Figure 1 shows an autonomoussystem (AS) across which many external routes are advertised. If externalroutes make up a significant portion of a topology database, you cansuppress the advertisements in areas that do not have links outsidethe network. By doing so, you can reduce the amount of memory thenodes use to maintain the topology database and free it for otheruses.
Figure 1: OSPF AS Network with StubAreas and NSSAs
To control the advertisement of external routes into an area,OSPF uses stub areas. By designating an area border router (ABR) interfaceto the area as a stub interface, you suppress external route advertisementsthrough the ABR. Instead, the ABR advertises a default route (throughitself) in place of the external routes and generates network summary(Type 3) link-state advertisements (LSAs). Packets destined for externalroutes are automatically sent to the ABR, which acts as a gatewayfor outbound traffic and routes the traffic appropriately.
 | Note: You must explicitly configure the ABR to generate a defaultroute when attached to a stub or not-so-stubby-area (NSSA). To injecta default route with a specified metric value into the area, you mustconfigure the default-metric option and specify a metricvalue. |
For example, area 0.0.0.3 in Figure 1 is not directly connected tothe outside network. All outbound traffic is routed through the ABRto the backbone and then to the destination addresses. By designatingarea 0.0.0.3 as a stub area, you reduce the size of the topologydatabase for that area by limiting the route entries to only thoseroutes internal to the area.
A stub area that only allows routes internal to the area andrestricts Type 3 LSAs from entering the stub area is often calleda totally stubby area. You can convert area 0.0.0.3to a totally stubby area by configuring the ABR to only advertiseand allow the default route to enter into the area. External routesand destinations to other areas are no longer summarized or allowedinto a totally stubby area.
| Note: If you incorrectly configure a totally stubby area, youmight encounter network connectivity issues. You should have advancedknowledge of OSPF and understand your network environment before configuringtotally stubby areas. |
Similar to area 0.0.0.3 in Figure 1, area 0.0.0.4 has no external connections. However, area0.0.0.4 has static customer routes that are not internal OSPF routes.You can limit the external route advertisements to the area and advertisethe static customer routes by designating the area an NSSA. In anNSSA, the AS boundary router generates NSSA external (Type 7) LSAsand floods them into the NSSA, where they are contained. Type 7 LSAsallow an NSSA to support the presence of AS boundary routers and theircorresponding external routing information. The ABR converts Type7 LSAs into AS external (Type 5 ) LSAs and leaks them to the otherareas, but external routes from other areas are not advertised withinthe NSSA.
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