OSPF is sometimes described as being partially link-state and partially distance vector, this is due to the fact that SPF calculations are only perfomed for intra-Area routing, while the ABR's advertised information is trusted for Inter-Area and External calculations.
SPF calculation is only done within the area not outside so a router for example in area 3 is connected to an ABR (ABR has an interface in another area say area 0 for example) does not need to know the detailed information about the path selection that is occuring behind the ABR so the router does not need to run SPF on the entire topology, which increases scalability, it only runs SPF only for rea 3. The router only need to know how to get to the ABR and that's it by calculating tje shortest Intra-Area path to reach ABR (Forward metric)
Does that make sense?
Francisco.
