Current network size and topology
As stated earlier static routing works well on small networks. At those networks get larger, routing takes longer, routing tables get very large, and general performance isn’t what it could be.
Topology is a concern as well. If all your computers are in one building, its much easier to stay with static routing longer. However, connecting a number of locations will be easier with the move to dynamic routing.
If you have a network of 20 computers, you can still likely use static routing. If those computers are in two or three locations, static routing will still be a good choice for connecting them. Also, if you just connect to your ISP and don’t worry about any special routing to do that, you are likely safe with just static routing.
If you have a network of 100 computers in one location, you can use static routing but it will be getting slower, more complex, and there won’t be much room for expansion. If those 100 computers are spread across three or more locations, dynamic routing is the way to go.
If you have 1000 computers, you definitely need to use dynamic routing no matter how many locations you have. Hopefully this section has given you an idea of what results you will likely experience from different sized networks using different routing protocols. Your choice of which dynamic routing protocol to use is partly determined by the network size, and topology.
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.