12.2. The link state database

A router has a separate link state database for every area to which it belongs. The link state database has been referred to elsewhere in the text as the topological database. All routers belonging to the same area have identical topological databases for the area.

The databases for each individual area are always dealt with separately. The shortest path calculation is performed separately for each area (see Section 16). Components of the area topological database are flooded throughout the area only. Finally, when an adjacency (belonging to Area A) is being brought up, only the database for Area A is synchronized between the two routers.

The area database is composed of router links advertisements, network links advertisements, and summary link advertisements (all listed in the area data structure). In addition, external routes (AS external advertisements) are included in all non-stub area databases (see Section 3.6).

An implementation of OSPF must be able to access individual pieces of an area database. This lookup function is based on an advertisement's LS type, Link State ID and Advertising Router.[13] There will be a single instance (the most up-to- date) of each link state advertisement in the database. The database lookup function is invoked during the link state flooding procedure (Section 13) and the routing table calculation (Section 16). In addition, using this lookup function the router can determine whether it has itself ever originated a particular link state advertisement, and if so, with what LS sequence number.

A link state advertisement is added to a router's database when either a) it is received during the flooding process (Section 13) or b) it is originated by the router itself (Section 12.4). A link state advertisement is deleted from a router's database when either a) it has been overwritten by a newer instance during the flooding process (Section 13) or b) the router originates a newer instance of one of its self-originated advertisements (Section 12.4) or c) the advertisement ages out and is flushed from the routing domain (Section 14). Whenever a link state advertisement is deleted from the database it must also be removed from all neighbors' Link state retransmission lists (see Section 10).