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cisco book download.19.1.1 Hierarchical design: areasTo provide scalability to very large networks, OSPF supports two important concepts: autonomous systems and zones. Autonomous Systems (AS) were discussed in Chapter 14. Within an AS, areas are used to provide hierarchical routing. An area is a set of adjacent networks; it is used to control how much routing information is shared between networks and under what circumstances. In a flat network design, such as one using IP RIP, if one router changes (possibly a rollover routing problem), it will affect all the routers in the entire network. In a properly designed hierarchical network, the impact of these changes is limited to a single area.Exam Essentials Don't forget that OSPF supports a hierarchical structure that contains two layers: the backbone (area 0 or 0.0.0.0) and the areas connected to the backbone.OSPF implements a hierarchical structure with two layers: the backbone and the areas connected to the backbone, as shown in Figure 19.1. This network contains the backbone and three regions connected to the backbone. Each region is assigned a unique number 32 bits long. Area numbers can be expressed in decimal (e.g., 1) or in dotted-decimal format, e.g., 0.0.0.1. Area 0 is a special area that represents the top level of the OSPF network hierarchy, often referred to as the backbone. By using the correct IP addressing scheme, routing information can be aggregated between areas, and by aggregating routing information (possibly one aggregated route per area), the amount of information a router needs to know can be reduced. For example, in Figure 19.1, theA separate Class B network number is assigned to each zone. By aggregating on the border routers between regions, other regions do not need to be informed of all Class B subnets, but only of the individual region172.16.0.0 area 2 area 3Area 0/ / 172.17.0.0 172.19X).0172.18.0.0 Area Map 19.1 Hierarchical Design of OSPF InternetSummary network number of the domain (Class B network number) oIn an OSPF, the backbone (region 0) is essential, while other regions (e.g., regions 1, 2, 3, etc.) must be connected to the backbone. This connection is preferably physical, but OSPF supports a dummy link feature that allows areas to connect to the backbone in a logical manner. Virtual links are beyond the scope of this book.