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Top-Down Network Design

Top-Down Network Design

4.0 1
by Priscilla Oppenheimer

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The purpose of Top-Down Network Design, Third Edition, is to help you design networks that meet a customer’s business and technical goals. Whether your customer is another department within your own company or an external client, this book provides you with tested processes and tools to help you understand traffic flow, protocol



The purpose of Top-Down Network Design, Third Edition, is to help you design networks that meet a customer’s business and technical goals. Whether your customer is another department within your own company or an external client, this book provides you with tested processes and tools to help you understand traffic flow, protocol behavior, and internetworking technologies. After completing this book, you will be equipped to design enterprise networks that meet a customer’s requirements for functionality, capacity, performance, availability, scalability, affordability, security, and manageability.



This book is for you if you are an internetworking professional responsible for designing and maintaining medium- to large-sized enterprise networks. If you are a network engineer, architect, or technician who has a working knowledge of network protocols and technologies, this book will provide you with practical advice on applying your knowledge to internetwork design.


This book also includes useful information for consultants, systems engineers, and sales engineers who design corporate networks for clients. In the fast-paced presales environment of many systems engineers, it often is difficult to slow down and insist on a top-down, structured systems analysis approach. Wherever possible, this book includes shortcuts and assumptions that can be made to speed up the network design process.


Finally, this book is useful for undergraduate and graduate students in computer science and information technology disciplines. Students who have taken one or two courses in networking theory will find Top-Down Network Design, Third Edition, an approachable introduction to the engineering and business issues related to developing real-world networks that solve typical business problems.


Changes for the Third Edition

Networks have changed in many ways since the second edition was published. Many legacy technologies have disappeared and are no longer covered in the book. In addition, modern networks have become multifaceted, providing support for numerous bandwidth-hungry applications and a variety of devices, ranging from smart phones to tablet PCs to high-end servers. Modern users expect the network to be available all the time, from any device, and to let them securely collaborate with coworkers, friends, and family. Networks today support voice, video, high-definition TV, desktop sharing, virtual meetings, online training, virtual reality, and applications that we can’t even imagine that brilliant college students are busily creating in their dorm rooms.


As applications rapidly change and put more demand on networks, the need to teach a systematic approach to network design is even more important than ever. With that need in mind, the third edition has been retooled to make it an ideal textbook for college students. The third edition features review questions and design scenarios at the end of each chapter to help students learn top-down network design.


To address new demands on modern networks, the third edition of Top-Down Network Design also has updated material on the following topics:

¿ Network redundancy

¿ Modularity in network designs

¿ The Cisco SAFE security reference architecture

¿ The Rapid Spanning Tree Protocol (RSTP)

¿ Internet Protocol version 6 (IPv6)

¿ Ethernet scalability options, including 10-Gbps Ethernet and Metro Ethernet

¿ Network design and management tools


Product Details

Pearson Education
Publication date:
Networking Technology
Sold by:
Barnes & Noble
File size:
6 MB

Read an Excerpt

Chapter 5: Designing a Network Topology

Designing a backup path that has the same capacity as the primary path can beexpensive and is only appropriate if the customer's business requirements dictate abackup path with the same performance characteristics as the primary path.

If switching to the backup path requires manual reconfiguration of any components,then Users will notice disruption. For mission-critical applications, disruption isprobably not acceptable. An automatic fallover is necessary for mission-criticalapplications. BY using redundant, partial-mesh network designs, you can speedautomatic recovery time when a link falls.

One other important consideration with backup paths is that they must be tested.Sometimes network designers develop backup solutions that are never tested until acatastrophe happens. When the catastrophe occurs, the backup links do not work. Insome network designs, the backup links are used for load balancing as well asredundancy. This has the advantage that the backup path is a tested solution that isregularly used and monitored as a part of day-to-day operations. Load balancing isdiscussed in more detail in the next section.

Load Balancing

The primary purpose of redundancy is to meet availability requirements. A secondarygoal is to improve performance by supporting load balancing across parallel links.

Load balancing must be planned and in some cases configured. Some protocols do notsupport load balancing by default. For example, when running Novell's Routing Protocol(RIP), an Internetwork Packet Exchange (IPX) router can remember only one route to aremote network. You can change this behavior on a Ciscorouter by using the ipx maximum-paths command.

In ISDN environments, You can facilitate load balancing by configuring channelaggregation. Channel aggregation on means that a router can automatically bring upmultiple ISDN B channels as bandwidth requirements increase. The Multilink Point-to-Point Protocol (MPPP) is an Internet Engineering Task Force (IETF) standard for ISDN B-channel aggregation. MPPP ensures that packets arrive in sequence at the receivingrouter. To accomplish this, data is encapsulated within the Point-to-point Protocol (PPP)and datagrams are given a sequence number. At the receiving router, PPP uses thesequence number to re-create the original data stream. Multiple channels appear as onelogical link to upper-layer protocols.Most vendor's implementations of IP routing protocols support load balancing acrossparallel links that have equal cost. (Cost values are used by routing protocols todetermine the most favorable path to a destination. Depending on the routing protocol,cost can be based on hop count, bandwidth, delay, or other factors.) Cisco supports loadbalancing across six parallel paths. With the IGRP and Enhanced [GRP protocols, Ciscosupports load balancing even when the paths do not have the same bandwidth (which isthe main metric used for measuring cost for those protocols). Using a feature calledvariance, IGRP and Enhanced IGRP can load balance across paths that do not haveprecisely the same aggregate bandwidth. Cost, metrics, and variance are discussed inmore detail in Chapter 7, "Selecting Bridging, Switching, and Routing Protocols."

Some routing protocols base cost on the number of hops to a particular destinationsThese routing protocols load balance over unequal bandwidth paths as long as thehop count is equal. Once a slow link becomes saturated, however higher capacitylinks cannot be filled. This is called Pinhole congestion. Pinhole congestion can be avoided by designing equal bandwidth links within one layer of the hierarchyusing a routing protocol that bases cost on bandwidth and has the variance feature.

Load balancing can be affected by advanced switching (forwarding) mechanismsimplemented in routers. Advanced switching processes often cache the path to remotedestinations to allow fast forwarding of subsequent packets to that destination. (Thecache obviates the need for the router CPU to look in the routing table for a path. Theresult of caching is that all packets destined to a particular destination take the same path.In this case, load balancing occurs across traffic flows to different destinations, but not ona packet-per-packet basis. Some newer technologies, such as Cisco Express Forwarding(CEF), can be configured to do packet-per-packet or destination-per-destination loadbalancing. Chapter 12, "Optimizing Your Network Design," covers CEF in more detail.


Campus network design topologies should meet a customer's goals for availability andperformance by featuring small broadcast domains, redundant distribution-laversegments, mirrored servers, and multiple ways for a workstation to reach a router for off-net communications. Campus networks should be designed using a hierarchical model sothat the network offers good performance, maintainability, and scalability.

Virtual LANs

A virtual LAN (VLAN) is an emulation of a standard LAN that allows data transfer totake place without the traditional physical restraints placed on a network. A networkadministrator can use management software to group users into a VLAN so they cancommunicate as if they were attached to the same wire, when in fact they are located ondifferent physical LAN segments. Because VLANs are based on logical instead ofphysical connections, they are very flexible.

Companies that are growing quickly cannot guarantee that employees working on thesame project will be located together. With VLANs, the physical location of a user doesnot matter. A network administrator can assign a user to a VLAN regardless of the user'slocation. In theory, VLAN assignment can be based on applications, protocols,performance requirements, security requirements, traffic-loading characteristics, or otherfactors.

VLANs allow a large flat network to be divided into subnets. This feature can be used todivide up broadcast domains. Instead of flooding all broadcasts out every port, a VLAN-enabled switch can flood a broadcast out only the ports that are part of the I same subnetas the sending station.

In the past, some companies implemented large switched campus networks with fewrouters. The goals were to keep costs down by using switches instead of routers, andprovide good performance because presumably switches were faster than routers. Withoutthe router capability of containing broadcast traffic, however, the companies neededVLANs. VLANs allow the large flat network to be divided into subnets. A router (or arouting module within a switch) was still needed for inter-subnet communication.

As routers become as fast as switches and Layer-3 functionality is added to switches,fewer companies will implement large, flat, switched networks, and there will be less of aneed for VLANs.

VLAN-based networks can be hard to manage and optimize. Also, when a VLAN isdispersed across many physical networks, traffic must flow to each of those networks,which affects the performance of the networks and adds to the capacity requirements oftrunk networks that connect VLANs....

Meet the Author

Priscilla Oppenheimer has been developing data communications and networking systems since 1980 when she earned her master’s degree in information science from the University of Michigan. After many years as a software developer, she became a technical instructor and training developer and has taught more than 3000 network engineers from most of the Fortune 500 companies. Her employment at such companies as Apple Computer, Network General, and Cisco gave her a chance to troubleshoot real-world network design problems and the opportunity to develop a practical methodology for enterprise network design. Priscilla was one of the developers of the Cisco Internetwork Design course and the creator of the Designing Cisco Networks course. Priscilla teaches network design, configuration, and troubleshooting around the world and practices what she preaches in her network consulting business.

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Top-Down Network Design 4 out of 5 based on 0 ratings. 1 reviews.
Boudville More than 1 year ago
[This is a review of the 3rd edition.] Oppenheimer directs the book at a network analyst who might have to design a large scale network for a client company. The discussion starts by suggesting an analysis of the client's industry and needs. This is reinforced by definitions of various network performance metrics, like MTBF, MTTR, capacity, throughput, delay (latency), delay variation, etc. Chapter 2 is distinguished by a comprehensive explanation of each metric. The explanations are fairly non-technical. You don't need a degree in computer science or electrical engineering to follow it. The text then goes into how to characterise any existing network. This is a pragmatic recognition that you typically do not have a blank slate, with no pre-existing network. These days, a company is likely to already have a network, which presumably is developing bottlenecks or other problems, such that you have been called in to suggest upgrades. Later in the book, the narrative does get more involved, delving into the design of a network topology, with associated switches and routers. Various common protocols are briefly but succinctly covered. As a network designer, you need thorough acquaintance with these and the text is an excellent discourse. I have never seen the 1st or 2nd editions, so I'm unsure exactly how the 3rd differs. I am guessing that much of the text is unchanged. For example, the protocols have been largely stable for several years. While the advice about network topologies could also have been largely unaltered. The most recent portions of this text may pertain to the latest capabilities of switches and routers. The text is also admirably ecumenical in its hardware descriptions. No lock in for Cisco hardware.