Area Sizing

Determining the number of routers to deploy within each OSPF area is extremely important and should be done with flexibility in mind. Factors that are hard to know during design (such as which links will flap) can be compensated for with flexibility in your design and implementation.

During initial network convergence, OSPF uses the CPU-intensive SPF algorithm. Experience has shown that 40 to 50 routers per area is the optimal upper limit for OSPF in the majority of networks.

This is not to say that there are not larger and smaller areas running just fine. The point here is that 40 to 50 routers per area is the proper size for most networks and routers. Of course, a network full of 12000s can do things differently than a network composed of 2500s.

The number of calculations that must be performed by the router, given that n is the number of link-state packets (LSPs), is proportional to n log n. As a result, the larger the area, the greater the likelihood for performance problems associated with OSPF routing recalculation, and the more unstable the area becomes.

Generally speaking, an area should have less than 100 routers. That does not mean that networks with 100 or more routers in an area won't function, but why experiment with stability if you don't need to? Areas with unstable links should be even smaller to reduce the impact of those links.

NOTE If you have a stable network infrastructure, you can most likely run many more routers per area—assuming that the router specifications (for example, LSDB processing capacity) allow it. Additionally, a well-designed OSPF structure is essential in making areas with larger numbers of routers more viable. This chapter is designed to enable you to make your OSPF implementation scalable, allowing you to surpass the limit set by the less-than-100-router rule.

One of the main problems with OSPF areas is that network administrators let their backbone area grow too large. Some administrators erroneously believe that most routes should belong in area 0. Use the following guidelines for the best results:

• Outline the logical view of the network from the beginning.

• Focus on which routes, routers, or networks belong in what area.

• Start creating nonrequired areas before they are needed.

A good rule of thumb is to plan for maximum growth coupled with long-term planning at the beginning of the network design process. Has this point been stressed enough yet? This has the added benefit of ensuring your network can handle rapid growth. In this case, planning for too much is not a bad thing to do.

192 Chapter 4: Design Fundamentals

The OSPF network sizing recommendations shown in Table 4-1 are made in accordance with Cisco recommendations regarding OSPF networks. As stated earlier, studies and real world implementations have gone further. For example, the statistics in Table 4-1 came from the old "IETF OSPF Standard Report." Although, that report is many years old now and does not take into account today's carrier class routers and thus has been adjusted. These are not hard and fast numbers but are good indicators for you so you can plan accordingly and understand when you might be stretching things a bit.

Table 4-1 OSPF Network Size Recommendations





Routers per domain




Routers per area




Areas per domain




Neighbors per router




Areas per router




OSPF has been thoroughly tested and can withstand substantial scaling.

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