Kevin Dooley - Designing Large-Scale LANs, Sieci(1)
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ISBN: 0-596-00150-9
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Table of Contents:
1. Networking Objectives
Business Requirements
OSI Protocol Stack Model
Routing Versus Bridging
Top-Down Design Philosophy
2. Elements of Reliability
Defining Reliability
Redundancy
Failure Modes
3. Design Types
Basic Topologies
Reliability Mechanisms
VLANs
Toward Larger Topologies
Hierarchical Design
Implementing Reliability
Large-Scale LAN Topologies
4. Local Area Network Technologies
Selecting Appropriate LAN Technology
Ethernet and Fast Ethernet
Token Ring
Gigabit and 10 Gigabit Ethernet
ATM
FDDI
Wireless
Firewalls and Gateways
Structured Cabling
5. IP
IP-Addressing Basics
IP-Address Classes
ARP and ICMP
Network Address Translation
Multiple Subnet Broadcast
General IP Design Strategies
DNS and DHCP
6. IP Dynamic Routing
Static Routing
Types of Dynamic Routing Protocols
RIP
IGRP and EIGRP
OSPF
BGP
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7. IPX
Dynamic Routing
General IPX Design Strategies
8. Elements of Efficiency
Using Equipment Features Effectively
Hop Counts
MTU Throughout the Network
Bottlenecks and Congestion
Filtering
Quality of Service and Traffic Shaping
9. Network Management
Network-Management Components
Designing a Manageable Network
SNMP
Management Problems
10. Special Topics
IP Multicast Networks
IPv6
Security
Appendix: Combining Probabilities
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Chapter 1. Networking Objectives
The American architect Louis Henry Sullivan described his design philosophy with the simple statement
"form follows function." By this credo he meant that a structure's physical layout and design should reflect
as precisely as possible how this structure will be used. Every door and window is where it is for a reason.
He was talking about building skyscrapers, but this philosophy is perhaps even more useful for network
design. Where building designs often include purely esthetic features to make them more beautiful to look
at, every element of a good network design should serve some well-defined purpose. There are no
gargoyles or frescos in a well-designed network.
The location and configuration of every piece of equipment and every protocol must be carefully
optimized to create a network that fulfills the ultimate purposes for which it was designed. Any sense of
esthetics in network design comes from its simplicity and reliability. The network is most beautiful when it
is invisible to the end user.
So the task of designing a network begins with a thorough study of the required functions. And the form
will follow from these business requirements.
1.1 Business Requirements
This is the single most important question to answer when starting a network design: why do you want to
build a network? It sounds a little silly, but frequently people seem confused about this point. Often they
start building a network for some completely valid and useful reason and then get bogged down in
technical details that have little or nothing to do with the real objectives. It is important to always keep
these real objectives in mind throughout the process of designing, implementing, and operating a network.
Too often people build networks based on technological, rather than business, considerations. Even if the
resulting network fulfills business requirements, it will usually be much more expensive to implement than
is necessary.
If you are building a network for somebody else, then they must have some reason why they want this
done. Make sure you understand what the real reasons are. Too often user specifications are made in terms
of technology. Technology has very little to do with business requirements. They may say that they need a
Frame Relay WAN, or that they need switched 100Mbps Ethernet to every desk. You wanted them to tell
you why they needed these things. They told you they needed a solution, but they didn't tell you what
problem you were solving.
It's true that they may have the best solution, but even that is hard to know without understanding the
problem. I will call these underlying reasons for building the network "business requirements." But I want
to use a very loose definition for the word "business." There are many reasons for building a network, and
only some of them have anything to do with business in the narrow sense of the word. Networks can be
built for academic reasons, or research, or for government. There are networks in arts organizations and
charities. Some networks have been built to allow a group of friends to play computer games. And there
are networks that were built just because the builders wanted to try out some cool new technology, but this
can probably be included in the education category.
What's important is that there is always a good reason to justify spending the money. And once the money
is spent, it's important to make sure that the result actually satisfies those requirements. Networks cost
money to build, and large networks cost large amounts of money.
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1.1.1 Money
So the first step in any network design is always to sit down and list the requirements. If one of the
requirements is to save money by allowing people to do some task faster and more efficiently, then it is
critical to understand how much money is saved.
Money is one of the most important design constraints on any network. Money forms the upper limit to
what can be accomplished, balancing against the "as fast as possible" requirement pushing up from below.
How much money do they expect the network to save them? How much money do they expect it will make
for them? If you spend more money building this network than it's going to save (or make) for the
organization, then it has failed to meet this critical business objective. Perhaps neither of these questions is
directly relevant. But in that case, somebody is still paying the bill, so how much money are they willing to
spend?
1.1.2 Geography
Geography is the second major requirement to understand. Where are the users? Where are the services
they want to access? How are the users organized geographically? By geography I mean physical location
on whatever scale is relevant. This book's primary focus is on Local Area Network (LAN) design, so I will
generally assume that most of the users are in the same building or in connected building complexes. But if
there are remote users, then this must be identified at the start as well. This could quite easily spawn a
second project to build a Wide Area Network (WAN), a remote-access solution, or perhaps a Metropolitan
Area Network (MAN). However, these sorts of designs are beyond the scope of this book.
One of the keys to understanding the local area geography is establishing how the users are grouped. Do
people in the same area all work with the same resources? Do they need access to the same servers? Are
the users of some resources scattered throughout the building? The answers to these questions will help to
define the Virtual LAN (VLAN) architecture. If everybody in each area is part of a self-contained work
group, then the network could be built with only enough bandwidth between groups to support whatever
small amounts of interaction they have. But, at the opposite extreme, there are organizations in which all
communication is to a centralized group of resources with little or no communication within a user area. Of
course, in most real organizations, there is most likely a mixture of these extremes with some common
resources, some local resources, and some group-to-group traffic.
1.1.3 Installed Base
The next major business requirement to determine is the installed base. What technology exists today?
Why does it need to be changed? How much of the existing infrastructure must remain?
It would be extremely unusual to find a completely new organization that is very large, has no existing
technology today, and needs it tomorrow. Even if you did find one, chances are that the problem of
implementing this new technology has been broken down among various groups. So the new network
design will need to fit in with whatever the other groups need for their servers and applications.
Installed base can cause several different types of constraints. There are geographical constraints, such as
the location and accessibility of the computer rooms and LAN rooms. There may be existing legacy
network technology that has to be supported. Or it may be too difficult, inconvenient, or expensive to
replace the existing cable plant or other existing services.
Constraints from an existing installed base of equipment can be among the most difficult and frustrating
parts of a network design, so it is critical to establish them as thoroughly and as early as possible.
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