Top-down network design lets you drill down to the components of the network design, and apply fundamental design principles to the components as well as the overall design. Hierarchy and redundancy, as mentioned in the previous sections, are fundamental network design concepts.
Another fundamental concept related to hierarchy is modularity. Large network design projects and large networks in general consist of different areas and modules. Each area should be designed using a systematic, top-down approach, applying hierarchy and redundancy where appropriate. Network solutions and services can be selected on a per-module basis, but validated as part of the overall network design.
Cisco Systems uses the Enterprise Composite Network Model to describe the different components or modules of a typical enterprise network. The next section briefly describes the model, which is part of Cisco's Secure Architecture for Enterprises (SAFE).
The Enterprise Composite Network Model is a blueprint that
network designers can use to simplify the complexity of a large internetwork.
The blueprint lets you apply a modular, hierarchical approach to network design.
With the Enterprise Composite Network Model, you can analyze the functional,
logical, and physical components of a network, and thus simplify the process of
designing an overall enterprise network.
The Enterprise Composite Network Model comprises three major areas, which can each be made up of smaller modules:
Enterprise campus. The enterprise campus includes the modules required to build a robust campus network that provides high availability, scalability, and flexibility. This area contains all the network elements for independent operation within one campus location. An enterprise can have more than one campus.
Enterprise edge. The enterprise edge aggregates the connectivity from the various elements at the edge of an enterprise network. The enterprise edge functional area filters traffic from the edge modules and routes it into the enterprise campus. The enterprise edge contains all the network elements for efficient and secure communication between the enterprise campus and remote locations, business partners, mobile users, and the Internet.
Service provider edge. The modules in this functional area are not implemented by the enterprise. The service provider edge modules are included to enable communication with other networks using different WAN technologies and Internet service providers (ISPs).