13.2. Installing link state advertisements in the database

Installing a new link state advertisement in the database, either as the result of flooding or a newly self-originated advertisement, may cause the OSPF routing table structure to be recalculated. The contents of the new advertisement should be compared to the old instance, if present. If there is no difference, there is no need to recalculate the routing table. (Note that even if the contents are the same, the LS checksum will probably be different, since the checksum covers the LS sequence number.)

If the contents are different, the following pieces of the routing table must be recalculated, depending on the new advertisement's LS type field:

Router links and network links advertisements

The entire routing table must be recalculated, starting with the shortest path calculations for each area (not just the area whose topological database has changed). The reason that the shortest path calculation cannot be restricted to the single changed area has to do with the fact that AS boundary routers may belong to multiple areas. A change in the area currently providing the best route may force the router to use an intra-area route provided by a different area.[16]

Summary link advertisements

The best route to the destination described by the summary link advertisement must be recalculated (see Section 16.5). If this destination is an AS boundary router, it may also be necessary to re-examine all the AS external link advertisements.

AS external link advertisements

The best route to the destination described by the AS external link advertisement must be recalculated (see Section 16.6).

Also, any old instance of the advertisement must be removed from the database when the new advertisement is installed. This old instance must also be removed from all neighbors' Link state retransmission lists (see Section 10).