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by Marc Menninger
IntroductionCisco takes network design very seriously. Many network problems can be avoided if the network is properly designed in the first place. That probably explains why Cisco has an entire design certification track with two different design exams: the Designing Cisco Networks exam (640-441 DCN) for your CCDA certification and the Cisco Internetworking Design exam (640-025 Design) for your CCDP certification.
Networking basics and layered communications are included in Cisco's exam topics for the CCNA exam (http://www.cisco.com/warp/public/10/wwtraining/certprog/testing/current_exams/640-507.html), so it's possible that you'll encounter some questions on the CCNA exam related to Cisco's Hierarchical Network Design Model. The Hierarchical Network Design Model is also an important component of the CCNP Switching exam (640-504), the Designing Cisco Networks exam (640-441 DCN), and Cisco Internetworking Design exam (640-025 Design).
This short tutorial covers everything you need to know about Cisco's Hierarchical Network Design Model for the CCNA exam as well as provides a solid foundation of understanding for the more advanced exams. This tutorial covers what the Hierarchical Network Design Model is, the layers of the model, Cisco network equipment you can expect to find in each layer, and some tips on remembering it all.
Cisco has defined four principles of good network design that it thinks you should follow:
Eliminate points of failure.
Characterize application and protocol traffic.
Analyze bandwidth availability.
Build networks using a hierarchical model.
The hierarchical model is therefore a key element of Cisco's concept of good network design.
Cisco's Hierarchical Network Design Model is a logical way of describing good network design. The purpose of this model is to simplify network design, implementation, and management. While you might think that using a design model is typically associated with designers, it is important that you understand the model as an implementer as well. If you understanding this model, you'll know what type of equipment and features are required for each device on your network.
Campus networks, such as the network for buildings on a corporate site, are designed using the hierarchical layer model. They are typically built with a "backbone" of high-speed network equipment at the center or "core" of the network. At the user level, higher data transfer speeds are not as critical, and bandwidth is often shared. Distributed network services, such as routing and security policies, normally fall between the high-speed backbone and the user level. This, in essence, describes Cisco's Hierarchical Network Design Model. Luckily, it follows common sense. For instance, you wouldn't want slower network equipment and shared bandwidth on your backbone. Likewise, you wouldn't want expensive, high-speed equipment at the user level (unless you had money to burn.)
Besides ordering network equipment, there are other aspects of network design that must be considered. For instance, complex applications force network designers to use traffic patterns in the network as a criteria for building the internetwork. If there is heavy use of network-intensive applications in one department, that must be accounted for in the network design. Therefore, networks can't always be divided into subnetworks based just on the number of users or the 80/20 rule, which states that 80% of network traffic will remain local to the network.
Servers that run global applications, such as e-mail or database servers, can have a noticeable impact on the traffic load across a network. More efficient routing and switching techniques may be required to deal with this higher traffic load across the entire network.
In order to view and address traffic pattern requirements on your network, it is especially important that your network follow Cisco's Hierarchical Network Design Model. The model has three layers organized as follows:
Core layer
Distribution layer
Access layer.
Figure 1. Cisco's Hierarchical Network Design Model
If you've ever been in a network closet in a corporate building and seen a rack with a lot of small switches with a larger switch at the top (or bottom) of the rack, you've seen Cisco's Hierarchical Network Design Model in action. The small switches were at the access layer and they each had one link to the larger switch that was at the distribution layer. Chances are that cables from the larger distribution switch (or switches) in that closet ran through a hole in the floor or ceiling to an even larger switch, probably on the first floor or in the basement of the building. That switch was at the core. If the network was designed well, that core switch had a redundant standby switch and both were connected to core switches elsewhere, probably in another building.
Like the OSI model, Cisco's Hierarchical Network Design Model is a logical model, so it doesn't necessarily correspond directly to physical devices. In other words, just because there are three layers doesn't mean you'll find three devices that correspond to these layers in all networks. You may find many devices in a single layer or a single device performing functions at two layers.
There are several benefits of using Cisco's Hierarchical Network Design Model:
Cost-effective. When you purchase network equipment and follow the hierarchical model, you'll buy equipment that is appropriate for each layer of the hierarchy, avoiding spending money on unnecessary features. Also, if you follow the hierarchical model you'll be able to control network management costs by distributing management systems and responsibility to different layers. Finally, because the hierarchical model is modular, you can accurately plan for capacity in each layer, reducing wasted bandwidth.
Simplification. When you follow the hierarchical model, your network will be modular. This helps ensure that your design is easy to understand and minimizes the amount of time and money required to train network operations personnel. A modular network is easier to implement and maintain. Testing your network design is also easier because there is a clear functionality at each layer that you can test. If you have problems in your network, network technicians can isolate the faults faster because they can recognize the transition points in the network.
Extensible. If you need to upgrade, you can often limit your upgrade costs to one layer of the network. If you didn't follow the hierarchical model, it would be harder to replace a single device or layer because complex interconnects could impact many subnetworks.
Scalable. As your network grows you can add more modular pieces of a hierarchical network. Each modular part of a network should be consistent, which makes expansion easier to plan and implement. For instance, if you need to add another floor of users to a network that follows a hierarchical model, you should be able to copy the design of an existing floor.
Let's look at each layer of Cisco's Hierarchical Network Design Model in more detail.
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Figure 2. Cisco's Hierarchical Network Design Model: Core Layer
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Figure 3. Cisco's Hierarchical Network Design Model: Distribution Layer
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Figure 4. Cisco's Hierarchical Network Design Model: Access Layer
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