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| 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:

- Core
- Distribution
- Access

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:

- A router
- A switch
- A link
- A combination of these

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 core layer provides an optimized and reliable transport structure by forwarding traffic at very high speeds. In other words, the core layer switches packets as fast as possible. Devices at the core layer should not be burdened with any processes that stand in the way of switching packets at top speed. This includes the following:

- Access-list checking
- Data encryption
- Address translation

The Distribution Layer
The distribution layer is located between the access and core layers and helps differentiate the core from the rest of the network. The purpose of this layer is to provide boundary definition using access lists and other filters to limit what gets into the core. Therefore, this layer defines policy for the network. A policy is an approach to handling certain kinds of traffic, including the following:

- Routing updates
- Route summaries
- VLAN traffic
- Address aggregation

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
The access layer supplies traffic to the network and performs network entry control. End users access network resources by way of the access layer. Acting as the front door to a network, the access layer employs access lists designed to prevent unauthorized users from gaining entry. The access layer can also give remote sites access to the network by way of a wide-area technology, such as Frame Relay, ISDN, or leased lines.

| Interactive Media Activity Point and Click: Layered Design Model In this media activity, the student will learn characteristics of the Layered Design Model. This is a point and click activity where the student will click the correct choice.

| Web Links Internetworking Design Basics http://www.cisco.com/univercd/cc/td/doc/ cisintwk/idg4/nd2002.htm