There are a number of problems associated with using OSPF. These problems are related to the network size.
The larger the network, the greater the probability of a network change, which would require a recalculation
of the whole area. This increases the frequency with which the SPF algorithm is being run. In addition, each
recalculation will take longer. As the network grows, the size of the routing table will increase. Although the
complete routing table is not sent out as in a distance vector routing protocol, the greater the size of the table,
the longer each lookup becomes. The memory requirements on the router will also increase. Furthermore,
the topological database will increase in size and will eventually become unmanageable. As the various
databases increase in size and the calculations become increasingly frequent, the CPU utilization will
increase as more of the available memory I consumed. This will have a negative impact on network response
time, not because of congestion on the line but because of congestion within the router itself.
Using multiple OSPF areas solves most of the common problems with running OSPF in larger networks.
The division of a large single area network into multiple areas allows routers in each area to maintain their
own topological databases. This limits the size of the topological databases within an area, which results in
routers requiring less memory and processing time to run SPF, and a decrease in convergence time.
Summary and external links ensure connectivity between areas and networks outside the autonomous area
(AS). This is achieved by creating areas from groups of subnets. Each area is treated internally as a small
entity on its own. It communicates with the other areas, exchanging routing information which is kept to a
minimum by allowing only that information that is required for connectivity.
There are two approaches to implementing multiple area networks. The first approach is to grow a single
area until it becomes unmanageable. This approach requires less initial work and configuration but care
should be put into the design of the network because this may cause problems in the future, particularly in
addressing. The second approach is to design the network with multiple areas, which are very small, in
anticipation that the networks will grow to fit comfortably into their areas. In practice, many companies
convert their networks into OSPF from a distance vector routing protocol when they realize that they have
outgrown the existing routing protocol. This allows the planned implementation of the second approach.