Two Tiered Topology

A two-tiered topology is a modified version of the basic star topology. Instead of a single concentrator router, two or more routers are used. This rectifies the basic vulnerability of the star topology without compromising its efficiency or scalability.

Figure 13-6 presents a WAN with a typical two-tiered topology. The worst-case hop count increases by one as a result of the extra concentrator (a.k.a. "backbone") router. However, unlike the peer-to-peer network presented in Figure 13-1, the hop count is not adversely affected every time a new location is added to the WAN.

Figure 13-6: A two-tiered WAN.

Figure 13-6: A two-tiered WAN.

A two-tiered WAN constructed with dedicated facilities offers improved fault tolerance over the simple star topology without compromising scalability. This topology can be implemented in a number of minor variations primarily by manipulating the number of concentrator routers and the manner with which they are interconnected. Having three or more concentrator routers introduced requires the network designer to select a subtopology for the concentrator tier. These routers can be either fully or partially meshed, or strung together peer to peer.

Regardless of the subtopology selected, hierarchical, multitiered topologies function best when some basic implementation principles are followed. First, the concentration layer of routers should be dedicated to the task. That is, they are not used to directly connect user communities. Second, the user-premises routers should only internetwork with concentrator nodes and not with each other in a peer-to-peer fashion. Third, the interconnection of user-premises routers to concentrator routers should not be done randomly. Some logic should be applied in determining their placement. Depending on the geographic distribution of the users and the transmission facilities used, it may be prudent to place the concentrator nodes to minimize the distances from the user premises. The reason for this is rooted in economics: Most WAN transmission facilities are priced according to their mileage. Longer transmission facilities are more costly than shorter transmission facilities. Therefore, arranging your topology to minimize the distances of transmission facilities will effectively reduce your network's monthly recurring facilities charges.

Given that one or more routers will be dedicated to route aggregation, this topology can be an expensive undertaking. This tends to limit the use of these topologies to larger companies.

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