Three Tiered Topology

WANs that need to interconnect a very large number of sites, or are built using smaller routers that can only support a few serial connections, may find the two-tiered architecture insufficiently scalable. Therefore, adding a third tier (or even a fourth or fifth tier) may provide the additional scalability they require. Figure 13-7 illustrates this topology.

Figure 13-7: The three-tiered WAN topology.

Figure 13-7: The three-tiered WAN topology.

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A three-tiered WAN constructed with dedicated facilities offers even greater fault tolerance and

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A three-tiered WAN constructed with dedicated facilities offers even greater fault tolerance and scalability than the two-tiered topology. Three-tiered networks are expensive to build, operate, and maintain. They should be used only for interconnecting very large numbers of locations. Given this, it is foolish to develop a WAN of this magnitude and not fully mesh the uppermost, or backbone, tier of routers.

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