A top-down approach to network design means that requirements are considered first, with the applications and network solutions that will run on the network driving the design.
A bottom-up approach would first select devices, features, cabling, and so on, and then try to fit the applications onto this network. A bottom-up approach can lead to redesign if the applications are not accommodated properly. This approach can also result in increased costs by including features or devices that are not required and would have been excluded had the network requirements analysis been completed.
After the alternative solutions have been developed, the optimal solution must be chosen. A systematic approach works bestall the options should be listed along with how well they meet (or don't meet) the design requirements and constraints. If no clear winner exists, the importance of the requirements (as determined in the requirements-gathering process) should be considered to select the optimal design.
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