Cisco Certified Network Associate (CCNA) Practice Exam

Why do large OSPF networks use a hierarchical design?

1. To increase redundancy

2. To speed up convergence and confine instability

3. To simplify routing protocols

4. To reduce the number of routers used

The correct answer is: To speed up convergence and confine instability

A hierarchical design in large OSPF networks is primarily utilized to speed up convergence and confine instability. In such designs, the network is divided into areas, allowing for more efficient routing and management. This means that any changes such as link failures or additions are localized to a specific area, which helps to prevent network-wide disruptions and speeds up the process of convergence. When a change occurs within a specific area, only the routers directly involved and those within that area need to recalibrate their routing tables, minimizing the overhead and time involved in this process. By confining instability to specific areas, the overall stability of the network is enhanced, as other areas are unaffected by changes, further aiding in faster convergence times across the network. In contrast, while redundancy, simplification of routing protocols, and reduction of router numbers can have their benefits in network design, they are not the primary reasons driving the need for hierarchical design in OSPF networks. The core goal of such a design is to improve convergence times and maintain localized stability amidst potential network changes.