Hello Peter.
That’s an excellent question. 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.
You can determine the use of resources using the following two commands:
show process cpu - This will show how much CPU usage each process is taking. Unfortunately, it might not list OSPF directly, but it can give you some idea.
show process memory- This will give you an overview of the memory usage in the router. Again, it might not break down usage by process.
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 typically 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 technology, it makes sense for each physical location to be assigned its own area, and the headquarters can be assigned the backbone, which is also 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.
I hope this has been helpful!
Laz