Now imagine a network with 900 routers, instead of only 11, and several thousand subnets. In that size of network, the sheer amount of processing required to run the complex SPF algorithm might cause convergence time to be slow just because of the time it takes each router to process all the math. Also, the routers might experience memory shortages. The problems can be summarized as follows:

■ A larger topology database requires more memory on each router.

■ The router CPU time required to run the SPF algorithm grows exponentially with the size of the LSDB.

■ With a single area, a single interface status change (up to down, or down to up) forces every router to run SPF again! OSPF provides a way to manage the size of the LSDB by breaking up a larger network into smaller pieces using a concept called OSPF areas.

OSPF then creates a separate and smaller LSDB per area, rather than one huge LSDB for all links and routers in the internetwork. With smaller topology databases, routers consume less memory and take less processing time to run SPF.

OSPF multi-area design puts all ends of a link—a serial link, and VLAN, and so on—inside an area. To make that work, some routers (Area Border Routers, or ABRs) sit at the border between multiple areas. Routers D1 and D2 serve as ABRs in the area design shown in Figure 20-16, which shows the same network as Figure 20-15, but with three OSPF areas (0, 1, and 2).

Figure 20-16 shows a sample area design and some terminology related to areas, but it does not show the power and benefit of the areas. By using areas, the OSPF SPF algorithm ignores the details of the topology in the other areas. For example, OSPF on router B1 (area 1), when doing the complex SPF math processing, ignores the topology information about area 0 and area 2. Each router has much less SPF work to do, so each router more quickly finishes its SPF work, finding the currently best OSPF routes.

	Created by Isaac Farias about 8 years ago

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Chapter 20 IPv4 Routing Protocols