5.1 Creating Multiple OSPF Areas
5.1.2 OSPF hierarchical routing
OSPF's capability to separate a large internetwork into multiple areas is referred to as hierarchical routing. Hierarchical routing enables you to separate large internetworks (autonomous system) into smaller internetworks that are called areas. With this technique, routing still occurs between the areas (called inter-area routing), but many of the smaller internal routing operations, such as recalculating the database, are restricted within an area. For example, if area 1 shown in the main figure is having problems with a link going up and down, routers in other areas do not need to run their Shortest Path First (SPF) calculation, since they are isolated from the problems in area 1.
Lab Activity
  In this lab, you will learn how to enable OSPF routing protocol for Multiple areas.

The hierarchical topology possibilities of OSPF have several important advantages:

  • Reduced frequency of SPF calculations --- Because detailed route information is kept within each area, it is not necessary to flood all link-state changes to all other areas. Thus, only those routers affected by a change need to run the SPF calculation.
  • Smaller routing tables --- When using multiple areas, detailed route entries for specific networks within an area are kept inside the area. Rather than advertising these explicit routes outside the area, you can have the routes summarized into one or more summary routes. Advertising these summaries reduces the amount of LSAs propagated between areas, but allows all networks to remain reachable.
  • Reduced Link State Update (LSU) overhead --- LSUs can contain a variety of LSA types, including link-state information and summary information. Rather than send an LSU about each network to every area, you can advertise a single route or a few summarized routes between areas to reduce the overhead associated with link-state updates that cross multiple areas.
Lab Activity
  In this lab, you will learn how to configure OSPF to calculate the correct cost of an interface.