OSPF (Open Shortest Path First) is a link-state interior gateway protocol using Dijkstra's algorithm to compute shortest paths. Supports areas (Area 0 = backbone) for scalability.
OSPF = interior routing (within an AS). BGP = exterior routing (between ASes). OSPF converges faster than RIP. MD5/SHA authentication prevents rogue routers from injecting false routes. Area 0 (backbone) must connect all other areas.