Access, Distribution, and Core Layers Explained

The term campus LAN refers to a LAN network that spans a single geographic location, such as a building or university campus. A campus LAN can be an entire network or part of an enterprise network. An enterprise network is a large network that may contain several campus networks spanning different geographic locations. If a campus network is part of an enterprise network, it allows end users and devices to access network services and resources within the same geographic area or in proximity.

The Cisco three-layer hierarchical model provides recommendations for designing campus LANs. Rather than implementing a flat network, this model endorses a hierarchical structure, which is generally easier to manage and troubleshoot. It contains three layers: core, distribution, and access. The core layer is the backbone of the network. It provides a high-speed connection between different distribution layer devices. The distribution layer connects the access layer to the core layer. The access layer provides initial connections to end users.

When designing a campus LAN, you may choose to implement all, some, or none of the Cisco three-layer model's recommendations. This flexibility allows for the creation of flat networks or hierarchical networks with two or three layers. A network without layers is called a flat network, while a network with layers is called a hierarchical network. A hierarchical network can use dedicated hardware for each layer, or a single piece of hardware for multiple layers.

A three-tier network utilizes dedicated hardware for the access, distribution, and core layers. A two-tier network combines hardware that supports the distribution and core layers. However, it uses separate hardware for the access layer. A one-tier network uses a single piece of hardware for all three layers. In a three-layer hierarchical model, a switch is named after the layer in which it works. For example, a switch that provides access-layer functionality is called an access switch, a switch that operates in the distribution layer is known as a distribution switch, and a switch that operates in the core layer is called a core switch.

The access layer

The access layer is the first layer of the Cisco three-layer hierarchical model. This layer allows end users to access the network. It connects user devices such as PCs, IP phones, wireless access points, printers, and scanners to the network. User devices connected to this layer use different protocols to discover each other, prevent loops, and exchange data. Various services and security policies are also configured and enforced at this layer.

The main functions of this layer are the following.

• Connecting various types of end devices to the LAN network.
• Providing layer-2 switching and implementing various layer-2 switching services such as spanning tree, virtual access control, QoS, PoE, and ARP.
• Preventing unauthorized devices from connecting to the LAN by enforcing various security policies such as port security, DHCP snooping, and static MAC address configuration.

Switches in this layer are called access switches. End devices connect to the LAN through the access switches. In other words, an access switch forwards traffic between connected devices and the rest of the LAN. The following image shows a network that contains two access switches.

The distribution layer

The distribution layer is the second layer of the Cisco three-layer hierarchical model. Switches connected in this layer are known as the distribution switches. Unlike access switches, distribution switches do not provide any service to end devices. Distribution switches connect the access switches. The following image shows how distribution switches connect the access switches in the network.

The main functions of distribution-layer switches are as follows.

• Providing connectivity between the access layer switches
• Aggregating LAN and WAN links and traffic
• If a separate core layer exists, it provides upstream services for the access layer switches.
• Controlling and filtering traffic by implementing ACLs
• Controlling broadcast through VLANs
• Providing redundancy and load balancing
• Providing routing services between different VLANs and routing domains
• Acting as a demarcation point between different LANs and broadcast domains

If the network includes a separate core layer, the distribution layer connects the access layer to the core. The following image shows how the distribution switches operate when a separate core layer exists.

The core layer

This is the third layer of the Cisco three-layer hierarchical model. Switches that operate at this layer are called core switches. Core switches connect distribution switches. In a large, complex network, core switches reduce cabling requirements and the number of switch ports while still allowing all devices to send data to all other devices on the LAN. Usually, small or medium LAN networks do not design the core layer. Instead of designing a separate core layer, they connect the distribution switches directly. This approach does not work in large networks. For example, if a LAN network has two distribution switches, it can connect them directly as shown in the above image (the first image of the distribution layer section).

But if a LAN has several distribution switches, they should not be connected directly to each other. To connect all distribution switches, a LAN requires N*N-1 connections and N-1 available ports on each distribution switch. For example, if the LAN has 8 distribution switches, it needs 8*8-1 = 56 links and 8-1 = 7 ports on each distribution switch. This requirement is without redundancy. For redundancy, if the LAN adds additional connections, the number of required connections will also increase in parallel.

The LAN can reduce the required number of connections and ports by connecting distribution switches through a few extra switches. A switch that connects the distribution switches is known as the core switch. The following image shows how the core switches connect the distribution switches.

Unlike the access and distribution layers, the core layer provides fewer services. The core layer has a single dedicated role in the Cisco three-layer model. It is responsible for forwarding traffic between the distribution switches.

Key points

• An access switch connects end-user devices to the LAN.
• An access switch forwards 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 them.
• A core switch aggregates distribution switches.
• Except for forwarding traffic at the fastest possible speed, core switches do not provide any other services.
• If core switches are not used, the network is known as the Tire-2 network.
• If core switches are used, the network is a Tier-3 network.

Conclusion

The Cisco three-layer hierarchical model provides a systematic framework for designing scalable, manageable, and efficient campus LANs. Dividing the network into access, distribution, and core layers simplifies troubleshooting, enhances performance, and supports robust security policies. Whether employing a two-tier or three-tier architecture, understanding the distinct role of each layer facilitates optimal resource allocation and prepares the network for future growth.