5. Protocol Data Structures

The OSPF protocol is described in this specification in terms of its operation on various protocol data structures. The following list comprises the top-level OSPF data structures. Any initialization that needs to be done is noted. OSPF areas, interfaces and neighbors also have associated data structures that are described later in this specification.

Router ID

A 32-bit number that uniquely identifies this router in the AS. One possible implementation strategy would be to use the smallest IP interface address belonging to the router. If a router's OSPF Router ID is changed, the router's OSPF software should be restarted before the new Router ID takes effect. Before restarting in order to change its Router ID, the router should flush its self-originated link state advertisements from the routing domain (see Section 14.1), or they will persist for up to MaxAge minutes.

Area structures

Each one of the areas to which the router is connected has its own data structure. This data structure describes the working of the basic algorithm. Remember that each area runs a separate copy of the basic algorithm.

Backbone (area) structure

The basic algorithm operates on the backbone as if it were an area. For this reason the backbone is represented as an area structure.

Virtual links configured

The virtual links configured with this router as one endpoint. In order to have configured virtual links, the router itself must be an area border router. Virtual links are identified by the Router ID of the other endpoint -- which is another area border router. These two endpoint routers must be attached to a common area, called the virtual link's Transit area. Virtual links are part of the backbone, and behave as if they were unnumbered point-to-point networks between the two routers. A virtual link uses the intra-area routing of its Transit area to forward packets. Virtual links are brought up and down through the building of the shortest-path trees for the Transit area.

List of external routes

These are routes to destinations external to the Autonomous System, that have been gained either through direct experience with another routing protocol (such as EGP), or through configuration information, or through a combination of the two (e.g., dynamic external information to be advertised by OSPF with configured metric). Any router having these external routes is called an AS boundary router. These routes are advertised by the router into the OSPF routing domain via AS external link advertisements.

List of AS external link advertisements

Part of the topological database. These have originated from the AS boundary routers. They comprise routes to destinations external to the Autonomous System. Note that, if the router is itself an AS boundary router, some of these AS external link advertisements have been self-originated.

The routing table

Derived from the topological database. Each destination that the router can forward to is represented by a cost and a set of paths. A path is described by its type and next hop. For more information, see Section 11.

TOS capability

This item indicates whether the router will calculate separate routes based on TOS. This is a configurable parameter. For more information, see Sections 4.5 and 16.9.

Figure 9 shows the collection of data structures present in a typical router. The router pictured is RT10, from the map in Figure 6. Note that Router RT10 has a virtual link configured to Router RT11, with Area 2 as the link's Transit area. This is indicated by the dashed line in Figure 9. When the virtual link becomes active, through the building of the shortest path tree for Area 2, it becomes an interface to the backbone (see the two backbone interfaces depicted in Figure 9).