Open Shortest Path First (OSPF) protocol

The Open Shortest Path First (OSPF) protocol is a shortest path first (SPF) or link-state protocol. OSPF is better suited than RIP for complex networks with many routers.

OSPF is an interior gateway protocol that distributes routing information between routers in a single autonomous system (AS). OSPF chooses the least cost path as the best path. Suitable for complex networks with a large number of routers, OSPF provides equal cost multipath routing where packets to a single destination can be sent via more than one interface simultaneously. In a link-state protocol, each router maintains a database describing the entire AS topology, which it builds out of the collected link-state advertisements of all routers. Each participating router distributes its local state (that is, the router's usable interfaces and reachable neighbors) throughout the AS by flooding. Each multi-access network that has at least two attached routers has a designated router and a backup designated router. The designated router floods a link-state advertisement for the multi-access network and has other special responsibilities. The designated router concept reduces the number of adjacencies required on a multi-access network.

OSPF allows networks to be grouped into areas. Routing information passed
between areas is abstracted, potentially allowing a significant reduction in routing traffic. OSPF uses four different types of route, listed here in order of preference: intra-area, inter-area, type 1 external, and type 2 external. Intra-area paths have destinations within the same area, inter-area paths have destinations in other OSPF areas, and Autonomous System External (ASE) routes are routes to destinations external to the AS. Routes imported into OSPF as type 1 routes are supposed to be from internal gateway protocols whose external metrics are directly comparable to OSPF metrics.
When a routing decision is being made, OSPF will add the internal cost to the AS border router to the external metric. Type 2 ASEs are used for exterior gateway protocols whose metrics are not comparable to OSPF metrics. In this case, only the internal OSPF cost to the AS border router is used in the routing decision.

From the topology database, each router constructs a tree of the shortest paths with itself as the root. This shortest-path tree gives the route to each destination in the AS. Externally derived routing information appears on the tree as leaves. The link-state advertisement format distinguishes between information acquired from external sources and information acquired from internal routers, so there is no ambiguity about the source or reliability of routes. Externally derived routing information (for example, routes learned from EGP or BGP) is passed transparently through the autonomous system and is kept separate from OSPF's internally derived data. Each external route can also be tagged by the advertising router, enabling a passing of additional information between routers on the borders of the autonomous system.

OSPF optionally includes type of service (TOS) routing and allows administrators to install multiple routes to a
given destination for each type of service (for example, low delay or high throughput). A router running OSPF uses the destination address and the type of service to choose the best route to the destination.

OSPF intra- and inter-area routes are always imported into the gated routing database with a preference of 10. (See ``gated preferences and route selection'' for a discussion of preferences.) It would be a violation of the protocol if an OSPF router did not participate fully in the area's OSPF, so it is not possible to override this. Although it is possible to give other routes lower preference values explicitly, it is ill-advised to do so.

OSPF areas are connected by the backbone area, the area with identifier 0.0.0.0. All areas must be logically contiguous and the backbone is no exception. To permit maximum flexibility, OSPF allows the configuration of ``virtual'' links to enable the backbone area to appear contiguous despite the physical reality.

All OSPF protocol exchanges are authenticated.

Authentication guarantees that routing information is imported only from trusted routers to protect the Internet and its users. A variety of authentication schemes can be used, but a single scheme must be configured for each area. This enables some areas to use much stricter authentication than others.

All routers in an area must agree on that area's parameters. A separate copy of the link-state algorithm is run for each area. Because of this, most configuration parameters are defined on a per-area basis. All routers belonging to an area must agree on that area's configuration. Misconfiguration will lead to adjacencies not forming between neighbors, and routing information might not flow or even loop.

OSPF is described in RFC 1583. The MIB is defined in RFC 1253. Other related documents are RFC 1245, RFC 1246, and RFC 1370.

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