4-4sends a hello packet through the OSPF interface, and the router that receives the hello packet checksparameters carried in the packet. If parameters of the two routers match, they become neighbors.Adjacency: A relationship formed between selected neighboring routers for the purpose of exchangingrouting information. Not every pair of neighboring routers become adjacent, which depends on networktypes. Only by synchronizing the LSDB via exchanging DD packets and LSAs can two routers becomeadjacent.OSPF Area Partition and Route SummarizationArea partitionIf all the routers on an ever-growing large network run OSPF, the large number of routers will result in anenormous LSDB, which will consume an enormous storage space, complicate the running of SPFalgorithm, and increase CPU load.Furthermore, as a network grows larger, it is more likely to have changes in the network topology.Hence, the network will often be “flapping”, and a great number of OSPF packets will be generated andtransmitted in the network. This will lower the network bandwidth utilization. Even worse, any change ofthe topology will cause all the routers on the network to re-perform route calculation.OSPF solves the above-mentioned problem by dividing an AS into multiple areas. Areas refer to groupsinto which routers are logically divided. Each group is identified by an Area ID, as shown in Figure 4-1.Figure 4-1 OSPF area partitionOn the border of an area is a router, which belongs to different areas. After area partition, area borderrouters perform route summarization to reduce the number of LSAs advertised to other areas andminimize the effect of topology changes.Classification of routersThe OSPF router falls into four types according to the position in the AS:1) Internal routerAll interfaces on an internal router belong to one OSPF area.
