Designing computer networks can be extremely complicated, but the Cisco three layer hierarchical model can help us design, implement, and maintain a reliable, scalable and cost-effective hierarchical internetwork.
The core layer is the core of the network and is responsible for transporting large amount of traffic both reliably and quickly. The distribution layer is the communication point between the access layer and the core. It determines the best path to quickly transport the request to the correct service. The access layer controls user and workgroup access to local network resources since the distribution layer handles any traffic for remote services.
We should consider the following in designing our internetwork:
- Design the core layer for high reliability, very little latency, and use routing protocols with lower convergence times. Consider data-link facilities that facilitate both speed and redundancy. Never do anything to slow traffic, and if performance becomes an issue, give preference to upgrade over expansion.
- Distribution layer should generally handle routing, VLANs and other workgroup support functions as well as implement access lists, packet filtering, queuing, security, network policies, address translation, firewall, workgroup access, and define broadcast and multicast domains. Distribution layer should also handle redistribution between routing protocols, including static routing.
- Access layer should create separate collision domains (segmentation), facilitate connectivity into the distribution layer, and continue from distribution layer the use of access control and policies.
Keep in mind that the three layers are logical and are not necessarily physical devices.