What prevents routing loops in a single-area OSPF network?

In OSPF, loop prevention within a single area is achieved through the fundamental nature of its link-state architecture. Unlike distance-vector protocols that rely on "routing by rumor," OSPF ensures that every router within an area maintains an identicalLink-State Database (LSDB). This database acts as a complete map of the network topology.

Once the LSDB is synchronized, each router independently executes theShortest Path First (SPF) algorithm, which is formally known as theDijkstra algorithm. This mathematical process treats the local router as the "root" of a tree and calculates the shortest path to every other node (router) and prefix in the area based on the cumulative interface costs. Because every router uses the same synchronized map (the LSDB) and the same deterministic algorithm, they all arrive at a consistent, loop-free view of the best paths.

According to Juniper Networks technical documentation, the Dijkstra algorithm is superior to theBellman-Ford algorithm(used by distance-vector protocols like RIP) in this regard. Bellman-Ford is susceptible to "count-to-infinity" problems and loops because routers only know the distance and direction to a destination provided by their neighbors, rather than the full topology. In OSPF, even if a link fails, the updated Link-State Advertisement (LSA) is flooded rapidly, and the Dijkstra algorithm is re-run to find a new loop-free path.Routing policies(Option B) are used to manipulate path selection or filter routes but are not the primary mechanism for fundamental loop prevention in OSPF. Similarly,forwarding policies(Option D) govern how traffic is handled at the data plane level rather than determining the control plane's loop-free topology.