3Com Switch 8800 Configuration Guide Chapter 19 OSPF Configuration19-25Table 19-28 Configure stub area of OSPFOperation CommandConfigure an area to be the stub area stub [ no-summary ]Remove the configured stub area undo stubConfigure the cost of the default route transmitted byOSPF to the stub area default-cost valueRemove the cost of the default route to the stub area undo default-costBy default, the stub area is not configured, and the cost of the default route to the stubarea is 1.19.2.22 Configuring NSSA Area of OSPFRFC1587 introduced a new type of area called NSSA area, and a new type of LSAcalled NSSA LSA (or Type-7 LSA).NSSA areas are virtually variations of Stub areas. They are similar in many ways.Neither of them generates or imports AS-External-LSA (namely Type-5 LSA), and bothof them can generate and import Type-7 LSA. Type-7 LSA is generated by ASBR ofNSSA area, which can only be advertised in NSSA area. When Type-7 LSA reachesABR of NSSA, ABR will select whether to transform Type-7 LSA into AS-External-LSAso as to advertise to other areas.For example, in the network below, the AS running OSPF comprises three areas: Area1, Area 2 and Area 0. Among them, Area 0 is the backbone area. Also, there are othertwo ASs respectively running RIP. Area 1 is defined as an NSSA area. After RIP routesof the Area 1 are propagated to the NSSA ASBR, the NSSA ASBR will generate type-7LSAs which will be propagated in Area 1. When the type-7 LSAs reach the NSSA ABR,the NSSA ABR will transform it into type-5 LSA, which will be propagated to Area 0 andArea 2. On the other hand, RIP routes of the AS running RIP will be transformed intotype-5 LSAs that will be propagated in the OSPF AS. However, the type-5 LSAs will notreach Area 1 because Area 1 is an NSSA. NSSAs and stub areas have the sameapproach in this aspect.Similar to a stub area, the NSSA cannot be configured with virtual links.area 0area 1NSSAarea 2RIPRIPNSSAASBRNSSAABRarea 0area 1NSSAarea 2RIPRIPNSSAASBRNSSAABRFigure 19-2 NSSA area
