Content

| 1.2 |  | Key Characteristics of Various Switching Technologies |
| 1.2.8 |  | Hierarchical design model for campus networks |

A hierarchical network design includes the following three layers:

- The core layer, which provides optimal connectivity between distribution blocks
- The distribution layer, which provides policy-based connectivity
- The access layer, which provides access-layer aggregation and L3/L4 services

A high-level view of the various aspects of a hierarchical network design is shown in Figure . A hierarchical network design presents three layers, core, distribution, and access. Each layer provides different functionality.

The Core Layer
The core layer is a high-speed switching backbone and should be designed to switch packets as fast as possible between all distribution layer devices in the campus network. The core layer should not perform any packet manipulation, such as filtering with access lists, that would slow down the switching of packets. It is now common for the core layer to be a pure Layer 3 switched environment, which means that VLANs and VLAN trunks (discussed in Module 2) are not present in the core. It also means that spanning tree loops (discussed in Module 3) are typically avoided in the core.

The Distribution Layer
The distribution layer of the network divides the access and core layers and helps to define and differentiate the core. This layer provides a boundary definition and is the place at which packet manipulation takes place. In the campus environment, the distribution layer can include several functions:

- Address or area aggregation
- Departmental or workgroup access connectivity to the backbone
- Broadcast/multicast domain definition
- Inter-VLAN routing
- Any media transitions that need to occur
- Security

In the noncampus environment, the distribution layer can be a redistribution point between routing domains or the demarcation between static and dynamic routing protocols. It can also be the point at which remote sites access the corporate network. The distribution layer can be summarized as the layer that provides policy-based connectivity.

Packet manipulation, filtering, route summarization, router filtering, route redistribution, inter-VLAN routing, policy routing, and security are some of the primary roles of the distribution layer.

The Access Layer
The access layer is the point at which local end users are allowed into the network. This layer may also use access lists to further optimize the needs of a particular set of users. In the campus environment, access-layer functions can include:

- Shared bandwidth
- Switched bandwidth
- MAC-layer filtering
- Microsegmentation

In the noncampus environment, the access layer can give remote sites access to the corporate network via wide-area technologies, such as public switched telephone network (PSTN), Frame Relay, ISDN, digital subscriber line (xDSL), or leased lines.

It is often mistakenly thought that the three layers (core, distribution, and access) must exist in clear and distinct physical entities, but this does not have to be the case. The layers are defined to aid successful network design and to represent functionality that must exist in a network. The way the layers are implemented depends on the needs of the network being designed. It is important to remember that for a network to function optimally and maintain scalability as growth occurs, hierarchy must be maintained.

| Interactive Media Activity Drag and Drop: Hierarchical Design Model for Campus Networks After completing this activity, the students will be able to understand campus networks.