Which of the following statements regarding OSPF is true?

Open Shortest Path First (OSPF) is designed for stability and accuracy. To ensure the Link-State Database (LSDB) on all routers in an area remains synchronized and to prevent stale information, OSPF periodically refreshes its Link-State Advertisements (LSAs). Each LSA is reflooded every 30 minutes (1800 seconds) by its originating router, even if no network topology changes have occurred. This process resets the LSA's age, confirming its validity and maintaining the integrity of the LSDB across the OSPF domain.

A. OSPF runs directly over the IP protocol (protocol number 89) and has its own built-in reliability mechanism, including explicit Link-State Acknowledgment (LSAck) packets. It does not use TCP.
C. OSPF is a link-state protocol that uses the Dijkstra (Shortest Path First) algorithm to calculate the best path, not the Bellman-Ford algorithm, which is used by distance-vector protocols like RIP.
D. OSPF floods Link-State Update (LSU) packets immediately upon detecting a network topology change, not at a fixed 5-second interval. The periodic refresh interval is 30 minutes.

1. Huawei HCIP-Datacom-Core Technology V1.0 Training Material. In the "OSPF Principles" section, it is stated that "To ensure LSDB synchronization, OSPF periodically floods LSAs every 30 minutes." This document also confirms OSPF uses the Dijkstra algorithm and has its own reliability mechanisms.

2. RFC 2328 - OSPF Version 2. Section 13, "The Aging of Link State Advertisements," specifies: "LSAs are also refreshed from time to time. Each LSA is reflooded every LSRefreshTime seconds. LSRefreshTime is a constant, set to 30 minutes."

3. RFC 2328 - OSPF Version 2. Appendix B, "Architectural Constants," defines LSRefreshTime as 1800 seconds (30 minutes).

4. Kurose, J. F., & Ross, K. W. (2017). Computer Networking: A Top-Down Approach (7th ed.). Chapter 5, "The Network Layer: Control Plane," explains that OSPF routers broadcast LSAs to all other routers in the domain and use Dijkstra's algorithm for path computation. It also clarifies that OSPF messages are carried directly over IP.