Describe the top-down approach to network design

Exam: 640-864 - Designing for Cisco Internetwork Solutions (DESGN)


A top down approach is used for designing phase of PPDIOO method. In this approach, the requirements of the organization are determined before determining the technologies to be used. Network designs are adequately tested before moving to the Implementation phase.

Top-Down Approach

Top down approach to design refers to starting the design beginning from the topmost layer and moving down. A top down approach is best suited for it allows a network & the relevant infrastructure to be adapted to the needs of the network application. In this approach, the technologies and devices are only selected after a detailed analysis of the requirements has been made. The following functions are important in a top down approach:

  • Analysis: The requirements of the applications and the organization are analyzed in detail;
  • Designing starting from topmost layer of the OSI model: This can be subdivided into two parts - defining the requirements of the upper layers and specifying the infrastructure for the lower layers.
  • Collecting additional information about the network
Top-Down Design Process

Top-Down Design Process

Pilot and Prototype Testing

The moment the design nears completion and before it is fully implemented, it is recommended that new network, solution be tested. The testing of a new solution can be carried out by adopting two methods:

  • Prototype or
  • Pilot Project

A prototype is a part of the complete design and the tests are carried out in isolation. It is not connected to an existing network. An advantage of going in for testing using a prototype is that it permits testing the network before its deployment.

A pilot project or site is a location that is live and serves the function of a testing site much before a solution is deployed on a large scale. A pilot site allows detection of problems of the real world.

Irrespective of the method chosen for testing, successful testing leads to a faith in the design, while a failure helps in correcting the designs and running the tests again.

Design Document

It is the design document that contains information about the requirements of the business, the existing architecture, requirements of the network, designing, planning and information necessary for configuring a new network.

Network analysts and architects use it to document the changes to the network and serve as a document. The important fields that a design document must contain are:

  • Introduction that briefly describes the reasons and the purpose behind the network;
  • Design Requirements where the requirements of the organization are listed along with the goals and the constraints;
  • Existing Network Infrastructure including the topology drawings, results of audit, health of the network, routing protocols, a detailed summary of the applications, list of devices like routers and switches etc.;
  • Design where the design information like logical topology, physical topology, routing protocols, addressing and security configurations are specified;
  • Proof of Concept is the result of prototype testing or a live pilot;
  • Implementation Plan where the steps to be followed in implementing a new installation are to be followed;
  • Appendixes contains information about network devices that exist, the configurations, and all extra information that is used while designing a network.

A number of steps are to be followed while implementing a network. It is important that every step contain information about:

  • Step description;
  • Reference to the design documentation;
  • Detailed guidelines for implementation;
  • Detailed guidelines for a rollback;
  • Time required for implementation.

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