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Chapter 5

Interconnecting OSPF Areas

The syntax for RouterB is similar to that used for RouterA. The primary

difference is that RouterB is connected to two areas:

RouterB(config)#router ospf 70

RouterB(config-router)#network 1.1.2.0 0.0.0.255 area 0

RouterB(config-router)#network 1.1.3.0 0.0.0.255 area 1

The syntax for RouterC is very similar to that of RouterA. The difference

is that RouterA is internal to Area 0, thereby classifying it as a backbone
router:

RouterC(config)#router ospf 70

RouterC(config-router)#network 1.1.3.0 0.0.0.255 area 1

RouterC(config-router)#network 1.1.4.0 0.0.0.255 area 1

Stub Area Configuration

ince the main purpose of having stub areas (and totally stubby 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:

Do not make the backbone area (Area 0) a stub area.

Since external routes are injected by autonomous system boundary
routers, do not make any area containing an ASBR a stub area.

Since 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. There-
fore, a stub area should usually only contain 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 5.4. We're going to make
Area 25 a stub area. In this example, we won't be concerned with the con-
figuration of RouterA, since it does not participate in Area 25. We will then
examine the syntax for RouterB, RouterC, and RouterD.

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