2 tier | 3 tier | collapsed core network architecture explained | Free CCNA 200-301 |

Building a Campus network is more than only interconnecting physical network infrastructure devices.
The most challenging and important part of it is the planning and design phases where different technical variables and technologies need to be considered that could even affect the product selection and the design entirely.
Network designers do not just plug in devices to any port and connect switches to each other in an arbitrary way, like you might do with a few devices on the same table in a lab.
Instead, there are known better ways to design the topology of a campus LAN,

Cisco’s hierarchical network design model breaks the complex problem of network design into smaller and more manageable.
Each layer or tier in the hierarchy is focused on specific set of roles.
This helps the network designer and architect to optimize and select the right network hardware, software and features to perform specific roles for that network layer
The two proven hierarchical design architectures for campus networks are the
1.Three-tier model
2.Two-tier model

A typical enterprise hierarchical campus network design includes the following three layers:
1.Core layer
2.Distribution layer
3.Access Layer

Three-tier layer model
This design model can be used in large campus networks where multiple distribution layer and buildings need to be interconnected
Access Layer
The access layer is the first tier or edge of the campus.
It grants end devices access to the network.
As shown in Figure, the access layer network generally incorporates Layer 2 switches and access points providing connectivity between workstations and servers.
The access layer provides the intelligent demarcation between the network infrastructure and the computing devices that leverage that infrastructure.
It is the first layer of defense in the network security architecture and the first point of negotiation between end devices and the network infrastructure.
When looking at the overall campus design, the access switch provides the majority of these access-layer services and is a key element in enabling multiple campus services

The Distribution Layer 
The distribution layer in the campus design has a unique role in that it acts as a services and control boundary between the access and the core. 
The distribution layer on the other hand serves multiple purposes.
 It is an aggregation point for all of the access switches and acts as an integral member of the access-distribution block providing connectivity and policy services for traffic flows within the access-distribution block
The distribution layer aggregates the data received from the access layer switches before it is transmitted to the core layer for routing to its final destination.
A distribution layer switch may provide upstream services for many access layer switches.
In Figure, the distribution layer is the boundary between the Layer 2 domains and the Layer 3 routed network.
Either a router or a multilayer switch is used to segment workgroups and isolate network problems in a campus environment.

Core layer
The core layer is also referred to as the Network Backbone
The campus core is in some ways the simplest yet most critical part of the campus.
The core layer consists of high-speed network devices such as the Cisco Catalyst 6500 or 6800.
. The core aggregates the traffic from all the distribution layer devices, and are designed to and interconnect multiple campus components.
As shown in Figure , the core layer is critical for interconnectivity between distribution layer devices (for example, interconnecting the distribution block to the WAN and Internet edge).
It provides a very limited set of services and is designed to be highly available and operate in an always-on mode.
In the modern business world, the core of the network must operate as a non-stop 7x24x365 service.
The key design objectives for the campus core are based on providing the appropriate level of redundancy to allow for near immediate data-flow recovery in the event of any component (switch, supervisor, line card, or fiber) failure.

Two-tier layer model
This design model, is more suitable for small to medium-size campus networks
Many small enterprise networks do not grow significantly larger over time as large organization grow.
In such network the core and distribution functions can be combined into one layer, also known as collapsed core-distribution architecture.
A “collapsed core” is when the distribution layer and core layer functions are implemented by a single device.
The primary motivation for the collapsed core design is reducing network cost, while maintaining most of the benefits of the three-tier hierarchical model.

In my next session i will be discussing about leaf and spine Architecture.

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Léa LOPEZ, Telecom System Administrator, Ambala College of Engineering and Applied Research, Ambala (INDIA)

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