Key Points about OSPF – Routing Protocol

Some key points 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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