Two‐tier & Three tier topologies in Network Design - connorethanjay/CCNP-Enterprise-Preparation GitHub Wiki

High level explanation of the need for Network Design

A network must be able to support the exchange of various types of traffic, eg. files, email, VoIP calling, and video applications (eg. Zoom calling).

Not only must a network be able to support critical applications, but a network must also be designed to support the varying speeds of video, voice, and data traffic, provide centralized control (in an administrative context), and support the diverse needs a business has, in the case the business grows, the network should be scalable.

What are two tier and three tier networks?

In hierarchical network design, there are the two tier and three tier models. The two tier model combines the lower two layers (core and distribution layers) for smaller networks.

  • Access layer: The access layer represents the network edge where traffic enters or exits the network. The access layer is used to connect end devices of varying types to the LAN network. The access layer provides layer-2 switching and various services such as Spanning Tree Protocol, QoS, PoE, and ARP. It also prevents unauthorized devices from connecting to the LAN by enforcing various security policies such as port security, DHCP snooping, and static MAC address configuration. Access layer switches connect to the next layer down, the distribution layer. image

  • Distribution layer: The distribution layer is the second / middle layer which does not provide services to the end devices in particular, however it does provide connectivity between the Access layer links and Core layer links. The distribution layer controls and filters traffic by implementing ACLs, controls broadcasts via VLANs, provides redundancy and load balancing, routing services between different VLANs and routing domains, and acts as the demarcation point between different LANs and broadcast domains.

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  • Core layer: The core layer is the third (lowest) layer, core switches connect to distribution switches, reducing cabling needs and switch ports while still allowing all devices to send data to all other devices in the LAN. Typically small or medium LAN networks do not need the core layer, but rather directly connect distribution switches. This approach does not work in a much larger network. See below (2nd photo).

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Key points of two tier / three tier topologies:

  • Access switches connect end-user devices to the LAN.
  • Access switches forward traffic between end-user devices and the rest of the lan.
  • An access switch does not connect two or more access switches.
  • A distribution switch connects the access switches.
  • A distribution switch does not connect end-user devices.
  • A distribution switch provides an aggregation point for access switches.
  • If the core switches exist, the distribution switches connect the access switches to the core switches.