Three Tier Architecture Cisco

1. What is the Three Tier Architecture Cisco Model?

As companies grow, theirs networks become more complex.

Without a well-defined network design, performance suffers, troubleshooting becomes a nightmare for network engineers and scalability is impossible.

To address all these challenges, large enterprises can use the Three Tier Architecture Cisco model, which was originally developed by Cisco to organize networks more efficiently.

As shown in the diagram above, this model organizes the network into three distinct layers:

This model is composed of three distinct layers:

• Core Layer
• Distribution Layer
• Access Layer

Each layer has a specific role and you need to understand every layer for your CCNA Exam.

Understanding the Three Tier Architecture Cisco model requires examining each layer individually, let’s dive into the Access Layer.

2. Access Layer

Let’s start with the Access Layer, the layer closest to your users.

The Access Layer mainly operates at Layer 2 of the OSI model (Switching). At this level, devices forward frames based on MAC addresses, not IP addresses.

The Access Layer is responsible for connecting end devices such as:

• PCs
• IP phones
• Printers
• Wi-Fi Access Point (AP)

This layer is the link between users and the network.

Essential functions used at this layer are:

• Physical network access links for endpoints and wireless access points (APs).
• Power over Ethernet (PoE) to power devices such as IP telephones or wireless APs.
• VLANs to segment the network and create smaller broadcast domains.
• Spanning Tree Protocol (STP/RSTP) to prevent loops and ensure a resilient network.
  
• Security mechanisms:
  • Port security to restrict access based on MAC addresses.
  • DHCP Snooping to prevent rogue DHCP servers.
  • Dynamic ARP Inspection (DAI) to protect against ARP spoofing attacks.
    
• Quality of Service (QoS) trust boundary established by applying QoS classification and marking at the edge of the network.

So it’s through the Access Layer that the user interacts with the network infrastructure.

3. Distribution Layer

Let’s move on to the Distribution Layer.

The Distribution Layer operates mainly at Layer 3 of the OSI model (Routing). Here, devices make forwarding decisions based on IP addresses, enabling communication between different parts of the network.

The Distribution Layer plays a key role: it acts as a link between the Access Layer and the Core Layer.

Its main functions are :

• Aggregation of links from the Access Layer.
• Application of security policies (ACLs) to control data flows.
• Quality of Service (QoS): to guarantee good quality of service for critical applications (voice, video, etc.).
• Inter-VLAN Routing: to enable communication between different VLANs.

In modern networks, the Distribution Layer often uses multilayer switches / L3 switches. These devices combine the ability of switching with the capabilities of routing !

The Distribution Layer is the brain of the corporate network: it’s here that traffic is controlled, secured, prioritized and routed.

4. Core Layer

Finally, we come to the pinnacle of the Three Tier Architecture Cisco model: the Core Layer.

The Core Layer serves as the network’s backbone. It connects all Distribution Layers together and ensures :

• High availability: to ensure that the network remains operational, even in the event of failure.
• Low latency: for rapid transport of large amounts of data.

To achieve this, the Core Layer typically uses:

• Fiber optic links for high bandwidth and long-distance connectivity,
• High-speed technologies such as 10 Gigabit Ethernet (10 Gb), 40 Gb, 100 Gb or even 400 Gb for large networks.
• Link Aggregation (LAG/EtherChannel) to combine multiple physical links into one logical link for redundancy and higher throughput.

At the Core Layer, it’s important to keep the design as simple and fast as possible: no ACLs, no NAT, no filtering ,just efficient and reliable packet forwarding.

5. How to Interconnect Multiple Buildings?

Now that we’ve seen the three layers of the Three Tier Architecture Cisco, a logical question arises:

How are the buildings of a large enterprise interconnected using this architecture?

As you can see from the diagram, the company has two main buildings and a Data Center in the middle.

To apply our Three-Tier architecture correctly :

• Each building has its own Access Layer and Distribution Layer.
• The Core Layer is centralized in the Data Center.

Here’s how it works:

• The Distribution Layers in the buildings are connected to the central Core Layer.
  
• This interconnection is provided by very high-speed optical fibers (often 10 GbE or more).
  
• Core switches are grouped together in the Data Center to guarantee better management, performance and reliability.
  
• To ensure high availability, each Distribution Layer is connected to two Core switches via EtherChannel or LAG (Link Aggregation Group) aggregated links.

6. Conclusion

To conclude this course, let’s take a final look at the summary table below to review the key concepts of the Three Tier Architecture Cisco model:

Layer	Main Function	OSI Layer	Key Technologies / Features
Access Layer	Connects end devices (PCs, printers, IP phones, APs)	Layer 2	Switching, VLANs, Port Security, PoE, Collision Domain Segmentation
Distribution Layer	Connects Access to Core, policy enforcement, routing between VLANs	Layer 3	Multilayer (L3) Switches, ACLs, QoS, Inter-VLAN Routing
Core Layer	High-speed backbone, interconnects Distribution Layers	Layer 3	Fiber Optics, 10/40/100/400 Gb Ethernet, Link Aggregation (LAG/EtherChannel)