Figure 56 shows an AS across which many external routes are advertised. If external routes make up a significant portion of a topology database, you can suppress the advertisements in areas that do not have links outside the network. By doing so, you can reduce the amount of memory the nodes use to maintain the topology database and free it for other uses.
Figure 56: OSPF AS Network with Stub Areas and NSSAs
To control the advertisement of external routes into an area, OSPF uses stub areas. By designating an area border router interface to the area as a stub interface, you suppress external route advertisements through the area border router. Instead, the area border router automatically advertises a default route (through itself) in place of the external routes. Packets destined for external routes are automatically sent to the area border router, which acts as a gateway for outbound traffic and routes them appropriately.
For example, area 0.0.0.3 in Figure 56 is not directly connected to the outside network. All outbound traffic is routed through the area border router to the backbone and then to the destination addresses. By designating area 0.0.0.3 a stub area, you reduce the size of the topology database for that area by limiting the route entries to only those routes internal to the area.
Like area 0.0.0.3 in Figure 56, area 0.0.0.4 has no external connections. However, area 0.0.0.4 has static customer routes that are not internal OSPF routes. You can limit the external route advertisements to the area and advertise the static customer routes by designating it a not-so-stubby area (NSSA). External routes are flooded into the NSSA and then leaked to the other areas, but external routes from other areas are not advertised within the NSSA.