OSPF = Open Shortest Path First. It is a protocol that allows routers to set routes between them. As links go up and down, OSPF allows the routers to change routes so that the route taken always is the shortest possible.

OSPF is link-state and distance-vector : link-state for intra-area computations and distance-vector for inter-area/external route computations. For external area route choosing, OSPF asks the ABR/ASBR on what is their view to the destination , taking for granted what the router reports in.

Ospf is a link state dynamic routing protocol. Instead of manually configuring static routes to every possible destination, routes are learned dynamically from ospf neighbors. Neighbor relationship between routers can be formed on point to point as well as broadcast segment. Passive interfaces can be configured with certain interfaces, for which no ospf neighbor adjacency is required. Routes are shared among neighbors using link state advertisement messages. Routers in the same area will need to have similar LSDB. This can cause problems of LSA flooding, if the number of ospf routers in the area grow. Large enterprises use multiple ospf areas to segment networks. Traffic from non-backbone area has to traverse the backbone area unless virtusl links are established among certain areas. Areas can be configured as stub area, non so stubby (nssa) area or totally stubby area. These kinds of areas help in reducing the size of the routing table, by allowing only certain routes to be advertized in this area.