standardization (ISO) to initiate the development of the OSI referencemodel.

Characteristics of the OSI Layers

To provide the reader with some examples of how the layers are spanned by a routing protocol, pleaserefer to Figure 1-3. You might also want to contact Network General, as their Protocol chart shows howalmost every single protocol spans the seven layers of the OSI reference model (see below).

Figure 1-3 provides a very good illustration to help the reader understand how the seven layers aregrouped together in the model, as previously discussed. For a larger picture of how protocols are laid inthe OSI reference model, go to the following locations and request a copy of their applicable posters:

Wandell & Golterman offer free OSI, ATM, ISDN, and Fiber Optics posters at http://www.wg.com

Network Associates offers a Guide to Communications Protocols at http://www.nai.com

Figure 1-4 shows the division between the upper and lower OSI layers.



http://www.iso.ch/http://www.ieee.com/http://www.wg.com/http://www.nai.com/

A cute little ditty to help you remember all seven OSI Layers and their order is as follows:

All Application

People Presentation

Seem Session

To Transport

Need Network

Data Data Link

Processing Physical

Understanding the Seven Layers of the OSIReference ModelThe seven layers of the OSI reference model can be divided into two categories: upper layers and lowerlayers. The upper layers of the model are typically concerned only with applications, and the lower layersprimarily handle data transportation.

Upper Layers (Layers 5, 6, 7--Handle Application Issues)

The upper layers of the OSI referece model are concerned with application issues. They are generallyimplemented only in software. The Application layer is the highest layer and is closest to the end user.Both users and Application layer processes interact with software applications containing acommunications component.

Note The term upper layer is often used to refer to any higher layer, relative to a given layer.

Layer 7--Application

Essentially, the Application layer acts as the end-user interface. This is the layer where interactionbetween the mail application (cc:Mail, MS Outlook, and so forth) or communications package and theuser occurs. For example, when a user desires to send an e-mail message or access a file on the server,this is where the process starts. Another example of the processes going on at this layer are things likeNetwork File System (NFS) use and the mapping of drives through Windows NT.

Layer 6--Presentation

The Presentation layer is responsible for the agreement of the communication format (syntax) betweenapplications. For example, the Presentation layer enables Microsoft Exchange to correctly interpret amessage from Lotus Notes. Another example of the actions occurring in this layer is the encryption anddecryption of data in PGP (Pretty Good Privacy).



Figure 1-3: OSI layer groupings.



Figure 1-4: How a protocol spans the OSI reference model.

Layer 5--Session



The Session layer is responsible for the Application Layer's management of information transfer, to theData Transport portion of the OSI reference model. An example is Sun's or Novell's Remote ProcedureCall (RPC), this functionality uses Layer 5.

Lower Layers (Layers 1, 2, 3, 4--Handle Data Transport Issues)

The lower layers of the OSI reference model handle data transport issues. The Physical and Data Linklayers are implemented in hardware and software. The other lower layers are generally implemented onlyin software.

Layer 4--Transport

The Transport layer is responsible for the logical transport mechanism, which includes functionsconforming to the mechanism's characteristics. For example, the Transmission Control Protocol (TCP), alogical transport mechanism, provides a level of error checking and reliability to the transmission of userdata to the lower layers of the OSI reference model. This layer is the only layer that provides truesource-to-destination end-to-end connectivity. This layer also supports multiple connections based uponport as found in TCP or UDP.

Layer 3--Network

The Network layer determines physical interface address locations. Routing decisions are made basedupon the locations of the Internet Protocol (IP) address in question. For example, IP addresses establishlogical topologies known as subnets. Applying this definition to a LAN workstation environment, theworkstation determines the location of a particular IP address and where its associated subnet residesthrough the Network layer. Therefore, a packet sent to IP address A.B.C.D will be forwarded through theworkstation's Ethernet card and out onto the network.

Note At this time it would be beneficial to give a brief high-level overview of the ARP process. AddressResolution Protocol (ARP) picks up where the IP address and the routing table fall short. As data travelsacross a network, it must obey the Physical layer protocols that are in use; however, the Physical layerprotocols do not understand IP addressing. The most common example of the Network layer translationfunction is the conversion from IP address to Ethernet address. The protocol responsible for this is ARP,which has been defined in RFC 826. ARP maintains a dynamic table of translations between IP addressesand Ethernet addresses. When ARP receives a request to translate an IP address it checks this table; if itis found, the Ethernet address is returned to the requestor. If it is not found, ARP broadcasts a packet toevery host on the Ethernet segment. This packet contains the IP address in question. If the host is found,it responds back with its Ethernet address, which is entered into the ARP table.

The opposite of this is Reverse Address Resolution Protocol (RARP). RARP translates addresses in theopposite direction as defined in RFC 903. RARP is used to enable a diskless workstation to learn its IPaddress because it has no disk from which to read its TCP/IP configuration. Nevertheless, every systemknows its Ethernet address because it is burned in on its Ethernet card. So the diskless workstation usesthe Ethernet broadcast ability to request its IP address from a server that looks it up by comparing theEthernet address to a table that can match it to the appropriate IP address. It is important to note that



RARP has nothing to do with routing data from one system to another, and it is often confused withARP.

Layer 2--Data Link

The Data Link layer provides framing, error, and flow control across the network media being used. Animportant characteristic of this layer is that the information that is applied to it is used by devices todetermine if the packet needs to be acted upon by this layer (that is, proceed to Layer 3 or discard). TheData Link layer also assigns a Media Access Control (MAC) address to every LAN interface on a device.For example, on an Ethernet LAN segment, all packets are broadcast and received by every device on thesegment. Only the device whose MAC address is contained within this layer's frame acts upon thepacket; all others do not. It is important to note at this point that serial interfaces do not normally requireMAC addresses unless it is necessary to identify the receiving end.

Note It is important to note that MAC addresses are 48-bits in size, three of which are dedicated forvendor identification and another three of which are for unique identification. Additional information onthis subject can be found at: http://www.Cisco.com/warp/public/701/33.html.

Layer 1--Physical

The Physical layer is the lowest layer and is closest to the physical network medium (the network cablingconnecting various pieces of network equipment, for example). It is responsible for actually placinginformation on the physical media in the correct electrical format (that is, raw bits). For example, anRJ45 cable is wired very differently from an Attachment Unit Interface (AUI); this means that thePhysical layer must place the information slightly differently for each media type. Figure 1-5 shows theactual relationship (peering) between the seven layers.

Figure 1-5: Detailed OSI layer relationships.



http://www.cisco.com/warp/public/701/33.html

OSI Reference Model Layers and InformationExchangeThe seven OSI layers use various forms of control information to communicate with their peer layers inother computer systems. This control information consists of specific

requests and instructions that are exchanged between peer OSI layers. Control information typically takesone of two forms:

Headers: Appended to the front of data passed down from upper layers. Trailers: Appended to the back of data passed down from upper layers.

An OSI layer is not necessarily required to attach a header or trailer to upper layer data.

Note Even though OSI is currently one of the most widely recognized frameworks, that was not alwaysthe case. Several other frameworks, such as the Digital Network Architecture (DNA), used to competewith ISO, but they did not stand the test of time.

Headers and Data

Headers (and trailers) and data are relative concepts, depending on the layer that is analyzing theinformation unit at the time.

For example, at the Network layer, an information unit consists of a Layer 3 header and data, known as



the payload. At the Data Link layer (Layer 2), however, all of the information passed down by theNetwork layer (the Layer 3 header and the data) is treated simply as data.

In other words, the data portion of an information unit at a given OSI layer can potentially containheaders, trailers, and data from all of the higher layers. This is known as encapsulation. Figure 1-6 showsthe header and data from one layer encapsulated in the header of the next lowest layer.

Figure 1-6: OSI packet encapsulation through the OSI layers.

How Does the OSI Reference Model Process Work?

Every person who uses a computer residing upon a network is operating under the OSI reference model.The following real world example takes this statement a step further.

You have written an e-mail message and want to send it a coworker (Dan) who is in another state. Thefollowing sequence illustrates how this transaction operates under the OSI reference model. Figure 1-7depicts the necessary sequence of events.

Figure 1-7: How the OSI reference model is used.



1. You finish writing your e-mail message and enter the send command.

2. The e-mail application determines how the workstation is configured to process this command.In this scenario, the workstation is connected via an Ethernet card to the LAN.

3. The e-mail application knows that the message needs to be formatted a certain way to be sent.The e-mail application knows how do this because its code is written to interpret the command andsends the data. The e-mail application begins the encapsulation process and sends the messagethrough the first three top layers of the OSI reference model: Application, Presentation, andSession.

4. Within the workstation, the encapsulated e-mail message is sent to the Ethernet card. The e-mailmessage becomes encapsulated in whatever protocol stack happens to be configured on the PC.For purposes of this discussion you will assume TCP/IP is configured.

5. The Ethernet card receives the message and knows that all outgoing traffic must be TCP, so itencapsulates the message accordingly (that is, the packet now contains the destination IP address).The message has now passed through Layer 4, the Transportation layer.

6. Further encapsulation takes place at the Network layer (Layer 3), which is IP in this scenario.The message is now further encapsulated in IP. Here, between Layers 3 and 4, ARP is executed tofind out the next hops IP address, and the information is added to the IP packet.

7. The message is now ready to leave the network card; however, the type of LAN on which themessage is going to be traveling must be determined (Ethernet, token ring, FDDI, and so on). In



this case, the LAN is Ethernet, so the Data Link layer (Layer 2) encapsulates the message to travelon an Ethernet segment.

8. Now the message needs to know the type of physical connection from which it has to enter theLAN segment. Let's say your workstation happens to use an RJ45 cable. Therefore, the very lastencapsulation is done at the Physical layer (Layer 1). The message is now transitioned to use theRJ45 physical connection type.

9. POOF! In a zing of electrons, the ones and zeros in the message to Dan now become a series ofvoltages and electrical impulses out onto your LAN ready for transmission.

10. The message enters the Ethernet interface as a series of bits that the interface can interpret andprocess, based upon a set of standards that define the interface.

11. The information that has been received is error-checked using a Cycle Redundancy Check(CRC). If the frame is received intact, the interface continues to process the packet by looking forthe destination address in the IP packet header. If the destination is not found, the frame isdiscarded and an error is registered on the interface. The end user will then need to resend themessage.

12. At this point, the interface acts as an interpreter for the binary transmissions, and forwards thedata based upon the logical destination address.

13. The device (router, bridge, hub, and so forth) continues to forward the message based upon thetype of media (Frame Relay, ISDN, ATM, and so on) needed to connect to Dan's LAN.

14. After the message reaches the device that is physically connected to Dan's LAN, steps 11, 12,and 13 are repeated inversely until the message is sent onto the LAN to which Dan's workstation isconnected.

15. Steps 1-9 are now repeated inversely as all of the information on how to send the data, how toroute the data, and so forth that is needed to deliver the message is transferred to Dan's e-mailapplication.

16. TADA! "You've Got Mail."

17. Now Dan determines the importance of the message and whether to read it now or wait untilhis schedule permits.

Open Systems Interconnection (OSI) Protocols

The OSI protocols are a suite of protocols that encompass all seven layers of the OSI reference model.They are part of an international program to develop data networking protocols that are based upon theOSI model as a reference. It is important to mention these briefly, but they are truly beyond the scope ofthis book. If you desire to learn more about them, read the following books to achieve a solidunderstanding:

Internetworking Technologies Handbook, published by Cisco Press.

Network Protocol Handbook, published by McGraw-Hill and authored by Mathew Naugle.



Intranet TopologiesThe preceding sections discussed the evolution of networks into today's intranets. The sections on theOSI reference model showed the essential means of how data is transported between the various layersrunning on all intranet devices. This section addresses the media operating on your Internet. Both local-and wide-area topologies will be discussed in the following sections.

Local Area Networks (LANs)

LANs connect workstations, servers, legacy systems, and miscellaneous network-accessible equipment,which are in turn interconnected to form your network. The most common types of LANs include thefollowing:

Ethernet. A communication system that has only one wire with multiple stations attached to thesingle wire and operates at a speed of 10Mbps.

Fast Ethernet. An improved version of Ethernet that also operates with a single wire withmultiple stations. However, the major improvement is in the area of speed as Fast Ethernetoperates at a speed of 100Mbps.

Gigabit Ethernet. Yet another version of Ethernet that allows for operational speeds of 1Gbps. Token Ring. Probably one of the oldest "ring" access techniques originally proposed in 1969. Ithas multiple wires that connect stations together forming a ring and operates at speeds of 4Mbpsand 16Mbps.

Fiber Distributed Data Internetworking (FDDI). A "dual" fiber optic ring that providesincreased redundancy and reliability. FDDI operates at speeds of 100Mbps.

Ethernet

Ethernet technology adheres to IEEE Standard 802.3. The requirements of the standard are that the LANsupports 10Mbps over coaxial cabling. Ethernet was originally developed by Xerox in the early 1970s toserve networks with sporadic, and occasionally heavy, network traffic.

Ethernet Version 2.0 was jointly developed by Digital Equipment Corp., Intel Corp., and Xerox Corp. Itis compatible with IEEE 802.3 Standards.

Ethernet technology is commonly referred to as Carrier Sense Multiple Access with Carrier Detect(CSMA/CD). What this means is that the Ethernet device will operate as long as it senses a carrier (or asignal) on the physical wire. When an Ethernet device wants to send a packet out of its interface, it willsense for traffic on the wire. If no other traffic is detected, the device will put its data onto the wire andsend it to all other devices that are physically connected to the LAN segment.

From time to time, two devices will send data out at the same time. When this occurs, the two packetsthat are on the wire have what is known as a collision. Built into Ethernet is a retransmission timerknown as a back-off algorithm. If an Ethernet device detects a collision, it will perform a randomcalculation based upon the back-off algorithm before it will send another packet (or resend the original)to prevent further collisions on the wire. Because each device that detects the original collision performsthis random calculation, each derives a different value for the resend timer; therefore, the possibility of



future collisions on the wire are reduced. Figure 1-8 illustrates a typical Ethernet LAN.

Figure 1-8: A typical Ethernet LAN.

If you need further information on this subject, a very good reference can be found at:http://wwwhost.ots.utexas.edu/ethernet/ethernet-home.html.

Token Ring

Token Ring is defined in IEEE Standard 802.5, developed by IBM in the 1970s. It is known as TokenRing because of its built-in token passing capability. Token Ring runs at speeds of 4 and 16Mbps. It alsopasses a small packet, known as a token, around the network. Whenever a workstation desires to sendinformation out on the wire (ring), it must first have possession of this token.

After the workstation has the token, it alters one bit (frame copied) within it and retransmits it back ontothe network. It is retransmitted as a start of frame sequence and is immediately followed with theinformation it wants to transmit. This information will circle the ring until the destination is reached, atwhich time it retrieves the information off the wire. The start of frame packet is then released to flowback to the sending workstation, at which time it changes it back to the original format and releases thetoken back onto to the wire. Then, the process begins again.

Token Ring technology has two fault management techniques:

Active monitoring in which a station acts as monitor for the ring and removes any frame that iscontinually flowing around the ring without being picked up.

A beaconing algorithm that detects and attempts to repair certain network failures.

Whenever a serious ring problem is detected, a beacon frame is sent out. This beacon frame commandsstations to reconfigure to repair the failure. Figure 1-9 illustrates a typical Token Ring LAN.

Figure 1-9: A typical Token Ring LAN.



http://wwwhost.ots.utexas.edu/ethernet/ethernet-home.html

Note If you need additional information on Ethernet or Token Ring operation and troubleshooting, referto the following additional resources: Dan Nassar's book at http:// www.lanscope.com orWandell & Goltermann's Ethernet and Token Ring Troubleshooting Guides: http://www.wg.com.

Fiber Distributed Data Internetworking (FDDI)

FDDI technology is an ANSI Standard, X3T9.5, developed in the mid-1980s in order to accommodatethe need for more local-area bandwidth. The standard was submitted to ISO, which created aninternational version of FDDI that is completely compatible with the ANSI version.

FDDI operates at a speed of 100Mbps. The technology is a token passing, dual-ring LAN using fiberoptic cable. The dual ring provides redundancy and reliability, with the increased operating speed overstandard Ethernet, making FDDI desirable for LAN backbones and interoffice infrastructure. FDDI alsouses a token passing technique in order to determine which station is allowed to insert information ontothe network.

The function of the second ring is for redundancy, as previously mentioned. If one of the fiber wires isbroken, the ring will mend itself by wrapping back toward the portion of the fiber wires that are intact.For this reason, FDDI is highly resilient. Figure 1-10 illustrates a typical FDDI LAN.

Figure 1-10: A typical FDDI LAN.



http://http://www.wg.com/

Wide Area Networks (WANs)

WANs are used to connect physically separated applications, data, and resources, thereby extending thereach of your network to form an intranet. The ideal result is seamless access to remote resources fromgeographically separated end users. The most common types of WAN connectivity technologies includethe following:

Frame Relay. A high-performance, connection-oriented, packet-switched protocol for connectingsites over a WAN.

Point-to-Point Protocol (PPP). A protocol that uses various standards via encapsulation for IPtraffic between serial links.

Asynchronous Transfer Mode (ATM). A fixed packet or cell protocol that emulates LANs forease of connectivity and transmission. This emulation is referred to LANE--LAN Emulation overATM.

X.25. A widely available transport that typically operates at T1 speeds. It has extensive errorchecking to ensure reliable delivery through its permanent and switched virtual circuits.

Integrated Systems Digital Network (ISDN). Consists of digital telephony and data transportservices using digitization over a specialized telephone network.

These WAN technologies are discussed in full detail in the sections that follow. Their connectivity andprotocol characteristics are also compared and contrasted. The tree shown in Figure 1-11 shows some ofthe basic differences and choices regarded when switching is involved.

Figure 1-11: Available WAN technology options.



Frame Relay

Frame Relay is a high performance WAN protocol that operates at the Physical and Data Link layers ofthe OSI reference model. Frame Relay is an example of a packet-switched technology. Frame Relay wasdeveloped in 1990 when Cisco Systems, Digital Equipment, Northern Telecom, and StrataCom formed aconsortium to focus on Frame Relay technology development. This was required because initialproposals submitted during the 1980s failed to provide a complete set of standards. Since that time, ANSIand CCITT have subsequently standardized their own variation, which is now more commonly used thanthe original version.

Packet-switched networks enable end stations to dynamically share the network media and its availablebandwidth. For example, this means that two routers, a type of end station, can communicate in bothdirections along the circuit simultaneously. Variable length packets are used for more efficient andflexible data transfers. The advantage of this technique is that it accommodates more flexibility and amore efficient use of the available bandwidth.

Devices attached to a Frame Relay WAN fall into two general categories: DTE and DCE devices, whichare logical entities. That is, DTE devices initiate a communications exchange, and DCE devices respond.Descriptions and examples of DTE and DCE devices follow.

Data terminal equipment (DTE): Customer-owned end-node and internetworking devices.Examples of DTE devices are terminals, personal computers, routers, and bridges.

Data circuit-terminating equipment (DCE): Carrier-owned internetworking devices. In mostcases, these are packet switches (although routers or other devices can be configured as DCE aswell). An important function of these devices is the capability to provide clocking, which is criticalto Layer 1's sequencing.

Note A good memory trick to remember which of the two types of equipment provides clocking isD-C-E (Data CLOCK Equipment)

Figure 1-12 illustrates the relationship between the two different types of devices (DTE and DCE).

Figure 1-12: The relationship between DTE and DCE devices.



Frame Relay provides connection-oriented Data Link layer communication. This connection isimplemented using virtual circuits. Virtual circuits provide a bi-directional communications path fromone DTE device to another. A Data Link Connection Identifier (DLCI) uniquely identifies them and theybecome locally significant. A Permanent Virtual Circuit (PVC) is one of two types of virtual circuitsused in Frame Relay implementations. PVCs are permanently established connections that are used whenthere is frequent and consistent data transfer between DTE devices across the Frame Relay network.Switched Virtual Circuits (SVCs) are the other types of virtual circuits used in Frame Relayimplementations. SVCs are temporary connections used in situations requiring only sporadic datatransfer between devices. These circuits are very similar in operation and function to ISDN (discussedlater in the chapter).

Frame Relay reduces network overhead by providing simple network congestion notification in the formof Forward Explicit Congestion Notification (FECN) and Backward Explicit Congestion Notification(BECN). Both types of congestion notification are controlled by a single bit within the Frame Relaypacket header. This bit also contains a Discard Eligible (DE) bit that, if set, will identify less importanttraffic that can be discarded during periods of congestion.

Note How are Discard Eligible (DE) packets determined?

If your contracted Committed Information Rate (CIR) is exceeded, the Frame Relay switch automaticallymarks the any frames above your CIR as Discard Eligible (DE). If the Frame Relay backbone iscongested, then the switch will discard them; otherwise, they will be allowed through. When the routerreceives them it will note them on the interface statistics.

Frame Relay uses a common error checking mechanism known as the Cyclic Redundancy Check (CRC).The CRC compares two calculated values to determine whether errors occurred during the transmissionfrom source to destination. Frame Relay reduces network overhead by implementing error checkingrather than error correction. Because Frame Relay is typically implemented on reliable network media,data integrity is not sacrificed because error correction can be left to higher-layer protocols, such asOSPF, which runs on top of Frame Relay.

The Local Management Interface (LMI) is a set of enhancements to the basic Frame Relay specification.The LMI offers a number of features (called extensions) for managing complex internetworks. Some of



the key Frame Relay LMI extensions include global addressing and virtual circuit status messages (seeFigure 1-13).

Figure 1-13: Typical Frame Relay connectivity.

Switched Virtual Circuits (SVCs)

SVC technology is the newest kid on the block, and MCI was the first carrier to offer SVCs to customersvia their Hyperstream Frame Relay network. SVCs, unlike PVCs, are set up and torn down on-the-fly asneeded. Through this capability, SVCs are able to save organizations thousands of dollars a month inservice charges when compared to PVCs. When used as a true bandwidth on demand, service routercapacity and management is conserved. This is done by putting one entry for each router in its routingtable, which allows the SVC to do the rest. For additional information refer tohttp://www.mci.com.

Point-to-Point Protocol (PPP)

PPP is an encapsulation protocol for transporting IP traffic over point-to-point links. It provides a methodfor transmitting packets from serial interface to serial interface. PPP also established a series of standardsdealing with IP address management, link management, and error checking techniques. PPP supportsthese many functions through the use of Link Control Protocol (LCP) and Network Control Protocols(NCP) to negotiate optional configuration parameters.

Asynchronous Transfer Mode (ATM)

ATM was originally developed to support video, voice, and data over WANs. ATM was developed bythe International Telecommunications Union Telecommunication Standardization Sector (ITU-T). ATMhas also been referred to as Broadband ISDN or B-ISDN.

ATM is a cell-switching and multiplexing technology that provides flexibility and efficiency forintermittent traffic, along with constant transmission delay and guaranteed capacity.

An ATM network consists of an ATM switch and endpoints that support the LAN Emulation (LANE)technology. LANE uses an ATM device to emulate a LAN topology by encapsulating the packet in anEthernet or Token Ring frame when going from media to media. Essentially, LANE enables an ATM



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device to behave as if it were in a standard LAN environment. LANE supports all versions of TokenRing and Ethernet but currently is not compatible with FDDI. The support for these technologies ispossible because these protocols use the same packet format regardless of link speed.

ATM can be configured to support either PVCs or SVCs. PVCs provide for a point-to point-dedicatedcircuit between end devices. PVCs do not require a call set up or guarantee the link will be available butare more manual in nature and require static addressing than SVCs. SVCs, however, are dynamicallyallocated and released. They remain in use only as long as data is being transferred. SVCs require a callset up for each instance of the circuit's connection. The switched circuits provide more flexibility andefficiency; however, they are burdened by the overhead associated with the call set up, in terms of theextra time and configuration. Figure 1-14 illustrates a typical ATM network.

Figure 1-14: A typical ATM network.

Integrated Systems Digital Network (ISDN)

ISDN is defined by ITU-T Standards Q.921 and Q.931. The Q.921 specification requires the user todesignate a network interface that is needed for digital connectivity. The Q.931 determines call setup andconfiguration. ISDN components include the following:

Terminals

Terminal adapters (TAs)

Network termination devices

Line termination equipment

Exchange termination equipment

It is important to point out that there is specialized ISDN equipment known as terminal equipment type 1(TE1). All other equipment that does not conform to ISDN Standards is known as terminal equipmenttype 2 (TE2). TE1s connect to the ISDN network through specialized cables. TE2s connect to the ISDNnetwork through a terminal adapter.



Another ISDN device is the network connection type--network termination type 1 or 2 devices. Thesetermination devices connect the specialized ISDN cables to normal two wire local wiring.

ISDN reference points define logical interfaces. Four reference points are defined:

R reference point. Defines the reference point between non-ISDN equipment and a TA. S reference point. Defines the reference point between user terminals and an NT2. T reference point. Defines the reference point between NT1 and NT2 devices. U reference point. Defines the reference point between NT1 devices and line-terminationequipment in a carrier network. (This is only in North America, where the NT1 function is notprovided by the carrier network.)

Figure 1-15 illustrates the various devices and reference points found in ISDN implementations, as wellas their relationship to the ISDN networks they support.

Figure 1-15: A typical ISDN configuration.

The ISDN Basic Rate Interface (BRI) service provides two B channels and one D channel. The BRIB-channel service operates at 64Kbps and carries data, while the BRI D-channel service operates at16Kbps and usually carries control and signaling information.

The ISDN Primary Rate Interface (PRI) service delivers 23 B channels and one 64Kbps D channel inNorth America and Japan for a total bit rate of up to 1.544Mbps. PRI in Europe and Australia carry 30 Bchannels and 1 D channel for a total bit rate of up to 2.048Mbps.

The ISDN network layer operation involves a series of call stages that are characterized by specificmessage exchanges. In general, an ISDN call involves call establishment, call termination, information,and miscellaneous messages.

The call stage characteristics define the way an ISDN call is initiated, acknowledged, and completed. Thespecifics of ISDN call stages and their supported characteristics are defined in the OSI reference modelNetwork layer definition of ISDN.

Formal call stage components include the following, in order:



SETUP

CONNECT

RELEASE

USER INFORMATION

CANCEL

STATUS

DISCONNECT

The formal call components as presented in the preceding list can also be tracked through a typical ISDNcall negotiation as shown in Figure 1-16.

Figure 1-16: A typical ISDN Network layer call negotiation.

SummaryThis chapter discussed how networks began and how they have been increasing in complexity ever since.You also learned about the physical layout of early networks as well as the issues surrounding theevolution of contemporary intranets and what the future holds for network engineers. This chapter also



established the physical foundations and needs of past, current, and future networks.

You also explored the OSI reference model down to each individual layer and learned how a typical datapacket flows up and down the OSI layers as well as the way it flows between geographically separatednetworks. At this point, you should understand the basic functions of the logical network through thediscussion and demonstrations illustrated.

The chapter continued with coverage of the LAN and WAN intranet topologies. The section on LANtopologies included coverage of the three most widely deployed topologies: Ethernet, Token Ring, andFDDI, as well as the standards and basic characteristics of each. The section on WAN topologiesincluded coverage of the three most widely deployed topologies: Frame Relay, PPP, ATM, and ISDN.Discussion of each topology included the standards applicable for each and some of the more importantaspects of each.

In conclusion, the reader should now understand the evolution of networks, intranet evolution, currentchallenges, physical and logical network fundamentals, popular LAN and WAN topologies. "NetworkingRouting Fundamentals," will build further upon the foundations of networking covered in this chapter.




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Table of Contents


Internet Protocol (IP) Addressing

Class A AddressesClass B AddressesClass C AddressesClass D AddressesClass E AddressesHow IP Addresses Are UsedHow IP Addresses Are ReadThe Role of IP AddressesIP Subnet AddressingSubnet MaskingSubnetting Restrictions

Explaining the Need for VLSM and CIDR

Route Summarization (Aggregation or Supernetting)Classful Routing

The Impact of Classful Routing

Classless Routing

Variable-Length Subnet Masks (VLSM)

VLSM Design Guidelines & Techniques

Classless Interdomain Routing (CIDR)

Validating a CIDRized NetworkWhat Do Those /16s and /24s Mean?Important CIDR TermsIP ClasslessCIDR Translation TableManually Computing the Value of a CIDR IP Prefix

Internetwork Components

NetworksBridgesGatewaysHubsSwitchesLAN SwitchesPacket SwitchesCSUDSURouterRoutingComponent Interaction with the OSI Model

Understanding Router Subinterfaces

Point-to-Point SubinterfacesMultipoint Subinterfaces




Network Protocols

TCP/IP Protocol Suite

TCP/IP PacketsCommon TCP/IP Routing Protocol Characteristics

Basic Protocol Operations

Chapter SummaryCase Study: Where Is the Network Broken?

Verifying the Physical Connection Between the DSU/CSU and the Router

Interface State: Serial x Is UpInterface State: Serial x Is Administratively DownInterface State: Serial x Is Down

Verifying That the Router and Frame Relay Provider Are Properly Exchanging LMI

Line Protocol State: Line Protocol Is UpLine Protocol State: Line Protocol Is Down

Verifying That the PVC Status Is Active

PVC Status: ActivePVC Status: InactivePVC Status: Deleted

Verifying That the Frame Relay Encapsulation Matches on Both Routers

Frequently Asked Questions (FAQs)

Networking Routing Fundamentals"Achievement: Unless you try to do something beyond what you have already mastered, you will never grow."--Successories

Routing with a network, whether it is the Internet or an intranet, requires a certain amount of "common" information. This chapterprovides a broad overview that covers some of the most essential points.

Internet Protocol (IP) addressing. An overview of IP addressing methodology and understanding, subnetting, variable-lengthsubnet masking, and classless interdomain routing is provided in this section. Why these techniques are needed will also be brieflydiscussed.

Internetwork components. This section provides an examination of the actual physical components that make use of the theoriespreviously discussed: OSI Model, IP addresses, subnet masks, and protocols.

Network protocols. Basic theory on network protocols is discussed, with emphasis on understanding the difference betweenrouted and routing protocols. Some of the fundamentals of protocol operation, with an emphasis on the evolution and operation ofthe Internet Protocol (IP), will be explained.

Internet Protocol (IP) AddressingThis section discusses IP addressing methodology and understanding, basic subnetting, variable length subnet masking (VLSM), andclassless interdomain routing (CIDR).

In a properly designed and configured network, communication between hosts and servers is transparent. This is because each deviceusing the TCP/IP protocol suite has a unique 32-bit Internet Protocol (IP) address. A device will "read" the destination IP address in thepacket and make the appropriate routing decision based upon this information. In this case, a device could be either the host or serverusing a default gateway or a router using its routing table to forward the packet to its destination.

IP addresses can be represented as a group of four decimal numbers, each within the range of 0 to 255. Each of these four decimalnumbers will be separated by a decimal point. This method of displaying these numbers is known as dotted decimal notation. It isimportant to note that these numbers can also be displayed in both the binary and hexadecimal numbering systems. Figure 2-1 illustratesthe basic format of an IP address as determined by using dotted decimal notation.

Figure 2-1: An IP address format as determined by dotted decimal notation.



IP addresses have only two logical components, network and host addresses, the use of which is extremely important. A network addressidentifies the network and must be unique; if the network is to be a part of the Internet, then it must be assigned by the Internet NetworkInformation Center (InterNIC). A host address, on the other hand, identifies a host (device) on a network and is assigned by a localadministrator.

Suppose a network has been assigned an address of 172.24, for example. An administrator then assigns a host the address of248.100. The complete address of this host is 172.24.248.100. This address is unique because only one network and one host canhave this address.

Note The network address component must be the same for all devices on that network, yet must be unique from all other networks.Additional information can be found in RFC 1600, which discusses reserved IP addresses.

Class A Addresses

In a class A address (also known as /8), the first octet contains the network address and the other three octets make up the node address.The first bit of a class A network address must be set to 0. Although mathematically it would appear that there are 128 possible class Anetwork addresses (remember the first is set to zero), the address 00000000 is not available, so there are only 127 such addresses. Thisnumber is further reduced because network 127.0.0.0 is reserved for loopback addressing purposes. There are only 126 class Aaddresses available for use. Each class A address, however, can support 126 networks that correspond to 16,777, 214 node addresses perclass A address.

Note Please note that the node addresses 00000000.00000000.00000000.00000000 and11111111.11111111.11111111.11111111 are not available in ANY address class, with the example shown being a class Aaddress. These node addresses translate into 0.0.0.0 and 255.255.255, respectively. These are typically used for protocoladvertisements, such as ARP, RIP, and broadcast packets. Also note that 127.x.x.x (where x is any number between 0 and 255) isreferred to as the local loopback address. A packet's use of this address will immediately result in it being sent back to the applicationfrom which it was sent. This information can be used to assist you in troubleshooting network problems.

Class B Addresses

In a class B (also known as /16) address, the network component uses the first two octets for addressing purposes. The first two bits of aclass B address are always 10; that is, one and zero, not ten. The address range would then be 128.1.0.0 to 191.254.0.0. Thisleaves you with the first six bits of the first octet and all eight bits of the second octet, thereby providing 16,384 possible class B networkaddresses. The remaining octets are used to provide you with over 65,534 hosts per class B address.

Class C Addresses

In a class C (also known as /24) address, the first three octets are devoted to the network compon

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