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1.3.LAN Design Fundamentals

Flat Network

A flat enterprise campus network is where all end devices (PCs, servers, printers …) are connected to each other using Layer 2 switches. A flat network does not use subnets for any design purposes. Layer 2 devices in a flat network provide little opportunity to control broadcasts or to filter undesirable traffic. As more devices and applications are added to a flat network, response times degrade until the network becomes unusable.

Hierarchical Network

To address the limitations of the flat network and the sizing needs of most campus networks, a hierarchical model is used. A typical enterprise hierarchical LAN campus network design includes the following three layers:

  • Access layer: Local and remote workgroup access.
  • Distribution layer: Policy-based connectivity.
  • Core layer: High-speed switching.

The benefit of dividing a flat network into smaller, more manageable blocks is that local traffic remains local. Only traffic that is destined for other networks is moved to a higher layer.

Functions at the access layer include:

  • Layer 2 switching
  • High availability
  • Security (Port security, ARP inspection, Port access lists …)
  • Spanning tree
  • PoE and auxiliary VLANs for VoIP

Functions at the distribution layer include:

  • Aggregation of LAN links
  • Policy-based security
  • Routing services
  • Redundancy and load balancing
  • Route aggregation and summarization
  • Broadcast domain control

Functions at the core layer include:

  • Providing high-speed switching
  • Providing reliability and fault tolerance
  • Avoiding CPU-intensive packet manipulation caused by security, inspection, QoS, or other processes



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