Cisco Internetworking Technology Overview.pdf

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Preface
Data communications technologies are evolving and expanding at an unparalleled rate. The growth
in demand for Internet access and intranet services continues to fuel rapid technical adaptation by
both implementers and developers. Unfortunately, creating an information resource such as the
Internetworking Technology Overview requires a certain recognition by its authors that some
information is likely to be obsolete the day it appears in print.
The authors of Internetworking Technologies Handbook approached its development with a
commitment to helping readers make informed technology decisions and develop a keen awareness
of this dilemma. We hope that this first release is a step in the correct direction, and that, together
with other books planned for the Cisco Press program, you will be able to identify technologies that
will accommodate working network solutions as your requirements change.
This chapter discusses the objectives, intended audiences, and overall organization of the
Internetworking Technology Overview, Second Edition.
Document Objectives
This publication provides technical information addressing Cisco-supported internetworking
technologies. It is designed for use in conjunction with other Cisco documents or as a stand-alone
reference.
The
Internetworking Technology Overview
is not intended to provide all possible information on the
included technologies. Because a primary goal of this publication is to help network administrators
configure Cisco products, the publication emphasizes Cisco-supported technologies; however,
inclusion of a technology in this publication does not necessarily imply Cisco support for that
technology.
Audience
The
Internetworking Technology Overview
is written for anyone who wants to understand
internetworking. Cisco anticipates that most readers will use the information in this publication to
assess the applicability of specific technologies for their environments.
Organization
This publication is divided into eight parts. Each part is concerned with introductory material or a
major area of internetworking technology and comprises chapters describing related tasks or
functions.
Preface
xv
Document Conventions
•
•
•
•
•
•
•
•
Part 1, “Introduction to Internetworking” presents concepts basic to the understanding of
internetworking and network management.
Part 2, “LAN Protocols,” describes standard protocols used for accessing network physical
media.
Part 3, “WAN Technologies” describes standard protocols used to implement wide-area
networking.
Part 4, “Bridging and Switching,” describes protocols and technologies used to provide Layer 2
connectivity between subnetworks.
Part 5, “Network Protocols,” describes standard networking protocol stacks that can be routed
through an internetwork.
Part 6, “Routing Protocols,” describes protocols used to route information through an
internetwork.
Part 7, “Internet Access Technologies” describes security network caching technologies and
directory services.
Part 8, “Network Management,” describes the architecture and operation of common network
management implementations.
Acknowledgments
This book was written as a collaborative effort. It represents several years of information compilation
and the integration of information products developed by Cisco Documentation developers.
Principal authors for this publication were Merilee Ford, H. Kim Lew, Steve Spanier, and Tim
Stevenson. During the last process of consolidation, Kevin Downes contributed to integrating the
material into this product.
The authors want to acknowledge the many contributions of Cisco subject-matter experts for their
participation in reviewing material and providing insights into the technologies presented here. Folks
who added to this compilation include Priscilla Oppenheimer, Aviva Garrett, Steve Lin, Manoj
Leelanivas, Kent Leung, Dave Stine, Ronnie Kon, Dino Farinacci, Fred Baker, Kris Thompson,
Jeffrey Johnson, George Abe, Yakov Rekhter, Abbas Masnavi, Alan Marcus, Laura Fay, Anthony
Alles, David Benham, Debra Gotelli, Ed Chapman, Bill Erdman, Tom Keenan, Soni Jiandani, and
Derek Yeung, among a number of other Cisco contributors. The authors appreciate the time and
critical reviews each of these participants provided in helping to develop the source material for the
Internetworking Technologies Handbook, Second Edition.
This publication borrows liberally from publications and training products previously developed by
Cisco Systems. In particular, the Internetworking Technology Overview publication and the Cisco
Connection Training multimedia CD-ROM provided the foundation from which this compilation
was derived.
Document Conventions
In this publication, the following conventions are used:
•
•
Commands and keywords are in
boldface.
New, important terms are
italicized
when accompanied by a definition or discussion of the term.
Note
Means
reader take note.
Notes contain helpful suggestions or references to materials not
contained in this manual.
xvi
Internetworking Technology Overview, June 1999
C HA P TER
1
Internetworking Basics
This chapter works with the next six chapters to act as a foundation for the technology discussions
that follow. In this chapter, some fundamental concepts and terms used in the evolving language of
internetworking are addressed. In the same way that this book provides a foundation for
understanding modern networking, this chapter summarizes some common themes presented
throughout the remainder of this book. Topics include flow control, error checking, and
multiplexing, but this chapter focuses mainly on mapping the
Open Systems Interconnect
(OSI)
model to networking/internetworking functions and summarizing the general nature of addressing
schemes within the context of the OSI model.
What is an Internetwork?
An internetwork is a collection of individual networks, connected by intermediate networking
devices, that functions as a single large network. Internetworking refers to the industry, products, and
procedures that meet the challenge of creating and administering internetworks. Figure 1-1
illustrates some different kinds of network technologies that can be interconnected by routers and
other networking devices to create an internetwork:
Figure 1-1
Different network technologies can be connected to create an internetwork.
FDDI
WAN
Ethernet
Token
Ring
Internetworking Basics
1-1
12308
Open Systems Interconnection (OSI) Reference Model
History of Internetworking
The first networks were time-sharing networks that used mainframes and attached terminals. Such
environments were implemented by both IBM’s System Network Architecture (SNA) and Digital’s
network architecture.
Local area networks (LANs) evolved around the PC revolution. LANs enabled multiple users in a
relatively small geographical area to exchange files and messages, as well as access shared resources
such as file servers.
Wide- area networks (WANs) interconnect LANs across normal telephone lines (and other media),
thereby interconnecting geographically dispersed users.
Today, high-speed LANs and switched internetworks are becoming widely used, largely because
they operate at very high speeds and support such high-bandwidth applications as voice and
videoconferencing.
Internetworking evolved as a solution to three key problems: isolated LANs, duplication of
resources, and a lack of network management. Isolated LANS made electronic communication
between different offices or departments impossible. Duplication of resources meant that the same
hardware and software had to be supplied to each office or department, as did a separate support
staff. This lack of network management meant that no centralized method of managing and
troubleshooting networks existed.
Internetworking Challenges
Implementing a functional internetwork is no simple task. Many challenges must be faced,
especially in the areas of connectivity, reliability, network management, and flexibility. Each area is
key in establishing an efficient and effective internetwork.
The challenge when connecting various systems is to support communication between disparate
technologies. Different sites, for example, may use different types of media, or they might operate
at varying speeds.
Another essential consideration, reliable service, must be maintained in any internetwork. Individual
users and entire organizations depend on consistent, reliable access to network resources.
Furthermore, network management must provide centralized support and troubleshooting
capabilities in an internetwork. Configuration, security, performance, and other issues must be
adequately addressed for the internetwork to function smoothly.
Flexibility, the final concern, is necessary for network expansion and new applications and services,
among other factors.
Open Systems Interconnection (OSI) Reference Model
The Open Systems Interconnection (OSI) reference model describes how information from a
software application in one computer moves through a network medium to a software application in
another computer. The OSI reference model is a conceptual model composed of seven layers, each
specifying particular network functions. The model was developed by the International Organization
for Standardization (ISO) in 1984, and it is now considered the primary architectural model for
intercomputer communications. The OSI model divides the tasks involved with moving information
between networked computers into seven smaller, more manageable task groups. A task or group of
tasks is then assigned to each of the seven OSI layers. Each layer is reasonably self-contained, so
that the tasks assigned to each layer can be implemented independently. This enables the solutions
offered by one layer to be updated without adversely affecting the other layers.
1-2
Internetworking Technology Overview, June 1999
Characteristics of the OSI Layers
The following list details the seven layers of the Open System Interconnection (OSI) reference
model:
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•
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Layer 7—Application layer
Layer 6—Presentation layer
Layer 5—Session layer
Layer 4—Transport layer
Layer 3—Network layer
Layer 2—Data Link layer
Layer 1—Physical layer
Figure 1-2 illustrates the seven-layer OSI reference model.
Figure 1-2
The OSI reference model contains seven independent layers.
7
6
5
4
3
2
1
Application
Presentation
Session
Transport
Network
Data Link
Physical
ith0102
Characteristics of the OSI Layers
The seven layers of the OSI reference model can be divided into two categories:
upper layers
and
lower layers.
The
upper layers
of the OSI model deal with application issues and generally are implemented only
in software. The highest layer, application, is closest to the end user. Both users and application-layer
processes interact with software applications that contain a communications component. The term
upper layer is sometimes used to refer to any layer above another layer in the OSI model.
The
lower layers
of the OSI model handle data transport issues. The physical layer and data link
layer are implemented in hardware and software. The other lower layers generally are implemented
only in software. The lowest layer, the physical layer, is closest to the physical network medium (the
network cabling, for example) , and is responsible for actually placing information on the medium.
Figure 1-3 illustrates the division between the upper and lower OSI layers.
Internetworking Basics
1-3

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