Stub Area Configuration

Stub Area Configuration
Because the main purpose of having stub areas is to keep such areas from carrying external routes,
we need to review some design guidelines before configuring a stub area or a totally stubby area:
 Area 0 (the backbone area) cannot be made a stub area.
 More than one area must exist.
Stub Area Configuration 185
 Because autonomous system boundary routers inject external routes, do not make any area
containing an ASBR a stub area. (However, see the discussion of NSSAs in this chapter.)
 Because routers within a stub area use a default route to get out of the stub area, typically
there is only one route out of the stub area. Therefore, a stub area should usually contain
only a single area border router. Keep in mind that since a default route is being used, if a
stub area contains more than one ABR, a non-optimal path may be used.
 If you decide to make a particular area a stub area, be sure to configure all the routers in
the area as stubby. If a router within a stub area has not been configured as stubby, it will
not be able to correctly form adjacencies and exchange OSPF routes.
With these guidelines in mind, let’s examine a sample configuration for a stub area. Consider
the network shown in Figure 6.5. We’re going to make Area 25 a stub area. In this example, we
won’t be concerned with the configuration of RouterA, because it does not participate in Area 25.
We will then examine the syntax for RouterB, RouterC, and RouterD.