Top-down Network Design 3rd Edition Pdf

[Ane Neemann]

This chapter serves as an introduction to the rest of the book by describingtop-down network design. The first section explains how to use a systematic,top-down process when designing computer networks for your customers. Dependingon your job, your customers might be other departments within your company,those to whom you are trying to sell products, or clients of your consultingbusiness.

After describing the methodology, this chapter focuses on the first step intop-down network design: analyzing your customer's business goals. Businessgoals include the capability to run network applications to meet corporatebusiness objectives, and the need to work within business constraints, such asbudgets, limited networking personnel, and tight timeframes.

This chapter also covers an important business constraint that some peoplecall the eighth layer of the Open Systems Interconnection (OSI) reference model:workplace politics. To ensure the success of your network design project, youshould gain an understanding of any corporate politics and policies at yourcustomer's site that could affect your project.

To paraphrase Einstein, networking professionals have the ability to createnetworks that are so complex that when problems arise they can't besolved using the same sort of thinking that was used to create the networks. Addto this the fact that each upgrade, patch, and modification to a network canalso be created using complex and sometimes convoluted thinking, and you realizethat the result is networks that are hard to understand and troubleshoot. Thenetworks created with this complexity often don't perform as well asexpected, don't scale as the need for growth arises (as it almost alwaysdoes), and don't match a customer's requirements. A solution to thisproblem is to use a streamlined, systematic methodology in which the network orupgrade is designed in a top-down fashion.

Many network design tools and methodologies in use today resemble the"connect-the-dots" game that some of us played as children. Thesetools let you place internetworking devices on a palette and connect them withlocal-area network (LAN) or wide-area network (WAN) media. The problem with thismethodology is that it skips the steps of analyzing a customer'srequirements and selecting devices and media based on those requirements.

Good network design must recognize that a customer's requirements embodymany business and technical goals including requirements for availability,scalability, affordability, security, and manageability. Many customers alsowant to specify a required level of network performance, often called aservice level. To meet these needs, difficult network design choices andtradeoffs must be made when designing the logical network before any physicaldevices or media are selected.

When a customer expects a quick response to a network design request, abottom-up (connect-the-dots) network design methodology can be used, if thecustomer's applications and goals are well known. However, networkdesigners often think they understand a customer's applications andrequirements only to discover, after a network is installed, that they did notcapture the customer's most important needs. Unexpected scalability andperformance problems appear as the number of network users increases. Theseproblems can be avoided if the network designer uses top-down methods thatperform requirements analysis before technology selection.

Top-down network design is a methodology for designing networks that beginsat the upper layers of the OSI reference model before moving to the lowerlayers. It focuses on applications, sessions, and data transport before theselection of routers, switches, and media that operate at the lower layers.

The top-down network design process includes exploring divisional and groupstructures to find the people for whom the network will provide services andfrom whom you should get valuable information to make the design succeed.

Top-down network design is also iterative. To avoid getting bogged down indetails too quickly, it is important to first get an overall view of acustomer's requirements. Later, more detail can be gathered on protocolbehavior, scalability requirements, technology preferences, and so on. Top-downnetwork design recognizes that the logical model and the physical design maychange as more information is gathered.

Because top-down methodology is iterative, some topics are covered more thanonce in this book. For example, this chapter discusses network applications.Network applications are discussed again in Chapter 4, "CharacterizingNetwork Traffic," which covers network traffic caused by application- andprotocol-usage patterns. A top-down approach lets a network designer get"the big picture" first and then spiral downward into detailedtechnical requirements and specifications.

Top-down network design is a discipline that grew out of the success ofstructured software programming and structured systems analysis. The main goalof structured systems analysis is to more accurately represent users'needs, which are unfortunately often ignored or misrepresented. Another goal isto make the project manageable by dividing it into modules that can be moreeasily maintained and changed.

A logical model is developed before the physical model. The logical modelrepresents the basic building blocks, divided by function, and the structure ofthe system. The physical model represents devices and specific technologies andimplementations.

With large network design projects, modularity is essential. The designshould be split functionally to make the project more manageable. For example,the functions carried out in campus LANs can be analyzed separately from thefunctions carried out in remote-access networks, virtual private networks(VPNs), and WANs.

Cisco Systems recommends a modular approach with its three-layer hierarchicalmodel. This model divides networks into core, distribution, and access layers.Cisco's Secure Architecture for Enterprises (SAFE) and Enterprise CompositeNetwork Model (ECNM), which are discussed in Part II of this book, "LogicalNetwork Design," are also modular approaches to network design.

With a structured approach to network design, each module is designedseparately, yet in relation to other modules. All the modules are designed usinga top-down approach that focuses on requirements, applications, and a logicalstructure before the selection of physical devices and products to implement thedesign.

Systems analysis students are familiar with the concept that typical systemsare developed and continue to exist over a period of time, often called asystems development life cycle. Many systems analysis books use the acronym SDLCto refer to the life cycle, which may sound strange to networking students whoknow SDLC as Synchronous Data Link Control, a bit-oriented, full-duplex protocolused on synchronous serial links, often found in a legacy Systems NetworkArchitecture (SNA) environment. Nevertheless, it's important to realizethat most systems, including network systems, follow a cyclical set of phases,where the system is planned, created, tested, and optimized.

Feedback from the users of the system causes the system to then be re-createdor modified, tested, and optimized again. New requirements arise as the networkopens the door to new uses. As people get used to the new network and takeadvantage of the services it offers, they soon take it for granted and expect itto do more.

Analyze requirements. In this phase, the network analystinterviews users and technical personnel to gain an understanding of thebusiness and technical goals for a new or enhanced system. The task ofcharacterizing the existing network, including the logical and physical topologyand network performance, follows. The last step in this phase is to analyzecurrent and future network traffic, including traffic flow and load, protocolbehavior, and quality of service (QoS) requirements.

Develop the logical design. This phase deals with a logicaltopology for the new or enhanced network, network layer addressing, naming, andswitching and routing protocols. Logical design also includes security planning,network management design, and the initial investigation into which serviceproviders can meet WAN and remote access requirements.

Develop the physical design. During the physical design phase,specific technologies and products to realize the logical design are selected.Also, the investigation into service providers, which began during the logicaldesign phase, must be completed during this phase.

Test, optimize, and document the design. The final steps intop-down network design are to write and implement a test plan, build aprototype or pilot, optimize the network design, and document your workwith a network design proposal.

These major phases of network design repeat themselves as user feedback andnetwork monitoring suggest enhancements or the need for new applications. Figure1-1 shows the network design and implementation cycle.

Cisco Systems teaches the Plan Design Implement Operate Optimize (PDIOO) setof phases for the life cycle of a network. It doesn't matter exactly whichlife cycle you use, as long as you realize that network design should beaccomplished in a structured, planned, modular fashion, and that feedback fromthe users of the operational network should be fed back into new networkprojects to enhance or redesign the network. Learning the Cisco steps isimportant if you are studying for a Cisco design certification. For that reason,the steps are listed here:

Plan. Network requirements are identified in this phase. Thisphase also includes an analysis of areas where the network will be installed andan identification of users who will require network services.

Operate. Operation is the final test of the effectiveness of thedesign. The network is monitored during this phase for performance problems andany faults, to provide input into the optimize phase of the network lifecycle.

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