Guidelines for Hierarchical Network Design
This section briefly describes some guidelines for hierarchical network design. Following these simple guidelines will help you design networks that take advantage of the benefits of hierarchical design.
The first guideline is that you should control the diameter of a hierarchical enterprise network topology. In most cases, three major layers are sufficient (as shown in Figure 5-4):
• The distribution layer
• The access layer
Controlling the network diameter provides low and predictable latency. It also helps you predict routing paths, traffic flows, and capacity requirements. A controlled network diameter also makes troubleshooting and network documentation easier.
Strict control of the network topology at the access layer should be maintained. The access layer is most susceptible to violations of hierarchical network design guidelines. Users at the access layer have a tendency to add networks to the internetwork inappropriately. For example, a network administrator at a branch office might connect the branch network to another branch, adding a fourth layer. This is a common network design mistake that is known as adding a chain. Figure 56 shows a chain.
Figure 5-6. A Chain and Backdoor at the Access Layer
In addition to avoiding chains, you should avoid backdoors. A backdoor is a connection between devices in the same layer, as shown in Figure 5-6. A backdoor can be an extra router, bridge, or switch added to connect two networks. Backdoors should be avoided because they cause unexpected routing and switching problems and make network documentation and troubleshooting more difficult.
NOTE
Sometimes there are valid reasons for adding a chain or a backdoor. For example, international network topologies sometimes get skewed by the availability of fiber-optic links, the ease and cost of provisioning new networks, and the availability of competent carriers. An international network might require a chain to add another country. A backdoor is sometimes added to increase performance and redundancy between two parallel devices in a layer. But, in general, other design options can usually be found that let the design retain its hierarchical structure. To maximize the benefits of a hierarchical model, chains and backdoor should usually be avoided.
Finally, one other guideline for hierarchical network design is that you should design the access layer first, followed by the distribution layer, and then finally the core layer. By starting with the access layer, you can more accurately perform capacity planning for the distribution and core layers. You can also recognize the optimization techniques you will need for the distribution and core layers.
You should design each layer using modular and hierarchical techniques and then plan the interconnections between layers based on your analysis of traffic load, flow, and behavior. To better understand network traffic characteristics you can review the concepts covered in Chapter 4. As you select technologies for each layer, as discussed in Part III of this book, "Physical Network Design," you might need to go back and tweak the design for other layers. Remember that network design is an iterative process. # phev
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Continue reading here: Redundant Network Design Topologies
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