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The Art of Network Architecture: Applying Modularity

What Is Hierarchical Design?

Hierarchical designs consist of three network layers: the core, the distribution, and the access, with narrowly defined purposes within each layer and along each layer edge.

Essentially, this definition takes one specific hierarchical design as the definition for all hierarchical design—we should never mistake one specific pattern for the whole design idea. What’s a better definition?

  • A hub-and-spoke design pattern combined with an architecture methodology used to guide the placement and organizations of modular boundaries in a network.

There are two specific components to this definition we need to discuss—the idea of a hub and spoke design pattern and this concept of an architecture methodology. What do these two mean?

A Hub-and-Spoke Design Pattern

An Architectural Methodology

Hierarchical network design reaches beyond hub-and-spoke topologies at the module level and provides rules, or general methods of design, that provide for the best overall network design. This section discusses each of these methods or rules—but remember these are generally accepted rules, not hard and fast laws. Part of the art of architecture is knowing when to break the rules.

Assign Each Module One Function

The first general rule in hierarchical network design is to assign each module a single function. What is a “function,” in networking terms?

  • User Connection: A form of traffic admission control, this is most often an edge function in the network. Here, traffic offered to the network by connected devices is checked for policy errors (is this user supposed to be sending traffic to that service?), marked for quality of service processing, managed in terms of flow rate, and otherwise prodded to ensure the traffic is handled properly throughout the network.
  • Service Connection: Another form of traffic admission control, which is most often an edge function as well. Here the edge function can be double sided; however, not only must the network decide what traffic should be accepted from connected devices, but it must also decide what traffic should be forwarded toward the services. Stateful packet filters, policy implementations, and other security functions are common along service connection edges.
  • Traffic Aggregation: Usually occurs at the edge of a module or a subtopology within a network module. Traffic aggregation is where smaller links are combined into bigger ones, such as the point where a higher-speed local area network meets a lower-speed (or more heavily used) wide area link. In a world full of high speed links, aggregation can be an important consideration almost any place in the network. Traffic can be shaped and processed based on the QoS markings given to packets at the network edge to provide effective aggregation services.
  • Traffic Forwarding: Specifically between modules or over longer geographic distances, this is a function that’s important enough to split off into a separate module; generally this function is assigned to core modules, whether local, regional, or global.
  • Control Plane Aggregation: This should happen only at module edges. Aggregating control plane information separates failure domains and provides an implementation point for control plane policy.

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