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The Three-Layer Network Design Model: An Analysis (+Examples)

 

The campus wired LAN uses a hierarchical design model to separate the network topology into modular groups or layers. Separating the design into layers allows each layer to implement specific functions, which simplifies the network design. This also simplifies the deployment and management of the network.
The campus wired LAN enables communications between devices in a building or group of buildings, as well as interconnection to the WAN and Internet edge at the network core. A hierarchical LAN design includes the access, distribution, and core layers as shown in the figure.

 

Hierarchical Design Model

Each layer is designed to meet specific functions.
The access layer provides endpoints and users direct access to the network. The distribution layer aggregates access layers and provides connectivity to services. Finally, the core layer provides connectivity between distribution layers for large LAN environments.
User traffic is initiated at the access layer and passes through the other layers if the functionality of those layers is required.
Even though the hierarchical model has three layers, some smaller enterprise networks may implement a two-tier hierarchical design. In a two-tier hierarchical design, the core and distribution layers are collapsed into one layer, reducing cost and complexity.

 

In flat or meshed network architectures, changes tend to affect a large number of systems. Hierarchical design helps constrain operational changes to a subset of the network, which makes it easy to manage as well as improve resiliency. Modular structuring of the network into small, easy-to-understand elements also facilitates resiliency through improved fault isolation.

Firewalls

Typically, a firewall with two interfaces is configured as follows:

  • Traffic originating from the private network is permitted and inspected as it travels toward the public network. Inspected traffic returning from the public network and associated with traffic that originated from the private network is permitted.
  • Traffic originating from the public network and traveling to the private network is generally blocked. 

 

Demilitarized zone

A demilitarized zone (DMZ) is a firewall design where there is typically one inside interface connected to the private network, one outside interface connected to the public network, and one DMZ interface, as shown in the figure.

  • Traffic originating from the private network is inspected as it travels toward the public or DMZ network. This traffic is permitted with little or no restriction. Inspected traffic returning from the DMZ or public network to the private network is permitted.
  • Traffic originating from the DMZ network and traveling to the private network is usually blocked.
  • Traffic originating from the DMZ network and traveling to the public network is selectively permitted based on service requirements.
  • Traffic originating from the public network and traveling toward the DMZ is selectively permitted and inspected. This type of traffic is typically email, DNS, HTTP, or HTTPS traffic. Return traffic from the DMZ to the public network is dynamically permitted.
  • Traffic originating from the public network and traveling to the private network is blocked.

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