Explanation/Reference:
Section: Design Methodologies Explanation
Explanation:
The bottom-up design approach is a design methodology that focuses on the devices and technologies that should be implemented in a design, instead of focusing on the applications and services that will be used on the network. The bottom-up design approach takes its name from the methodology of starting with the lower layers of the Open Systems Interconnection (OSI) model, such as the Physical, Data Link, Network, and Transport layers, and working upward toward the higher layers. Because the bottom-up approach does not use a detailed analysis of an organization's requirements, the bottom-up approach can be much less time-consuming than the top-down design approach. However, the bottom-up design approach can often lead to costly network redesigns because the design does not provide a "big picture" overview of the current network or its future requirements. In addition, the bottom-up approach relies on previous experience rather than on a thorough analysis of organizational requirements or projected growth.
By contrast, the top-down design approach takes its name from the methodology of starting with the higher layers of the OSI model, such as the Application, Presentation, and Session layers, and working downward toward the lower layers. The top-down design approach requires a thorough analysis of the organization's requirements. As a result, the top-down design approach is a more time-consuming process than the bottom-up design approach. With the top-down approach, the designer obtains a complete overview of the existing network and the organization's needs. With this "big picture" overview, the designer can then focus on the applications and services that meet the organization's current requirements. By focusing on the applications and services required in the design, the designer can work in a modular fashion that will ultimately facilitate the implementation of the actual design. In addition, the flexibility of the resulting design is typically much improved over that of the bottom-up approach because the designer can account for the organization's projected needs.
Reference:
CCDA 200-310 Official Cert Guide, Chapter 1, Top-Down Approach, pp. 24-25 Cisco: Using the Top-Down Approach to Network Design: 4. Top-Down and Bottom-Up Approach Comparison (Flash)