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Chapter 1
Introduction to Network Design


ü Demonstrate an understanding of the steps for designing internetwork solutions.
ü Analyze a client’s business and technical requirements and select appropriate internetwork technologies and topologies.
ü Construct an internetwork design that meets a client’s objectives for internetwork functionality, performance, and cost.
ü Define the goals of internetwork design.
ü Define the issues facing designers.
ü List resources for further information.
ü Identify the origin of design models used in the course.
ü Define the hierarchical model.

Network design is one of the more interesting facets of computing. While there are many disciplines in information technology, including help desk, application development, project management, workstation support, and server administration, network design is the only one that directly benefits from all these other disciplines. It incorporates elements of many disciplines into a single function. Network designers frequently find that daily challenges require a certain amount of knowledge regarding all of the other IT disciplines.

The network designer is responsible for solving the needs of the business with the technology of the day. This requires knowledge of protocols, operating systems, departmental divisions in the enterprise, and a host of other areas. The majority of network design projects require strong communication skills, leadership, and research and organizational talents. Project management experience can also greatly benefit the process, as most network design efforts will require scheduling and budgeting with internal and external resources, including vendors, corporate departments, service providers, and the other support and deployment organizations within the enterprise.

This text will both provide an introduction to network design and serve as a reference guide for future projects. Its primary purpose is to present the objectives for the CCDP: Cisco Internetwork Design examination and to prepare readers to pass this certification test. However, it would be unfortunate to read this book only in the context of passing the exam. A thorough understanding of network design not only assists administrators in troubleshooting, but enables them to permanently correct recurrent problems in the network. An additional perk is the satisfaction that comes with seeing a network that you designed and deployed—especially a year later when only minor modifications have been needed and all of those were part of your original network design plan.

Having said that, it is important to note that in “real world” network designs virtually no individual does all the work. Vendors, business leaders, and other administrators all will, and should, play a significant role in the design process. This is obviously true when planning server-based services, such as DHCP (Dynamic Host Configuration Protocol). Though many beautiful network designs have been conceived without consideration and consultation of the user community, the end result is an expensive “It should have worked!” After reading this text, and specifically this chapter, no one should ever make this mistake.

Overview of Network Design

It has been stated that network design is 50 percent technology, 50 percent diplomacy, and 50 percent magic. While written examinations will likely ignore the last item, mastery of the first two is critical in exam preparation.

In actuality, network design is simply the implementation of a technical solution to solve a nontechnical problem. Contrary to expectations, network design is not as basic as configuring a router, although we will address this critical component. Rather, as presented in this first chapter, network design is a multifaceted effort to balance various constraints with objectives.

Network design encompasses three separate areas: conception, implementation, and review. This chapter will elaborate on these areas and expand the scope of each. It’s important to remember that each phase is unique and requires separate attention. The final phase of network design—review—is perhaps more important than any other phase, as it provides valuable information for future network designs and lessons for other projects. Readers should consider how they might design networks deployed with the technology referenced in this text—the easiest methodology is to establish a list of metrics from which to make a comparison. Designers who meet the original metrics for the project usually find that the network is successful in meeting the customer’s needs.

Each design, whether the simple addition of a subnet or the complete implementation of a new international enterprise network, must address the same goals: scalability, adaptability, cost control, manageability, predictability, simplicity of troubleshooting, and ease of implementation. A good design will both address current needs while effectively accommodating future needs. However, two constraints limit most designs’ ability to address these goals: time and money. Typical network technology lasts only 24 to 60 months, while cabling and other equipment may be expected to remain for over 15 years. The most significant constraint, though, will almost always be financial.

The actual expected life of a cable plant is subject to some debate. Many networks are already coming close to the 15-year mark on the data side, and the voice side already has upwards of 60 years. The trend has been for copper cable to have some built-in longevity, and such efforts as Digital Subscriber Line (DSL), Category 5E, and Gigabit Ethernet over copper are solid evidence that corporations will continue to regard this copper infrastructure as a long-term investment.

With that said, let’s focus on some of the theory behind network designs.

Network Design Goals

Network designers should strive to address a number of objectives in their designs. Readers should focus on these goals and consider how they might relate to the typical corporate environment. (Later in this chapter, we will more fully explore the importance of the business relationship.) In addition, designers should pay specific attention to the relationships between the design goals, noting that addressing one goal will frequently require compromising another. Let’s look at these goals in detail.


Scalability refers to an implementation’s ability to address the needs of an increasing number of users. For example, a device with only two interfaces will likely not provide as much service and, therefore, not be as scalable as a device with 20. Twenty interfaces will likely cost a great deal more and will undoubtedly require greater amounts of rack space, and so scalability is often governed by another goal—controlling costs. Architects are often challenged to maintain future-proof designs while maintaining the budget.

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