The hierarchical model follows the same basic concept as the OSI Reference Model, which is layering. Because each layer is responsible for a particular function, or sets of functions, a layered approach simplifies the tasks required for hosts to communicate, as well as other basic networking tasks such as troubleshooting connectivity issues between the hosts. The LAN hierarchical model is no different.
By using a hierarchical network design, network changes are easier to make and implement. Additionally, such a design allows network engineers to create design elements that can be replicated as the network grows. As each element in the network design requires change, the cost and complexity of making the upgrade is constrained to a small subset of the overall network, whereas in a large, flat or meshed network, such changes tend to impact a large number of systems. The LAN hierarchical model is comprised of the following three layers:
- The Core Layer
- The Distribution Layer
- The Access Layer
The core, or backbone, layer provides optimal transport between sites. It is a high-speed switching backbone and should be designed to switch packets as fast as possible. This layer of the network should not perform any packet manipulation, such as access lists and filtering, that would slow down the switching of packets.
The distribution layer provides policy-based connectivity; that is, the distribution layer is where packet manipulation takes place. The distribution layer provides boundary definition and is the demarcation point between the access and core layers. This layer also helps to define and differentiate the core layer. In a campus network environment, the distribution layer can include several functions, as follows:
- Address or area aggregation
- Departmental or workgroup access
- VLAN routing
- Broadcast or Multicast domain definition
- Media transitions
In a non-campus environment, the distribution layer can be a redistribution point between routing domains or the demarcation between static and dynamic routing protocols. The distribution layer can also be the point at which remote sites access the corporate network.
The access layer provides workgroup or user access to the LAN. In other words, the access layer is the point at which local users physically connect to the network. The access layer may also use access lists or filters, such as MAC address filters, to optimize the needs of a particular set of users or to provide security. In a campus network environment, access layer functions can include the following:
- Shared bandwidth (i.e., via hub connectivity)
- Switched bandwidth (i.e., using LAN switches)
- MAC layer and MAC address filtering
In the non-campus environment, the access layer can give remote sites access to the corporate network via WAN technologies, such as Frame Relay. Figure 1-2 below illustrates the interaction of these three layers in a typical enterprise LAN:
It is commonly believed that the three layers must exist as clear and distinct physical entities; in fact, this is not always practical or applicable. For example, in medium-sized networks, it is common to find the core and distribution layer functions incorporated into the same physical devices, resulting in a collapsed core. This concept is illustrated in Figure 1-3 below:
Based on the diagram illustrated above, it is important to understand and remember that the layers in the hierarchical model are implemented based on the needs of the network being designed. A medium-sized network, as illustrated above, may have only a collapsed core and access layer, while an even smaller network may have only a single switch performing the functions of the access layer, distribution layer, and core layer at the same time.
In a manner similar to the OSI Model, the layers in the hierarchical model are defined to assist with successful network design and represent the functionality that should exist in a switched network. Additionally, this model also simplifies the identification of failures or problems by structuring the network into smaller, easy-to-understand elements.