Cisco Hierarchical Network Model

Prior to any discussion of the architecture models proposed in the IIN vision, it is necessary to step back to a discussion of a somewhat older model advocated for network scalability, the Cisco Hierarchical Network Model. Figure 1-2 illustrates the model for purposes of discussion.

Figure 1-2 Cisco Hierarchical Network Model

Data Wireless Voice

VLAN VLANs VLAN

Data Wireless Voice

VLAN VLANs VLAN

As is evident in the figure, the essential layers of the network are divided into three layers: Core, Distribution, and Access. This provides a repeatable, or "cookie-cutter," model that is easily reproduced site to site. The model also has the benefit of being scalable from hundreds to thousands of devices in a campus network. Additionally, this model supports the integration of SONA Interactive Services Layer applications and services, facilitating an improved experience in the interaction between the clients and applications/services provided by the network.

Each layer has its prescribed function, as described here:

■ Access Layer—Devices deployed throughout the network with the express purpose of providing user access to the network, generally through switch port access. Access layer switches are generally located near the user populous they serve.

■ Distribution Layer—Devices deployed as aggregation points for Access layer devices. Distribution layer devices can be used to segment workgroups or departments in a campus environment. The Distribution layer devices also provide for WAN aggregation connectivity at the Campus Edge and provide policy-based connectivity.

■ Core Layer (a.k.a. Backbone Layer)—Devices that carry the weight of the network. They are designed to switch packets as fast as possible. The Core layer must be highly available and redundant to ensure that no loss or degradation of service is experienced in the event of a network outage.

This model can be applied to any network of any size regardless of the technologies and connectivity options it presents. This includes LAN, WAN, MAN, wireless, VPN, and other networks. In smaller networks, it is feasible that one or more of these layers might be combined into a multi-functional layer. In the discussions to follow, and throughout nearly any networking technology-related book, these three layers are referenced quite frequently.

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