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Internetwork Design Overview


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Internetwork Design Overview



Internetwork Design Goals



Key Design Issues and Requirements



Design Methodology






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Cisco Internetwork Design

From: Cisco Internetwork Design
Author: Matthew Birkner
Publisher: Cisco Press (53)
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1. Internetwork Design Overview

Nothing in this world can take the place of persistence. Talent will not; nothing is more common than unsuccessful people with talent. Genius will not; unrewarded genius is almost a proverb. Education will not; the world is full of educated derelicts. Persistence and determination alone are omnipotent. The slogan “Press On” has solved and always will solve the problems of the human race.

--Calvin Coolidge

Upon completion of this chapter, you will be able to do the following:

  • List internetwork design goals

  • Identify key requirements of internetwork design

  • Describe a methodology for internetwork design

If you have ever been tasked with a network design project, whether large or small, undoubtedly you have had to make several difficult design decisions. You may have even found several “right answers” to the problem, but then needed to select the best “right answer.” What, then, is the best “right answer?” It depends on how well you and your team know your customer and their requirements. The truth is, you can approach a network design in several ways. However, most network designs (the successful ones, at least) follow some fundamental guidelines.

This chapter focuses on the goals of internetwork design, including the technical and business trade-offs you must understand prior to making design choices. Sometimes, you and your team may be diverted from the topic into business issues that may not necessarily be technically oriented. As a network designer, you must remember to keep the team focused and gather the relevant information to make the design meet the customer's goals. This chapter provides you with an internetwork design methodology road map for use when approaching an internetwork design to keep your project on track.

Internetwork Design Goals

An internetwork can be generally defined as two or more local-area networks (LANs) interconnected by one or more Layer 3 devices (ordinarily routers). An internetwork may be contained within a single building, or may span the globe. Although there are specific differences between LANs and wide-area networks (WANs), in general terminology (and in this book) the word network refers to either type.

The first step in designing an internetwork is to establish and document the goals of the design. These goals will be particular to each organization or situation. Certain requirements tend to always show up in any good network design, however. They are as follows:

  • Functionality

  • Scalability

  • Adaptability

  • Manageability

  • Cost effectiveness


First and foremost, you cannot design a network without first fully knowing what you are trying to accomplish. Gathering all the requirements is often a very difficult task, but when the network is deployed, it must work as designed. There is absolutely no room for negotiation here. The network must enable users to meet their individual job requirements in such a way that the overall business requirements of the organization are met. The network must provide end-to-end application availability at some specified level of service (defined by management as the optimal compromise between functionality and cost). An MCI executive once told me that, before you embark on a new challenge, you must know the answer to the question, “How do you measure your success?” I urge you to ask yourself this same question of your network design on a regular basis, because it leads to an overall understanding of the tasks you are trying to achieve and keeps you focused on success.


The network must be able to grow as the organization grows, and as more of the organization is included in the network. The initial design must be scalable across several orders of magnitude of network growth.

Take company XYZ, for example. They have been acquiring new companies at a rate of three to five per year. As a measure of your success, the network infrastructure must scale in a modular fashion so that new acquisitions “snap in” to their existing infrastructure. If scalability is not present in company XYZ's network, the network absorption of this new company will undoubtedly fail or will become a management nightmare. I have seen many networks in the past which, due to poor planning for scalability, had become a jumbled “spaghetti net.” It is vital, therefore, to ensure that a design will scale in the future, even if large-scale expansion is not necessarily required today.


The network should be designed with an eye toward future technologies and should not include any design elements that would limit adoption of new technologies as they become available. There may be trade-offs between this and cost effectiveness throughout a network design/implementation. For example, Voice over IP (VoIP) and multicast are new technologies rapidly being adopted in many internetworks. Network designs should certainly be able to support these technologies without requiring a “forklift upgrade.” This is done by provisioning hardware and software that has future-proofed options for expansion and upgradability.


The network should be designed to facilitate proactive network monitoring and management, to ensure ongoing stability of operation and availability of resources (see the sidebar later in this chapter, “Total Cost of Ownership”). You should consider a network management strategy as carefully as you consider the network design. What this means is that the network must work as designed, but it also must be supportable. If a highly complex design is delivered to the network-management team, it may require an excessive amount of your time and support to work with network-operations personnel. The key is to remember that another organization or individual may support the network you designed. Your reputation as a designer is on the line and, as the adage says, “Perception is reality.”

Cost Effectiveness

The benefits of the network to the organization, however quantified, must equal or outweigh the costs. The cost of implementing the network design must be within agreed-upon budgetary constraints. For example, a design requirement may implicitly exceed the financial commitment of the customer. If that is the case, you must identify this and find an alternative solution, if one exists. If an alternative does not exist, this must be communicated back to the customer with his options so that he can make an informed decision.


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