Explanation/Reference:
Section: Design Methodologies Explanation
Explanation:
The top-down approach to network design is recommended over the bottom-up approach, and the top- down approach is more time-consuming than the bottom-up approach. The top-down design approach takes its name from the methodology of starting with the higher layers of the Open Systems Interconnection (OSI) model, such as the Application, Presentation, and Session layers, and working downward toward the lower layers. The top-down design approach is more time-consuming than the bottom-up design approach because the top-down approach requires a thorough analysis of the organization's requirements. Once the designer has obtained a complete overview of the existing network and the organization's needs, in terms of applications and services, the designer can provide a design that meets the organization's current requirements and that can adapt to the organization's projected future needs. Because the resulting design includes room for future growth, costly redesigns are typically not necessary with the top-down approach to network design.
By contrast, the bottom-up approach can be much less time-consuming than the top-down design approach. The bottom-up design approach takes its name from the methodology of starting with the lower layers of OSI model, such as the Physical, Data Link, Network, and Transport layers, and working upward toward the higher layers. The bottom-up approach relies on previous experience rather than on a thorough analysis of organizational requirements or projected growth. In addition, the bottom-up approach focuses on the devices and technologies that should be implemented in a design, instead of focusing on the applications and services that will actually use the network. Because the bottom-up approach does not use a detailed analysis of an organization's requirements, the bottom-up design approach can often lead to costly network redesigns. Cisco does not recommend the bottom-up design approach, because the design does not provide a "big picture" overview of the current network or its future requirements.
Reference:
CCDA 200-310 Official Cert Guide, Chapter 1, TopDown Approach, pp. 24-25 Cisco: Using the TopDown Approach to Network Design: 4. TopDown and BottomUp Approach Comparison (Flash)