Chapter 2 Advanced Routing for FortiOS 5.0 : Dynamic Routing Overview : Comparison of dynamic routing protocols : When to adopt dynamic routing : Expected network growth
  
Expected network growth
You may not be sure if your current network is ready for dynamic routing. However, if you are expecting rapid growth in the near future, it is a good idea to start planning for that growth now so you are ready for the coming expansion.
Static routing is very labor intensive. Each network device’s routing table needs to be configured and maintained manually. If there is a large number of new computers being added to the network, they each need to have the static routing table configured and maintained. If devices are being moved around the network frequently, they must also be updated each time.
Instead, consider putting dynamic routing in place before those new computers are installed on the network. The installation issues can be worked out with a smaller and less complex network, and when those new computers or routers are added to the network there will be nowhere near the level of manual configuration required. Depending on the level of growth, this labor savings can be significant. For example, in an emergency you can drop a new router into a network or AS, wait for it to receive the routing updates from its neighbors, and then remove one of the neighbors. While the routes will not be the most effective possible, this method is much less work than static routing in the same situation, with less chance of mistakes.
Also, as your network grows and you add more routers, those new routers can help share the load in most dynamic routing configurations. For example if you have 4 OSPF routers and 20,000 external routes those few routers will be overwhelmed. But in a network with 15 OSPF routers they will better be able to handle that number of routes. Be aware though that adding more routers to your network will increase the amount of updates sent between the routers, which will use up a greater part of your bandwidth and use more bandwidth overall.