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The partial-mesh model does not constrain the designer with a predefined number of circuits per nodes in the network, which permits some latitude in locating and provisioning circuits. However, this flexibility can cause reliability and performance problems. The benefit is cost—fewer circuits can support the entire enterprise while providing specific data paths for higher priority connections.

The Two-Tier Network Model

The two-tier model shares many attributes with the partial-mesh model, but the design has some additional benefits. This design typically evolves from the merger of two companies—each of small size and using historical star topologies. However, the design may also merit use in the initial deployment of a medium-sized network. Figure 1.6 illustrates the two-tier model. This model is sometimes used in metropolitan settings where a number of buildings require connectivity but only two buildings have WAN connections—this design reduces total costs yet provides some redundancy. The two core installations in Figure 1.6 would incorporate the WAN links.

Notice that the two-tier model introduces a single, significant point of failure: the link between the primary locations. However, if designed for each side (east/west) to be independent of the other, the model can work effectively.

This solution works best when both locations have strong support organizations and the expenses associated with complete integration are high. Because of the limited connectivity between the two primary sites and the lack of any other connections, this solution typically provides the lowest cost and is the simplest approach. When a single core location is selected, the alternate primary location can move to the distribution layer (explained in the next section) or can provide a distributed core for redundancy.

FIGURE 1.6  The two-tier model

The Three-Tier (Hierarchical) Network Model

Most modern networks are designed around a form of the three-tier model. As shown in Figure 1.7, this network model defines three levels (functions) of the network: core, distribution, and access. The highest level is the network core, which interconnects the distribution layer resources. Access routers connect to the distribution layer moving up the model and to workstations and other resources moving down the model.

This design affords a number of advantages, although the costs are greater than those for the previous models. The biggest advantage to this design is scalability.

FIGURE 1.7  The three-tier model

Virtually all scalable networks follow the three-tier model for network design. This model is particularly valuable when using hierarchical routing protocols and summarization, specifically OSPF, but it is also helpful in reducing the impact of failures and changes in the network. The design also simplifies implementation and troubleshooting, in addition to contributing to predictability and manageability. These benefits greatly augment the functionality of the network and the appropriateness of the model to address network design goals. These benefits, which are typically incorporated in hierarchical designs, are either not found inherently in the other models or not as easily included in them. Following is a closer look at the benefits just mentioned:

Scalability As shown in the previous models, scalability is frequently limited in network designs that do not use the three-tier model. While there may still be limitations in the hierarchical model, the separation of functions within the network provides natural expansion points without significantly impacting other portions of the network.
Easier implementation Because the hierarchical model divides the network into logical and physical sections, designers find that the model lends itself to implementation. A setback in one section of the network build-out should not significantly impact the remainder of the deployment. For example, while a delay in connecting a distribution layer to the core would affect all of the downstream access layer nodes, the setback would not preclude continued progress between the access layer and the distribution layer. In addition, other distribution and access layers could be installed independently. Project managers typically build out the core and distribution layers first in a new deployment and then proceed with the access layer; however, if immediate service is needed at the access layer, the designer may adopt a plan that focuses on that tier and then interconnects with the infrastructure at a later time. This means that the designer may be required to provide a connection between two locations that are remote—locations that would typically be located in the access layer. When the core and distribution layers are completed, the designer can move the circuits used for the temporary connection, bringing the smaller network into the larger one. Better still, many architects try to place the distribution in one of the two temporary link locations—reducing the expense and providing a termination point for other access layer locations.
Easier troubleshooting Given the logical layout of the model, hierarchical networks are typically easier to troubleshoot than other networks of equal size and scope. Reducing the possibility of routing loops further aids troubleshooting, and hierarchical designs typically work to reduce the potential number of loops.
Predictability Capacity planning is generally easier in the hierarchical model, since the need for capacity usually increases as data moves toward the core. Akin to a tree, where the trunk must carry more nutrients to feed the branches and leaves, the core links all the other sections of the network and thus must have sufficient capacity to move data. In addition, the core typically connects to the corporate data center via high-speed connections to supply data to the various branches and remote locations.
Manageability Hierarchically designed networks are usually easier to manage because of these other benefits. Predictable data flows, scalability, independent implementations, and simpler troubleshooting all simplify the management of the network.

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