📘 CCNA 200-301 v1.1, CompTIA Network+ (N10-009)
1.2 Describe Characteristics of Network Topology Architectures
1.2.b Three-Tier Network Architecture
What is Three-Tier Network Architecture?
In enterprise networks (like in offices, data centers, or campus networks), devices and users are connected in a structured way to make the network easier to manage, scale, and troubleshoot.
The Three-Tier Architecture is a hierarchical design model that divides the network into three layers (tiers):
- Core Layer
- Distribution Layer
- Access Layer
Each layer has a specific role and responsibility, and together they provide efficient, fast, and reliable data communication.
1️⃣ Access Layer
Purpose:
The Access Layer is the first layer that connects end devices (hosts) to the network.
Functions:
- Provides network access for end-user devices such as:
- PCs, laptops, printers, IP phones, wireless access points, etc.
- Handles Layer 2 switching (MAC addresses).
- Implements port security and VLAN assignments.
- Can provide Power over Ethernet (PoE) for devices like IP phones and APs.
- Acts as the entry point to the network.
Devices used:
- Access switches (Layer 2 or Layer 3 switches)
- Sometimes wireless access points
Key Features:
| Feature | Description |
|---|---|
| VLANs | Used to segment network traffic for users/departments. |
| Port Security | Limits the number of MAC addresses on a port for security. |
| QoS (Quality of Service) | Prioritizes traffic, e.g., voice over data. |
| PoE | Supplies power to IP devices. |
| Link aggregation (EtherChannel) | Increases bandwidth and redundancy between switches. |
2️⃣ Distribution Layer
Purpose:
The Distribution Layer connects the Access Layer to the Core Layer.
It acts as a control boundary for the network and performs routing, filtering, and policy enforcement.
Functions:
- Provides inter-VLAN routing (communication between VLANs).
- Implements security policies like ACLs (Access Control Lists).
- Performs traffic filtering and aggregation from access switches.
- Acts as the communication point between access and core layers.
- Offers redundancy and load balancing.
Devices used:
- Layer 3 switches or routers
Key Features:
| Feature | Description |
|---|---|
| Routing (Layer 3) | Moves traffic between VLANs or subnets. |
| ACLs | Control which traffic can enter or leave certain VLANs. |
| Policy-based routing | Directs traffic based on rules (e.g., VoIP over faster links). |
| Redundancy protocols | HSRP, VRRP, GLBP provide gateway redundancy. |
| Load balancing | Spreads traffic evenly for better performance. |
3️⃣ Core Layer
Purpose:
The Core Layer is the high-speed backbone of the network.
It is responsible for fast and reliable transportation of data between different distribution layers, data centers, and external networks (like the internet).
Functions:
- Provides high-speed switching and routing.
- Ensures low latency and high availability.
- Connects distribution layers, data centers, and WAN routers.
- Focuses on performance and redundancy, not security or filtering.
Devices used:
- High-performance Layer 3 switches or core routers
Key Features:
| Feature | Description |
|---|---|
| High-speed links | Typically 10G, 40G, or higher. |
| Redundancy | Multiple paths between devices to prevent downtime. |
| Fast convergence | Rapid recovery from link or device failure. |
| Minimal filtering | Avoids ACLs or heavy processing to ensure speed. |
Summary of the Three Layers
| Layer | Function | Device Type | Main Features |
|---|---|---|---|
| Core | High-speed backbone | Core routers/switches | Speed, redundancy, reliability |
| Distribution | Policy control, inter-VLAN routing | L3 switches/routers | Routing, ACLs, load balancing |
| Access | Connects end devices | L2/L3 switches | VLANs, security, PoE |
Advantages of Three-Tier Architecture
| Benefit | Description |
|---|---|
| Scalability | Easy to add more devices without redesigning the entire network. |
| Performance | Traffic is efficiently handled at each layer. |
| Manageability | Easier to configure and troubleshoot due to clear layer functions. |
| Security | Policies and controls can be applied at the distribution layer. |
| Redundancy | Multiple links and paths ensure high availability. |
Where It’s Commonly Used in IT Environments
In enterprise and campus networks, especially medium to large organizations, this model is ideal:
- Access Layer: connects employees’ computers and VoIP phones on each floor.
- Distribution Layer: aggregates each floor’s access switches.
- Core Layer: connects distribution switches from different buildings and connects to the data center or internet routers.
Difference Between Two-Tier and Three-Tier
| Feature | Two-Tier (Collapsed Core) | Three-Tier |
|---|---|---|
| Layers | Access + Distribution (combined) | Access, Distribution, Core |
| Complexity | Simpler | More complex |
| Scalability | Suitable for small/medium networks | Suitable for large enterprise networks |
| Cost | Lower | Higher |
| Performance | Moderate | Very high |
Exam Tip for CCNA 200-301
✅ Remember these points:
- Three-tier = Access, Distribution, Core
- Access Layer: end-user connection, VLANs, security
- Distribution Layer: routing, ACLs, policies
- Core Layer: backbone, high-speed, redundancy
- Used in large enterprise or campus networks
- Two-tier (Collapsed Core) = combination of distribution and core layers for smaller networks
