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This chapter is from the book


In this chapter, you learned about establishing OSPF neighbor relationships, building the OSPF link-state database, optimizing OSPF behavior, configuring OSPFv2 and OSPFv3. Some key points in this chapter are:

  • OSPF uses a two-layer hierarchical approach dividing networks into a backbone area (area 0) and nonbackbone areas.
  • For its operation, OSPF uses five packet types: Hello, DBD, LSR, LSU, and LSAck.
  • OSPF neighbors go through several different neighbor states before adjacency results in Full state.
  • OSPF elects DR/BDR routers on a multiaccess segment to optimize exchange of information.
  • The most common OSPF network types are point-to-point, broadcast, nonbroadcast, and loopback.
  • OSPF uses several different LSA types to describe the network topology.
  • LSAs are stored in an LSDB, which is synchronized with every network change.
  • OSPF calculates interface costs based on default reference bandwidth and interface bandwidth.
  • Using SPF, OSPF determines the total lowest cost paths and selects them as the best routes.
  • Intra-area routes are always preferred over interarea routes.
  • Route summarization improves CPU utilization, reduces LSA flooding, and reduces routing table sizes.
  • The area range command is used summarize at the ABR. The summary-address command is sued to summarize at the ASBR.
  • Default routes can be used in OSPF to prevent the need for specific route to each destination network.
  • OSPF uses the default-information originate command to inject a default route.
  • There are several OSPF area types: normal, backbone, stub, totally stubby, NSSA, and totally stubby NSSA.
  • Use the area area-id command to define an area as stubby.
  • Use the area area-id stub command with the no-summary keyword only on the ABR to define an area as totally stubby.
  • For stub areas, external routes are not visible in the routing table, but are accessible via the intra-area default route.
  • For totally stubby areas, interarea and external routes are not visible in the routing table, but are accessible via the intra-area default route.
  • OSPFv3 for IPv6 supports the same basic mechanisms that OSPFv2 for IPv4, including the use of areas to provide network segmentation and LSAs to exchange routing updates.
  • OSPFv3 features two new LSA types and has renamed two traditional LSA types.
  • OSPFv3 uses link-local addresses to source LSAs.
  • OSPFv3 is enabled per-interface on Cisco routers.
  • New-style OSPFv3 and traditional OSPFv3 for IPv6, configured with ipv6 router ospf, can coexist in the network to provide IPv6 routing.
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