Using a hierarchical model for network design can help you build a better network. Such a design allows you to build your network in layers, with each layer performing a specific function.

What’s the benefit? You can choose which features and equipment should be used at each layer. A hierarchical network design can make complex networks easier to manage, troubleshoot, and support. Other benefits include:

  • Scalability
  • Performance
  • Fault tolerance
  • Cost savings

Cisco’s hierarchical network design consists of three layers:

  • Core layer
  • Distribution layer
  • Access layer

The layers are logical and do not necessarily require separate physical devices. Each layer may consist of several hubs, switches, and routers, or a single network device can provide the functionality of all three layers.

The core layer
The core layer is at the top of the hierarchy and is the network backbone. It’s responsible for high-speed communications and moving large payloads throughout the internetwork.

The core layer is crucial to the network infrastructure. If the core layer goes down, every network user is affected. Since a failure at the core can affect everyone on the network, it is very important to implement fault tolerance at this level.

Core design
Now that we know the functions of the core, let’s look at some design rules. The core is required to provide high bandwidth possessing high-speed and low latency, so technologies such as Fast Ethernet, ATM, and FDDI should be deployed. Also, whenever possible, high-speed switching should be used.

Only routing protocols with fast convergence times should be used at the core level. Nothing should be done that would slow down traffic at this level. Therefore, access lists, VLANs, and packet filtering should not be performed in the core layer. Remember, the two most important functions of the core are speed and redundancy.

The distribution layer
The responsibilities of the distribution layer are security and control. The distribution layer must direct and filter traffic from the access layer to the core layer. Functions at the distribution layer include:

  • Packet filtering
  • Routing between VLANs
  • Media translation (e.g., Token Ring to Ethernet)
  • Routing between domains
  • Policy-based routing and security
  • Firewalls
  • Address translation
  • Queuing
  • Access to resources at the core

The access layer
The access layer, as its name implies, provides users with access to the internetwork. There is switched and shared network bandwidth, and creation of separate collision domains and network segmentation occur at this layer. Additionally, it is through this layer that workgroups gain access to the distribution layer and core layer resources.

Warren Heaton CCDA, CCNA, MCSE+I, is the Cisco Program Manager for A Technological Advantage in Louisville, KY.

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