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10. OSPF Special Areas

1. Technical background

Early routers relied on CPU calculations for forwarding. Due to hardware technology limitations, resources were not particularly sufficient. Therefore, in order to conserve resource usage, planning was very necessary.

OSPF routers need to maintain intra-domain routing, inter-domain routing, and external routing information databases at the same time. As the network scale continues to expand, the LSDB scale will also continue to grow. OSPF can reduce the scale of the LSDB of routers in the area by dividing the area. OSPF is divided into backbone areas and non-backbone areas.

Dividing the area does effectively reduce the number of LSA entries, but some routers in non-backbone areas will still receive a large number of Type 3, Type 4, and Type 5 LSAs.

The routers in the backbone area of ​​OSPF generally have stronger performance and can carry and maintain more LSDBs, but some routers in non-backbone areas may not be able to carry too large LSDBs, which is still unbearable for low-end routers in non-backbone areas. In addition, some routers in non-backbone areas do not need to provide traffic transfer for other areas, so the routers in this area do not need to maintain LSDBs outside this area. Therefore, in order to further reduce the scale of router LSDBs and the number of LSAs, we can introduce two special areas, Stub and Nssa areas, to further reduce the number of LSAs and the scale of routing tables.

Three types of OSPF traffic

① Intra-domain traffic: traffic consisting of data packets exchanged between routers within a single area

② Inter-domain traffic: traffic consisting of data packets exchanged between routers in different regions

③ External traffic: traffic consisting of data packets exchanged between routers within the OSPF domain and routers outside the OSPF area or in another autonomous system

2. Overview of Special Areas

In an OSPF network, the Stub area, Totally-stub area, NSSA area, and Totally-NSSA area are special area types.Used to implement specific routing aggregation and transmission strategies in the networkThey can helpSimplify routing tables, reduce network overhead, and improve network stability，And adapt to different network designs and requirements。

Precautions：Only non-backbone areas can be configured as special areas，The backbone area cannot be configured as a special areaEven if it is configured, it will not take effect because the backbone area connects multiple areas and, as a transit station for LSA, it must transmit LSA.

main effect

① Reduce the number of special area LSAs