4-8Figure 4-6 Route summarizationOSPF has two types of route summarization:2) ABR route summarizationTo distribute routing information to other areas, an ABR generates Type-3 LSAs on a per networksegment basis for an attached non-backbone area. If contiguous network segments are available in thearea, you can summarize them with a single network segment. The ABR in the area distributes only thesummary LSA to reduce the scale of LSDBs on routers in other areas.3) ASBR route summarizationIf summarization for redistributed routes is configured on an ASBR, it will summarize redistributedType-5 LSAs that fall into the specified address range. If in an NSSA area, it also summarizes Type-7LSAs that fall into the specified address range.If this feature is configured on an ABR, the ABR will summarize Type-5 LSAs translated from Type-7LSAs.Route typesOSPF prioritizes routes into four levels:z Intra-area routez Inter-area routez type1 external routez type2 external routeThe intra-area and inter-area routes describe the network topology of the AS, while external routesdescribe routes to destinations outside the AS. OSPF classifies external routes into two types: type1and type2.A type1 external route is an IGP route, such as a RIP or static route, which has high credibility andwhose cost is comparable with the cost of an OSPF internal route. The cost from a router to thedestination of the type1 external route= the cost from the router to the corresponding ASBR+ the costfrom the ASBR to the destination of the external route.A type2 external route is an EGP route, which has low credibility, so OSPF considers the cost from theASBR to the destination of the type2 external route is much bigger than the cost from the ASBR to anOSPF internal router. Therefore, the cost from the internal router to the destination of the type2 externalroute = the cost from the ASBR to the destination of the type2 external route. If two routes to the samedestination have the same cost, then take the cost from the router to the ASBR into consideration.OSPF Network TypeFour OSPF network typesOSPF divides networks into four types by link layer protocols:z Broadcast: If Ethernet or FDDI is adopted, OSPF defaults the network type to broadcast. In abroadcast network, protocol packets are sent in multicast (224.0.0.5 and 224.0.0.6) by default.
