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C

HAPTER

 31: OSPF C

ONFIGURATION

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Backup Designated Router (BDR)

If the DR fails for some faults, a new DR must be elected and synchronized with 
other routers on the segment. This process will take a relatively long time, during 
which, the route calculation is incorrect. To shorten the process, BDR is brought 
forth in OSPF. In fact, BDR is a backup for DR. DR and BDR are elected in the 
meantime. The adjacencies are also established between the BDR and all the 
routers on the segment, and routing information is also exchanged between 
them. After the existing DR fails, the BDR will become a DR immediately.

The network size grows increasingly larger. If all the routers on a huge network are 
running OSPF, the large number of routers will result in an enormous LSDB, which 
will consume an enormous storage space, complicate the SPF algorithm, and add 
the CPU load as well. Furthermore, as a network grows larger, the topology 
becomes more likely to take changes. Hence, the network will always be in 
"turbulence", and a great deal of OSPF packets will be generated and transmitted 
in the network. This will lower the network bandwidth utility. In addition, each 
change will cause all the routes on the network to recompute the route.

OSPF solves the above problem by partition an AS into different areas. Areas are 
logical groups of routers. The borders of areas are formed by routers. Thus, some 
routers may belong to different areas. A router connects the backbone area and a 
non-backbone area is called Area Border Router (ABR). An ABR can connect to the 
backbone area physically or logically.

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Backbone Area

After the area partition of OSPF, not all the areas are equal. In which, an area is 
different from all the other areas. Its area-id is 0 and it is usually called the 
backbone area.

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Virtual link

Since all the areas should be connected to the backbone area, virtual link is 
adopted so that the physically separated areas can still maintain the logic 
connectivity to the backbone area.

An AS is divided into different areas that are interconnected via OSPF ABRs. The 
routing information between areas can be reduced through route summary. Thus, 
the size of routing table can be reduced and the calculation speed of the router 
can be improved. After calculating an intra-area route of an area, the ABR 
summarizes multiple OSPF routes into an LSA and sends it outside the area 
according to the configuration of summary.

For example, as shown in Figure 73, the Area 19 has three area intra-area routes: 
19.1.1.0/24, 19.1.2.0/24 and 19.1.3.0/24. The three routes are summarized into 
one route 19.1.0.0/16 after you configured route summary. The RTA only 
generates an LSA, describing the summarized route.