The algorithm in Section 12.4.3 is optional when Area A is an OSPF stub area. Area border routers connecting to a stub area can originate summary link advertisements into the area according to the above Section's algorithm, or can choose to originate only a subset of the advertisements, possibly under configuration control. The fewer advertisements originated, the smaller the stub area's link state database, further reducing the demands on its routers' resources. However, omitting advertisements may also lead to sub- optimal inter-area routing, although routing will continue to function.
As specified in Section 12.4.3, Type 4 link state advertisements (ASBR summary links) are never originated into stub areas.
In a stub area, instead of importing external routes each area border router originates a "default summary link" into the area. The Link State ID for the default summary link is set to DefaultDestination, and the metric set to the (per- area) configurable parameter StubDefaultCost. Note that StubDefaultCost need not be configured identically in all of the stub area's area border routers.