3Com 8807 Configuration Manual page 306

306 C 31: OSPF C
HAPTER ONFIGURATION
Backup Designated Router (BDR)
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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.
Area
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.
Backbone area and virtual link
Backbone Area
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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.
Virtual link
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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.
Route summary
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.