OSPF LSA Refresh Interval and Protocol Mechanics

The question tests fundamental OSPF protocol mechanics, specifically regarding transport, timers, algorithms, and packet flooding. The correct answer is B.

First, evaluate the transport mechanism. OSPF packets are encapsulated directly in IP with Protocol Number 89; they do not use TCP or UDP. This makes Option A incorrect because OSPF implements its own reliable transmission mechanisms (Hello packets, LSUs, LSRs, LSacks) rather than relying on upper-layer protocols.

Next, consider the database synchronization process. OSPF routers maintain a Link State Database (LSDB). To ensure consistency, each LSA has an age field that increments over time. When an LSA reaches its MaxAge (60 minutes), it is flushed. However, to prevent unnecessary flooding of stable routes, routers refresh LSAs periodically. The default refresh interval for self-originated LSAs is 1800 seconds, which equals 30 minutes. This periodic refresh ensures that all routers in the area have up-to-date information without waiting for the LSA to expire. Therefore, Option B accurately describes this standard behavior.

Regarding the routing algorithm, OSPF is a Link-State protocol that uses the Dijkstra Shortest Path First (SPF) algorithm to calculate the loop-free shortest path tree. In contrast, distance-vector protocols like RIP use the Bellman-Ford algorithm. Thus, Option C is factually incorrect regarding the algorithm type.

Finally, examine the flooding interval. While OSPF does flood Link State Updates (LSUs), the default retransmission interval for unacknowledged LSUs is typically 5 seconds only on slow networks (like Frame Relay) or when configured specifically. On broadcast multi-access networks, the Hello interval is usually 10 seconds, and LSU retransmissions are handled via specific timers, but stating generally that OSPF floods an LSU at an interval of 5s as a universal truth is misleading compared to the definitive nature of the 30-minute refresh rule. Moreover, Option B is a precise definition of the LSA refresh mechanism found in standard documentation.

In summary, Option B correctly identifies the default LSA refresh interval, while A, C, and D contain fundamental errors about OSPF's architecture.