Determining the Number of Areas per ABR

ABRs keep a copy of the database for all areas that they service. For example, if a router is connected to five areas, it must keep five different databases. It is better not to overload an ABR; rather, you should spread the areas over other routers. The ideal design is to have each ABR connected to two areas only—the backbone and another area—with three to five areas being the upper limit. Figure 4-19 shows the difference between one ABR holding five different databases, including area 0 (part a), and two ABRs holding three databases each (part b).

These are just guidelines; the more areas you attach per ABR, the lower the performance you get from that router. This is a simple case of resource management. In some cases, network administrators will accept the lower performance; but usually, end users won't see it that way. As you do this, remember to check your routers' memory and CPU utilization, as there is a point in which every router ceases to function properly if asked to do too much.

Figure 4-19 How Many Areas Should Be Connected per ABR?

194 Chapter 4: Design Fundamentals

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