Routing is simultaneously the most complicated function of a network and the most important. Most
knowledgeable people agree that networking and routing technologies have been around about 25 years.
The concept of routing actually dates back to the late 1950s, when computing was still an arcane science
in its infancy. Precious few organizations had a single computer, much less multiple computers that
needed to be linked together. Internetworking, the interconnection of multiple computers, was still more
of a futuristic vision than a reality. This vision predicted a day when computers would be widely
implemented and interconnected via a ubiquitous global internetwork: the Internet.
The challenge in building and using a global internetwork is developing the means to find, access, and
communicate with remote hosts. Ostensibly, a global internetwork would offer redundancy. In other
words, there could be many different physical paths through a network between any given pair of hosts.
Mechanisms would be needed that could discover remote networks and hosts and explore the different
possible paths (or routes) through the network to those networks and hosts.
Finally, some way to apply either logic or mathematics would be needed. Logically, if there are many
different routes to a specific destination, they can't all be equal. Some routes would likely offer either
shorter overall paths or better performance than others. Thus, it would be logical to compare all the
possible routes and then select the best route or routes. In time, these mechanisms would become known
as routers. The process of discovering, calculating, and comparing routes to remote networks and hosts is
routing.
This book will help you explore the mechanics of routers and routed and routing protocols, and build
internetworks using routing technologies. Although this book was designed primarily for the novice, it
contains detailed technical examinations of many of today's leading routing protocols. These
examinations are sufficiently detailed to be valuable to technical professionals at all levels of expertise.
Consequently, you will find this book an indispensable technical reference long after you have mastered
the basic theory and mechanics of routing and routing protocols.
Part I of this book, "Internetworking Fundamentals," provides an overview of internetworking, including
the implications of using routers in both LANs and WANs. This overview is provided using the Internet
Protocol (IP), which is the predominant routed protocol in use today. IP has grown substantially since its
inception approximately two decades ago. Its once simple addressing architecture has become quite
complicated during its life. An entire chapter is devoted to examining IP's addressing. This includes the
original class-based address architecture, subnet numbers, and classless interdomain routing (CIDR)
addresses. This chapter also provides a glimpse at how IP's addresses will change with the IPv6, the next
generation of IP. IP addresses are used extensively throughout the book to present you with specific
examples of the various routing concepts that are introduced.
Part II of the book, "The Inner Workings of Routers," delves into a slightly deeper level of detail. Instead
of looking at internetworking from a high level, Part II looks at the inner workings of a router. This
includes a side-by-side comparison of the two versions of IP, IPv4 (the current version) and IPv6 (the
next generation); the various transmission technologies that a router can use for communications; and the
mechanics of routing protocols.
There are different types of routing protocols. Generally speaking, they fall into two categories: those
that calculate routes based on some measurement of distance, and those that calculate routes based on
some measurement of the state of the links that comprise a route. The first type is known as a
distance-vector routing protocol and the second type is a link-state routing protocol. An appreciation of
the basic functional differences between these two types of routing protocols will prepare you for Part III
of this book.
Part III, "Routing Protocols," presents a detailed examination of today's leading routing protocols. You
will see exactly how RIP, RIP-2, OSPF, IGRP, and EIGRP operate in an internetwork. Understanding the
mechanics of a routing protocol will help you design better networks and more effectively troubleshoot
and fine-tune an existing network.
The detailed examination of routers and routing in the first three sections of the book provides the
context for the last section. The last section of this book emphasizes the implementation of routing
technologies and provides insight into the future of routing.
The first chapter of Part IV, "Implementation Issues," focuses on building internetworks. An
internetwork must accommodate different types of needs. These needs vary considerably from network
to network but encompass some specific attributes. These attributes include scalability, geographic
distance between the locations in the network, traffic volumes, performance delays, and monetary costs
of operating and maintaining the network. The implications of each of these are explored, along with
sample network topologies and guidelines for selecting transmission technologies.
One of the more challenging aspects of building an internetwork is coping with multiple protocols.
Precious few networks have the luxury of using a single routed and/or routing protocol. There are many
reasons for this, including merger and acquisitions, extranets, and even migrations to new technologies.
Regardless of the reason, the challenge lies in overcoming the dissimilarities of the routed and/or routing
protocols. Chapter 14 examines the options for internetworking with dissimilar protocols (both routed
and routing). This chapter includes a look at the implications of a migration to IPv6 from IPv4 and some
strategies for successfully conducting such a migration.
The book concludes with an assessment of the future of routing. This is necessary, as technological
advances have created substantial confusion, and even doubts, about routers and routing! For example,
Microsoft's Windows NT operating system can enable a client or server-grade computer to function as a
router. What does this mean for the future of stand-alone routers?
Additional confusion about the future of routers has been caused by the technological developments that
blur the previous distinctions between LANs and WANs. Switching, in particular, is rapidly being
implemented for both network types, and it can forward Layer 3 packets as easily as Layer 2 frames.
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