In networking, a hierarchical design involves dividing the network into discrete layers. Each layer, or tier, in the hierarchy provides specific functions that define its role within the overall network. This helps the network designer and architect to optimize and select the right network hardware, software, and features to perform specific roles for that network layer. Hierarchical models apply to both LAN and WAN design.
A typical enterprise hierarchical LAN campus network design includes the following three layers:
- Access layer - Provides workgroup/user access to the network.
- Distribution layer - Provides policy-based connectivity and controls the boundary between the access and core layers.
- Core layer - Provides fast transport between distribution switches within the enterprise campus.
The benefit of dividing a flat network into smaller, more manageable blocks is that local traffic remains local. Only traffic that is destined for other networks is moved to a higher layer.
Layer 2 devices in a flat network provide little opportunity to control broadcasts or to filter undesirable traffic. As more devices and applications are added to a flat network, response times degrade until the network becomes unusable.
Click play in Figure 1 to view a transition of a flat network to a hierarchical network design.
Another sample three-layer hierarchical network design is displayed in Figure 2. Notice that each building is using the same hierarchical network model that includes the access, distribution, and core layers.
Note: There are no absolute rules for the way a campus network is physically built. While it is true that many campus networks are constructed using three physical tiers of switches, this is not a strict requirement. In a smaller campus, the network might have two tiers of switches in which the core and distribution elements are combined in one physical switch. This is referred to as a collapsed core design.