![]() ![]() |
1.1 | ![]() |
The Hierarchical Network Design Model | |
1.1.1 | ![]() |
The three-layer hierarchical design model |
A hierarchical network design model breaks the complex problem of
network design into smaller, more manageable problems. Each level, or
tier in the hierarchy addresses a different set of problems. This
helps the designer optimize network hardware and software to perform specific
roles. For example, devices at the lowest tier are optimized to accept
traffic into a network and pass that traffic to the higher layers. Cisco offers a three-tiered hierarchy as the preferred
approach to network design.
![]() In the
three-layer network design model, network devices and links are
grouped according to three layers:
The three-layer model is a conceptual framework. It is an abstract picture of a network similar to the concept of the Open System Interconnection (OSI) reference model. Layered models are useful because they facilitate modularity. Devices at each layer have similar and well-defined functions. This allows administrators to easily add, replace, and remove individual pieces of the network. This kind of flexibility and adaptability makes a hierarchical network design highly scalable. At the same time, layered models can be difficult to comprehend because the exact composition of each layer varies from network to network. Each layer of the three-tiered design model may include the following:
Some networks may combine the function of two layers into a single device or omit a layer entirely. The following sections discuss each of the three layers in detail. The Core Layer
The Distribution Layer
Use these policies to secure networks and to preserve resources by preventing unnecessary traffic. If a network has two or more routing protocols, such as Routing Information Protocol (RIP) and Interior Gateway Routing Protocol (IGRP), information between the different routing domains is shared, or redistributed, at the distribution layer. The Access Layer
|