Which of the following features are provided by OSPF? (Select all that apply.)
Correct Answer: A,B,C
Detailed Explanation: Open Shortest Path First (OSPF) is a widely used link-state routing protocol that offers a rich set of features designed to enhance scalability, efficiency, and reliability in enterprise networks. Let’s break down each option: A. Hierarchical Network Design Using Areas: OSPF introduces a hierarchical design model by dividing the network into multiple areas. The backbone area (Area 0) serves as the core, and all other areas connect to it. This segmentation limits the scope of route recalculations and reduces the overall size of the routing table on each router. By confining the flooding of link-state advertisements (LSAs) within areas, OSPF improves network scalability and reduces overhead. B. Support for Variable Length Subnet Masking (VLSM): VLSM allows network administrators to use subnets of different sizes within the same network. OSPF fully supports VLSM, enabling more efficient utilization of IP address space. This flexibility is critical in modern networks where addressing schemes must be tailored to the specific needs of different subnets, preventing wastage of IP addresses. C. Use of the Dijkstra Algorithm for Route Calculation: OSPF uses the Shortest Path First (SPF) algorithm—commonly known as the Dijkstra algorithm—to compute the shortest path tree for each router. This algorithm considers the cost associated with each link (which is usually based on the link’s bandwidth) and determines the optimal routes to all destinations. The use of the Dijkstra algorithm is one of the key characteristics that distinguishes OSPF from distance-vector protocols. D. Reliance on Hop Count as the Primary Metric (Incorrect): Unlike distance-vector protocols such as RIP, which use hop count as their metric, OSPF’s primary metric is the cost, which is calculated based on the bandwidth of the links. This allows OSPF to make more nuanced decisions that reflect the actual performance capabilities of different network paths. Hop count is a simplistic metric that does not account for link speed or other important factors, so relying on hop count would be insufficient for modern network needs. Importance in Network Design and Troubleshooting: Understanding these OSPF features is essential for network designers and administrators. The hierarchical area-based design helps in managing large networks, while VLSM support allows for flexible and efficient IP addressing. The Dijkstra algorithm’s use ensures that OSPF can determine the best available paths based on multiple link characteristics rather than just counting hops. Practical Scenarios: For example, if a network experiences frequent topology changes or requires rapid convergence, OSPF’s design—with its area segmentation and cost-based route calculation—provides the necessary robustness. In networks where bandwidth varies significantly between different segments, using OSPF ensures that traffic is routed over higher-capacity links rather than simply following the path with the fewest hops. Conclusion: OSPF provides several key features, including a hierarchical network design (Option A), support for VLSM (Option B), and the use of the Dijkstra algorithm (Option C) for route calculation. It does not, however, rely on hop count as its primary metric (Option D is incorrect). These features collectively make OSPF a versatile and efficient routing protocol for complex, modern networks.
This CCNA practice question helps students prepare for Cisco networking certification exams by testing knowledge of network fundamentals, routing, switching, and network security concepts.