Collapsed Core Cisco Architecture

Course Contents

1. What is Collapsed Core Cisco Architecture?

The Collapsed Core Cisco Architecture, also called the Two-Tier Architecture, is a network design model commonly used for small to medium-sized enterprise networks.

It simplifies the network structure by reducing the number of layers compared to the traditional Three-Tier Architecture.

Here’s what a typical Collapsed Core Cisco network looks like;

Collapsed Core Cisco network architecture diagram showing core and access layers with switch connections

As you can see the Collapsed Core Architecture is made up of two layers:

Collapsed Core Layer
Access Layer

To help you understand this architecture, let’s compare it to a traditional three-layer design:

Comparison between Three-Tier Architecture and Collapsed Core Cisco design showing separate and merged network layers

On the left: Three-Tier Architecture separates the network into Core, Distribution, and Access layers.

On the right: Collapsed Core Cisco Architecture combines the Core and Distribution layers into a single layer.

This new design reduces the number of layers, simplifying the network.

This leads to an important question:

Cartoon character asking about the advantages of the Collapsed Core Cisco architecture

It’s a good question, let’s break it down together !

The Collapsed Core Cisco Architecture offers several key advantages, but it also has some limitations that you need to consider:

Advantages	Limitations
✅ Simplified design: easier to deploy and manage.	❌ Limited scalability: not ideal for very large networks.
✅ Cost-effective: no need for expensive dedicated core switches.	❌ May cause bottlenecks: if too many access switches are added.

When is Collapsed Core Architecture a good choice ?

This kind of design is perfect for:

Small and medium-sized businesses that don’t expect massive network growth.
Simple campus networks with a limited number of users and devices.
Organizations on a tighter budget that want solid performance without overspending.

In short: if you want a network that’s easy to build, affordable and manageable, the Collapsed Core Cisco Architecture is a great fit.

Now that you have the big picture, let’s dive into the details, starting with the Access Layer.

2. Access Layer

If you’ve already heard about the Access Layer in the Three-Tier Architecture, you’ll find it’s pretty much the same here.

But don’t worry, if that’s new for you let’s review the purpose of the Access Layer :

Collapsed Core Cisco diagram showing access layer switches connected to core switches in a two-tier network design

The Access Layer is where users connect their devices such as laptops, IP phones, printers or wireless access points to the network.

It’s the front door of the network and it handles a few essential jobs:

Physical network access links to plug in endpoints and Wi-Fi access points.
Power over Ethernet (PoE) to provide power directly over the network cables for devices like IP phones and wireless APs.
VLANs to logically segment the network.
Spanning Tree Protocol (STP/RSTP) to prevent switching loops.

On top of that, the Access Layer plays a major role in security:

Port Security to control which devices can connect to the network based on their MAC addresses.
DHCP Snooping to block unauthorized DHCP servers.
Dynamic ARP Inspection (DAI) to protect against ARP spoofing attacks.

And finally, it’s at the Access Layer that we set up the Quality of Service (QoS) trust boundary marking and classifying traffic right at the edge to make sure important data like voice and video get priority across the network.

As you can see, this layer heavily relies on key Layer 2 technologies !

Now that we’ve seen how end devices connect through the Access Layer, let’s look at how the Collapsed Core handles aggregation and routing.

3. Collapsed Core Layer

In the Collapsed Core Cisco Architecture, the Collapsed Core Layer merges two traditional layers you might also know: the Distribution Layer and the Core Layer.

Comparison of Collapsed Core Cisco architecture and traditional three-tier network model showing merged core and distribution layers

That’s why we call it collapsed: we merge two layers into one to simplify the network.

Responsibilities of the Collapsed Core Layer

The Collapsed Core Layer plays several critical roles:

Collapsed Core Cisco diagram showing Layer 3 core switches and Layer 2 access switches in a two-tier network design

Aggregates traffic coming from all Access Layer switches.

Routes traffic between different VLANs.

Provides exit points for traffic heading to the WAN, Internet, or other external networks.

Transitions traffic from Layer 2 (Access Layer) to Layer 3 (routed network).

In short: it is both a high-speed backbone and a routing layer for the entire network.

Collapsed Core Cisco network diagram showing SVI gateways, HSRP redundancy, and STP for loop prevention between access and core layers

Technologies Used in the Collapsed Core Layer

Let’s dive into the key technologies you’ll find at this layer:

Technology	Purpose
SVIs (Switched Virtual Interfaces)	Act as default gateways for VLANs; allow inter-VLAN communication.
OSPF (Open Shortest Path First)	Dynamic routing protocol used between Collapsed Core switches; ensures fast and resilient route computation.
HSRP / VRRP / GLBP	Provide gateway redundancy; ensure failover in case a switch goes down.
STP (Spanning Tree Protocol)	Active between Access and Core layers; prevents Layer 2 loops by blocking redundant paths.

4. Conclusion

To conclude, remember that the Collapsed Core Cisco Architecture is ideal for small to medium-sized enterprises looking to reduce networking costs without sacrificing an efficient network design.

However, keep in mind that this architecture has its drawbacks, particularly its limited scalability which makes it less suitable for very large or rapidly growing networks.