Scaling OSPF Through Hierarchical Design
OSPF can be used in some networks with very little thought about design issues. You just turn on OSPF in all the routers, and it works. However, in large networks, engineers need to think about and plan how to use several OSPF features that allow it to scale well in larger networks.
* A larger topology database requires more memory on each router.
* Processing the larger-topology database with the SPF algorithm requires processing power that grows exponentially with the size of the topology database.
* A single interface status change (up to down or down to up) forces every router to run SPF again.
In networks with at least 50 routers and at least a few hundred subnets, engineers should use OSPF scalability features to reduce problems. Although these are major generalizations.