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The following list comprises the top-level OSPF data structures:

• Router ID

  A 32-bit number that uniquely identifies a router in the AS. One possible strategy would be to use the smallest IP interface address belonging to the router.

• Area Structures

  Each one of the areas to which the router is connected has its own data structure. This structure describes the working of the basic OSPF algorithm.

• Backbone (Area 0) structure

  This special structure is responsible for the dissemination of inter-area routing information.

• Virtual Links

  For area border routers that are configured as virtual link endpoints. Virtual links are identified by the Router ID of the other endpoint, which is also another area border router.

• List of external routes

  Routes to destination external to the AS, that have been gained through direct experience with another routing protocol (such as BGP) or through configuration information, or through a combination of the two.

• List of AS-external-LSAs

  Part of the Link-State database. These have originated from the AS boundary routers. They comprise routes to destinations external to the AS.

• The routing table

  Derived from the LS-database. Each entry in the routing table is indexed by a destination, and contains the destination's cost and a set of paths to use in forwarding packets to the destination. A path is described by its type and next hop.

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