Open Shortest Path First.
The OSPF is a (link state) routing protocol used by the Internet community. OSPF is classified as an Interior Gateway Protocol (IGP), this means that it distributes routing information between routers belonging to a single Autonomous System. OSPF is used over IP. That means that an OSPF packet is transmitted with an IP data packet header. The PROTOCOL field in the IP header is set to 89 for OSPF.
OSPF is designated to be run internal to a single Autonomous System. Each OSPF router maintains an identical database describing the Autonomous System's topology. From this database, a routing table is calculated by constructing a shortest path tree. OSPF recalculates routes quickly in the face of topological changes, utilizing a minimum of routing protocol traffic. Separate routes can be calculated for each IP type of service.
OSPF allows sets of networks to be grouped together. Such a grouping is called an area and it's topology is hidden from the rest of the Autonomous System. This information hiding enables a significant reduction in routing traffic. An area is a generalization of an IP subnetted network.
All OSPF routing protocol exchanges are authenticated. This means that only trusted routers can participate in the AS's routing.
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