6. The Area Data Structure

6. The Area Data Structure

   The area data structure contains all the information used to run the basic OSPF routing algorithm. Each area maintains its own link-state database. A network belongs to a single area, and a router interface connects to a single area. Each router adjacency also belongs to a single area.

   The OSPF backbone is the special OSPF area responsible for disseminating inter-area routing information.

   The area link-state database consists of the collection of router- LSAs, network-LSAs and summary-LSAs that have originated from the area's routers. This information is flooded throughout a single area only. The list of AS-external-LSAs (see Section 5) is also considered to be part of each area's link-state database.

   Area ID A 32-bit number identifying the area. The Area ID of 0.0.0.0 is reserved for the backbone.

   List of area address ranges In order to aggregate routing information at area boundaries, area address ranges can be employed. Each address range is specified by an [address,mask] pair and a status indication of either Advertise or DoNotAdvertise (see Section 12.4.3).

   +----+ |RT10|------+ +----+ +-------------+ / \ |Routing Table| / \ +-------------+ /
   +------+ / \ +--------+ |Area 2|---+ +---|Backbone| +------+***********+ +--------+ / \ * /
   / \ * /
   +---------+ +---------+ +------------+ +------------+ |Interface| |Interface| |Virtual Link| |Interface Ib| | to N6 | | to N8 | | to RT11 | +------------+ +---------+ +---------+ +------------+ | / \ | | | / \ | | | +--------+ +--------+ | +-------------+ +------------+ |Neighbor| |Neighbor| | |Neighbor RT11| |Neighbor RT6| | RT8 | | RT7 | | +-------------+ +------------+ +--------+ +--------+ | | +-------------+ |Neighbor RT11| +-------------+

   Figure 9: Router RT10's Data structures

   Associated router interfaces This router's interfaces connecting to the area. A router interface belongs to one and only one area (or the backbone). For the backbone area this list includes all the virtual links. A virtual link is identified by the Router ID of its other endpoint; its cost is the cost of the shortest intra-area path through the Transit area that exists between the two routers.

   List of router-LSAs A router-LSA is generated by each router in the area. It describes the state of the router's interfaces to the area.

   List of network-LSAs One network-LSA is generated for each transit broadcast and NBMA network in the area. A network-LSA describes the set of routers currently connected to the network.

   List of summary-LSAs Summary-LSAs originate from the area's area border routers. They describe routes to destinations internal to the Autonomous System, yet external to the area (i.e., inter-area destinations).

   Shortest-path tree The shortest-path tree for the area, with this router itself as root. Derived from the collected router-LSAs and network-LSAs by the Dijkstra algorithm (see Section 16.1).

   TransitCapability This parameter indicates whether the area can carry data traffic that neither originates nor terminates in the area itself. This parameter is calculated when the area's shortest-path tree is built (see Section 16.1, where TransitCapability is set to TRUE if and only if there are one or more fully adjacent virtual links using the area as Transit area), and is used as an input to a subsequent step of the routing table build process (see Section 16.3). When an area's TransitCapability is set to TRUE, the area is said to be a "transit area".

   ExternalRoutingCapability Whether AS-external-LSAs will be flooded into/throughout the area. This is a configurable parameter. If AS-external-LSAs are excluded from the area, the area is called a "stub". Within stub areas, routing to AS external destinations will be based solely on a default summary route. The backbone cannot be configured as a stub area. Also, virtual links cannot be configured through stub areas. For more information, see Section 3.6.

   StubDefaultCost If the area has been configured as a stub area, and the router itself is an area border router, then the StubDefaultCost indicates the cost of the default summary-LSA that the router should advertise into the area. See Section 12.4.3 for more information.

   Unless otherwise specified, the remaining sections of this document refer to the operation of the OSPF protocol within a single area.