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As the name implies, the Cisco model was developed by Cisco Systems to simplify the construction of large networks by dividing them into three layers. The model dictates specific network configurations that, if followed, will ensure network scalability and efficiency.

The three layers of the model are as follows:

Core Layer
Placed at the top of the hierarchy, the core layer switches large amounts of network traffic. It forms the basic links responsible for the overall integrity of the network, and thus a failure at this level can be catastrophic. Traffic speed at this layer is extremely important, as slowdowns will effect large amounts of users. Because of this, routing and other network-layer functions should not be used. High-speed data link technologies such as FDDI and Fast Ethernet should be implemented here.

Distribution Layer
This layer functions as an intermediary between the upper and lower layers. It is responsible for deciding which packets will access the core layer, as well as the implementation of Access Contol Lists (ACLs) and other packet filtering technologies. Packet routing should occur at this layer, as well as security features such as firewalls and VLANs. Broadcast domains are also created at this layer.

Access Layer
At the bottom of the hierarchy, this layer controls user and workgroup access to the network. Local network resources should be placed on this layer, as well as security features that require a more localized scope. Collision domains are created here by the use of routers and switches. Because of the simplicity at this level, static routing can often be used in place of dynamic routing protocols such as RIP and IGRP.

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