Summary
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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