Design your OSPF network with areas to make the network scalable and efficient. Areas have been discussed in previous chapters, but a brief review at this point is helpful. Keep areas simple and stubby, with less than 100 routers (optimally 40 to 50) and have maximum summarization for ease of routing. The network illustrated in Figure 4-7 demonstrates these suggestions.
NOTE Let me digress for a moment into the design debate surrounding the number of routers per area. Everyone has an opinion on this topic, and in actual implementation, you can go higher than the numbers I quote here. However, and I stress this point, you must take into consideration all the factors here to make this successful. You are now probably wondering if I will put a number on how far we can grow a single area and what I have experienced. Well, I am not going to do that, sorry. There are just too many variables to consider in a forum like this, but I will say that I have done a much higher number than Cisco recommends!
While I am also discussing highly opinionated topics, let me also mention that OSPF areas should also have a logical correlation to their placement, thus making the addressing and subnetting much easier to handle. One of the benefits here is that you can grow the areas much higher.
What does your network gain by adding stub areas? Stub areas summarize all external LSAs into one default route, which provides a path to external routes for all traffic inside the stub area. The stub ABR forwards LSAs for inter-area routes but not external routes and floods them to other area 0 routers. The stub ABR keeps the LSA database for the stub area with this additional information and the default external route. Figure 4-8 illustrates the operations in a stub area. An ASBR cannot be part of a stub area, and redistribution of routes from other protocols cannot take place in this area.
176 Chapter 4: Design Fundamentals
Figure 4-8 Stub Area Operation
Note that Area 1 is now a Totally Stubby Area (TSA).
Area Border Router (ABR)
OSPF Stub Area
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