Chapter 1: Planning for Complex Networks

Network Design Models

Today’s networks typically include voice, video, network management, mission-critical, and routing traffic in addition to bulk user traffic. Each type of traffic has different performance (bandwidth, delay, and jitter) and security requirements. Network design models provide a framework for integrating the many different types of traffic into the network.

Over the years, several models have been used to help describe how a complex network functions. These models are useful for designing a network and for understanding traffic flow within a more complex network. This section covers three models: the traditional Hierarchical Model, the Enterprise Composite Model, and the Cisco Enterprise Model.  

Hierarchical Design Model

Network designers used the three-level Hierarchical Design Model for years. This older model provided a high-level idea of how a reliable network might be conceived, but it was largely conceptual because it didn’t provide specific guidance. Figure 1-1 shows the Hierarchical Design Model.

This is a simple drawing of how the three-layer model might be built out for a campus network. A distribution Layer-3 switch is used for each building on campus, tying together the access switches on the floors. The core switches link the various buildings together.

This same three-layer hierarchy can be used in the WAN with a central headquarters, division headquarters, and units.

Figure 1-1
Hierarchical Design Model

The layers break a network in the following way:

• Access layer: Provides network access to workgroup end stations.

• Distribution layer: Intermediate devices provide connectivity based on policies.

• Core layer: Provides a high-speed switched path between distribution elements.

Redundant distribution and core devices, with connections, make the model more fault-tolerant. This early model was a good starting point, but it failed to address key issues, such as

• Where do wireless devices fit in?

• How should Internet access and security be provisioned?

• How do you account for remote access, such as dial-up or VPN?

• Where should workgroup and enterprise services be located?

Enterprise Composite Model

A newer Cisco model—the Enterprise Composite Model—is significantly more complex and attempts to address the shortcomings of the Hierarchical Design Model by expanding the older version and making specific recommendations about how and where certain network functions should be implemented. This model is a component of the Cisco Security Architecture for Enterprise (SAFE) Reference Architecture.