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1.6.- Stub Areas  
OSPF allows certain areas to be configured as stub areas. AS-external-LSAs are not flooded into stub areas; routing to AS external networks is based on a single per-area default route. This reduces the LS-database size and memory requirements for routers within stub areas.
An area can be configured as a stub when there is a single exit point from the area, or when the choice of exit point need not be made on a per-external-destination basis. For example, in our example, Area 3 could be configured as a stub area because all external traffic must travel through router RT11. If it would be the case, RT11 would advertise a default route for distribution inside Area 3, instead of flooding AS-external-LSAs to networks N12-N15 into the area.

Some important points:
  1. OSPF ensures that all routers belonging to an area agree on whether the area has been configured as a stub.
  2. Virtual links cannot be configured as a stub.
  3. AS boundary routers cannot be placed on stub areas.
  

In order to take advantage of the stub area support, default routing must be used in the stub area. Then one stub area border router (normally it should be just one) must advertise a default route into the stub area via a summary-LSA. This summary default route should be used for any destination that is not explicity reachable by an intra-area or inter-area path.
 

 
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