3Com 3C13636 Configuration Manual page 598

3Com Router 3000 Ethernet Family
Configuration Guide
If a DR becomes invalid due to some fault, it must be reelected and synchronized. It
takes time and meanwhile the route calculation is incorrect. In order to speed up this
process, OSPF puts forward the concept of BDR. In fact, BDR is a backup for DR. DR
and BDR are elected in the mean time. The adjacencies are also established between
the BDR and all the routers on the segment, and routing information is also exchanged
between them. Once a DR becomes invalid, the BDR will immediately turn into a DR,
and a new BDR will be reelected.
III. Area
When a large number of OSPF routers are present on a network, LSDBs may become
so large that a great amount of storage room is occupied and CPU resources are
exhausted performing SPF computation. In addition, as the topology of a large network
is prone to changes, enormous amount of OSPF packets may be created, decreasing
bandwidth utilization.
To solve this problem, OSPF splits an AS into multiple areas, which are identified by
area IDs. In logical sense, the areas put routers into different groups. The area 0, also
known as backbone area, is of the most significance.
The backbone area achieves routing information exchange between the non-backbone
areas. The backbone area and other non-backbone areas must be physically
consecutive. Otherwise, you have to configure virtual links to make them consecutive.
The router connecting the backbone area and another area is called area border router
(ABR).
There also exists the autonomous system boundary router (ASBR) in OSPF, which
connects OSPF routing domain and the router running other routing protocols can be
taken as the router redistributing OSPF external routing information.
IV. Route aggregation
An AS is divided into areas that are interconnected via OSPF ABRs. The routing
information between areas can be reduced through route aggregation. 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 will look up the routing table
and encapsulate each OSPF route into an LSA and send it outside the area.
For example, in Figure 4-1, there are three intra-area routes in Area 19, 19.1.1.0/24,
19.1.2.0/24 and 19.1.3.0/24. If route aggregation is configured and the three routes are
aggregated into one route 19.1.0.0/16, only one LSA, which describes the route after
aggregation, is generated on RTA.
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Chapter 4 OSPF Configuration