Typical Hierarchical Design Patterns
There are two traditional hierarchical design patterns: two layer networks and three layer networks. These have been well covered in network design literature (for instance, see Optimal Routing Design), so we will provide only a high level overview of these two design patterns here. Figure 7-4 illustrates two- and three-layer designs.
Figure 7-4 Two- and Three-Layer Hierarchical Design Patterns
In the traditional three-layer hierarchical design:
- The core is assigned the function of forwarding traffic between different modules within the distribution layer. Little to no control or data plane policy should be configured or implemented in the core of a traditional three-layer hierarchical design.
- The distribution layer is assigned the functions of forwarding policy and traffic aggregation. Most control plane policy, including the aggregation of reachability and topology information, should be configured in the distribution layer of the traditional three layer hierarchical design. Blocking access to specific services, or forwarding plane filtering and policy, should be left out of the distribution layer, however, simply to keep the focus on each module narrow and easy to understand.
- The access layer is assigned the functions of user attachment, user traffic aggregation, and data plane policy implementation. The access layer is where you would mark traffic for specific handling through quality of service, block specific sources from reaching specific destinations, and implement other policies of this type.
In the traditional two-layer hierarchical design:
- The core is assigned the function of forwarding traffic between different modules within the aggregation layer. The core edge, facing toward the aggregation layer, is also where any policy or aggregation toward the edge of the network is implemented.
- The aggregation layer is assigned the functions of user attachment, user traffic aggregation, and data plane policy implementation. The aggregation layer is where you would mark traffic for special handling through quality of service, block access to specific services, and otherwise implement packet and flow level filters. The edge of the aggregation layer, facing the core, is also where any policy or aggregation at the control plane is implemented moving from the edge of the network toward the core.
It’s easy to describe the two-layer network design as simply collapsing the distribution layer into the edge between the core and aggregation layers, or the three-layer design as an expanded two-layer design. Often the difference between the two is sheer size—three-layer designs are often used when the aggregation layer is so large that it would overwhelm the core or require excessive links to the core. Or if it’s used in a campus with multiple buildings containing large numbers of users. Geography often plays a part in choosing a three-layer design, such as a company that has regional cores connecting various sites within a given geographical area, and a global core connecting the various regional cores.
Hierarchical network design doesn’t need to follow one of these design patterns, however. It’s possible to build a hierarchical network using layers of layers, as illustrated in Figure 7-5.
Figure 7-5 Layers Within Layers
Is the network shown in Figure 7-5 a four-layer design, a three-layer design with two layers within each aggregation module, a three-layer design with the distribution layer collapsed into the core, or a two-layer design with layers within each module? It really doesn’t matter, so long as you’re following the basic rules for hierarchical network design.