OSPF design - when to create a new area
When designing an OSPF topology, it is important to understand under what circumstances a new area should be created. OSPF uses the concept of a backbone area which must exist on all OSPF topologies. However, how should you determine if and when a new area should be created? There are several factors that can help in this design.
One reason you would want to create a new area in OSPF is if you have too many routes being exchanged in area 0 (on the order of hundreds or even thousands) and your routing tables and your OSPF databases are getting too large. How large is too large? Well, if they begin slowing down your routers and you see a lot of memory usage and slow reconvergence times that are affecting the performance of OSPF on your network, then you know that it’s time to separate the network into multiple areas. Indeed, you should do this well before you perceive any impact on the functionality of your network.
But there are other reasons to separate your network into areas, and if you follow these, it is unlikely that you will reach the threshold of OSPF resource usage. Typically, the architecture of a network naturally separates it into large sections, and you can assign a different area to each section.
For example, a medium-sized enterprise will have a large headquarters, several regional locations, and a series of branch offices. If the networks of those locations are interconnecting using MPLS or some other WAN technology, it makes sense for each physical location to be assigned its own area, and the headquarters can be assigned the backbone, which would also typically be the largest area. This way, you minimize OSPF update sizes being sent over the WAN.
For larger enterprises, it may even be beneficial to separate a particular campus into several areas, where each building is an OSPF area. What you ultimately choose to do depends much more on the hierarchical structure of the sections of a network rather than just the number of routers within an area.
Just for argument’s sake, how many routers are too many? A general rule of thumb for OSPF is 50 routers in an area, but this is not a hard and fast rule. It depends on many more factors such as the number of routes, the CPU and memory power of the routers used, as well as the IP addressing scheme being used. But again, separating your areas according to the natural network design you are using is your best bet to ensure you don’t run into “too many” routers or routes being exchanged in a single area.