246 CHAPTER 16: IP R OUTING P ROTOCOL OPERATIONgenerated on the ABR, even though the default route 0.0.0.0 is not in the routingtable. On an ASBR, however, the default type-7 LSA route can be generated only ifthe default route 0.0.0.0 is in the routing table.Executing the no-import-route command on the ASBR prevents the external routesthat OSPF imported through the import-route command from advertising to theNSSA. Generally, if an NSSA router is both ASBR and ABR, this argument is used.The default-cost command is used on the ABR attached to the NSSA. Using thiscommand, you can configure the default route cost on the ABR to NSSA.By default, the NSSA is not configured, and the cost of the default route to the NSSAis 1.Configuring the Route Summarization of OSPF AreaRoute summary means that ABR can aggregate information of routes of the sameprefix and advertise only one route to other areas. An area can be configured withmultiple aggregate segments allowing OSPF can summarize them. When the ABRtransmits routing information to other areas, it will generate Sum_net_Lsa (type-3LSA) per network. If continuous networks exist in this area, you can use theabr-summary command to summarize these segments into one segment. Thus, theABR only needs to send an aggregate LSA, and all the LSAs in the range of theaggregate segment specified by the command are not transmitted separately.Once the aggregate segment of a certain network is added to the area, all theinternal routes of the IP addresses in the range of the aggregate segment will nolonger be separately advertised to other areas. Only the route summary of the wholeaggregate network will be advertised. But if the range of the segment is restricted bythe parameter not-advertise, the route summary of this segment is not advertised.This segment is represented by IP address and mask.Route summarization can take effect only when it is configured on ABRs.Perform the following configuration in OSPF Area View.By default, the inter-area routes will not be summarized.Configuring Summarization of Imported Routes by OSPFPerform the following configurations in OSPF View.By default, summarization of imported routes is disabled.Table 236 Configuring the route summarization of OSPF areaOperation CommandConfigure the Route Summarizationof OSPF Areaabr-summary ip_address mask [ advertise |not-advertise ]Cancel the route summarization ofOSPF Areaundo abr-summary ip_address maskTable 237 Configuring summarization of imported routes by OSPFOperation CommandConfigure summarization ofimported routes by OSPFasbr-summary ip_address mask [ not-advertise |tag value ]Remove summarization ofroutes imported into OSPFundo asbr-summary ip_address mask
