General Design Rules for Each Layer

A clear understanding of the traffic patterns within the organization—who is connecting to whom and when—helps to ensure the appropriate placement of client and servers, and eases the implementation of filtering at each layer. The filtering that is imposed by the router creates the network hierarchy illustrated in Figure 2-3. Without hierarchy, networks have less capacity to scale because the traffic must traverse every path to find its destination, and manageability becomes an issue.

It is important for each layer to communicate only with the layer above or below it. Any connectivity or meshing within a layer impedes the hierarchical design.

Organizations often design their networks with duplicate paths. This is to build network resilience so that the routing algorithm can immediately use the alternative path if the primary line fails. If this is the design strategy of your company, care should be taken to ensure that the hierarchical topology is still honored.

Figure 2-3 shows an illustration of the appropriate design and traffic flow.

Figure 2-3 Redundant Connections Between Layers

Figure 2-3 Redundant Connections Between Layers

Redundant meshing within a layer

WARNING Unless you have a profound knowledge of the current network and the placement of the servers, it is impossible to design a new network with the proper hierarchy.

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