Which of the following statements regarding Open Shortest Path First (OSPF) are correct? (Select all that apply.)
Correct Answer: A,B,C,E
Detailed Explanation: Open Shortest Path First (OSPF) is one of the most widely implemented Interior Gateway Protocols (IGPs) in enterprise networks. As a link-state routing protocol, OSPF builds a comprehensive map of the network by exchanging information about the state of each link with all its neighbors (Option A). This approach differs from distance-vector protocols by allowing each router to construct a complete picture of the network topology and subsequently run a shortest path first (SPF) algorithm to determine the best routes. A key mechanism in OSPF is the use of hello packets (Option B). These packets are transmitted at regular intervals to discover neighboring routers and establish adjacencies. The hello process is critical for ensuring that routers within the same area maintain consistent neighbor relationships, which in turn guarantees that link-state databases remain synchronized. If a router stops receiving hello packets from a neighbor, it will eventually assume that the neighbor is down and trigger a recalculation of the routing topology. Another advantage of OSPF is its support for Variable Length Subnet Masking (VLSM) (Option C). VLSM allows network administrators to allocate IP addresses more efficiently by varying the subnet mask lengths across different subnets. This flexibility ensures that IP address space is used optimally, avoiding the wastage that can occur with fixed-length subnetting schemes. With VLSM, large networks can be divided into subnets that match the precise number of hosts required, improving both scalability and management. OSPF’s scalability is further enhanced by its ability to divide large networks into smaller, more manageable areas (Option E). Each OSPF area is a logical grouping of routers that share the same link-state information, reducing the amount of routing data that each router must process. This hierarchical structure minimizes the SPF calculation load on individual routers and helps contain the scope of network changes. For instance, a topology change in one area does not necessitate a complete recalculation of the entire network, thus improving convergence times and overall network stability. Option D is incorrect because OSPF does not use a distance-vector algorithm. Instead, it employs a link-state algorithm based on Dijkstra’s Shortest Path First (SPF) method. This fundamental difference allows OSPF to calculate the shortest path through a network using a full view of the network’s topology rather than relying solely on information received from immediate neighbors. In summary, OSPF’s design—as a link-state routing protocol that leverages cost metrics, hello packets, VLSM, and area-based organization—enables it to provide efficient, scalable, and robust routing for complex enterprise networks. Its ability to quickly converge on optimal paths in dynamic environments makes it a preferred choice for many network professionals.
This CCNA practice question helps students prepare for Cisco networking certification exams by testing knowledge of network fundamentals, routing, switching, and network security concepts.