6. The Area Data Structure

The area data structure contains all the information used to run the basic routing algorithm. Each area maintains its own topological 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 has all the properties of an area. For that reason it is also represented by an area data structure. Note that some items in the structure apply differently to the backbone than to non-backbone areas.

                              +----+
                              |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

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

Area ID

A 32-bit number identifying the area. 0.0.0.0 is reserved for the Area ID of the backbone. If assigning subnetted networks as separate areas, the IP network number could be used as the Area ID.

List of component address ranges

The address ranges that define the area. Each address range is specified by an [address,mask] pair and a status indication of either Advertise or DoNotAdvertise (see Section 12.4.3). Each network is then assigned to an area depending on the address range that it falls into (specified address ranges are not allowed to overlap). As an example, if an IP subnetted network is to be its own separate OSPF area, the area is defined to consist of a single address range - an IP network number with its natural (class A, B or C) mask.

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 structure 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 links advertisements

A router links advertisement is generated by each router in the area. It describes the state of the router's interfaces to the area.

List of network links advertisements

One network links advertisement is generated for each transit multi-access network in the area. A network links advertisement describes the set of routers currently connected to the network.

List of summary link advertisements

Summary link advertisements originate from the area's area border routers. They describe routes to destinations internal to the Autonomous System, yet external to the area.

Shortest-path tree

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

AuType

The type of authentication used for this area. Authentication types are defined in Appendix D. All OSPF packet exchanges are authenticated. Different authentication schemes may be used in different areas.

TransitCapability

Set to TRUE if and only if there are one or more active virtual links using the area as a Transit area. Equivalently, 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, and is used as an input to a subsequent step of the routing table build process (see Section 16.3).

ExternalRoutingCapability

Whether AS external advertisements will be flooded into/throughout the area. This is a configurable parameter. If AS external advertisements are excluded from the area, the area is called a "stub". Internal to 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 link that the router should advertise into the area. There can be a separate cost configured for each IP TOS. See Section 12.4.3 for more information.

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