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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
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March 1999
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Design and ImplementationPublications focusing on network design
and implementation strategies.
Internet Routing ArchitecturesISBN: 1-56205-652-2By Bassam
HalabiExplores the ins and outs of interdomainrouting network
designs.
Designing Campus NetworksISBN: 1-57870-030-2By Terri Quinn-Andry
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Thomas IIPresents detailed, applied coverage of OpenShortest Path
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IProuting protocol. Essential reading! Contentnot available.
Networking FundamentalsSupport publications providing technology
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Internetworking Technologies Handbook(2nd Edition)ISBN:
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and protocols.
Internetworking TroubleshootingHandbookISBN: 1-56205-024-8By
Cisco Staff and Kevin DownesSummarizes connectivity and
performanceproblems, helps develop a strategy forisolating
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IP Routing PrimerISBN: 1-57870-108-2By Robert WrightTechnical
tips and hints focusing on howCisco routers implement IP
functions.
IP Routing FundamentalsISBN: 1-57870-071-XBy Mark
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routing protocols.
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Design and Implementation SeriesCisco Router Configuration
OSPF Network Design Solutions
Internet Routing Architectures
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Cisco Router ConfigurationIntroduction
Getting Started in Internetworking
The Basics of Device Configuration
The Basics of Device Interfaces
TCP/IP Basics
AppleTalk Basics
IPX Basics
Basic Administration and Management Issues
Comprehensive IOS Configuration for ZIP Network
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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.
Foreword
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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
Vice President and General Manager
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.
Posted: Wed Aug 2 16:28:13 PDT 2000Copyright 1989-2000Cisco
Systems Inc.Copyright 1997 Macmillan Publishing USA, a Simon &
Schuster Company
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Table of Contents
Foundations of Networking
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.
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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
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(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
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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.
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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
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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.
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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
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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
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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.
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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).
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Figure 1-3: OSI layer groupings.
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Figure 1-4: How a protocol spans the OSI reference model.
Layer 5--Session
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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
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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.
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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
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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.
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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
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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.
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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
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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.
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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.
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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.
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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.
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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
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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.
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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:
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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
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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.
Posted: Wed Aug 2 16:09:50 PDT 2000Copyright 1989-2000Cisco
Systems Inc.Copyright 1997 Macmillan Publishing USA, a Simon &
Schuster Company
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Table of Contents
Networking Routing Fundamentals
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
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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.
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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