Campus and data centre architectures: three-tier, collapsed core and spine-leaf
In short: Campus networks use a three-tier or collapsed-core hierarchy built for users. Data centres, dominated by server-to-server traffic, use spine-leaf fabrics where every leaf connects to every spine, giving equal, predictable paths. Overlays such as VXLAN stretch Layer 2 segments across a routed fabric.
Two very different jobs
A campus network connects people: laptops, phones, printers and Wi-Fi, mostly talking north-south to the internet, cloud and data centre. A data centre network connects servers that mostly talk to each other (east-west traffic): web servers to databases, storage to compute. Different traffic patterns need different architectures.
Campus designs
- Three-tier (access, distribution, core): for large campuses with many buildings. Each building has a distribution pair; the core joins them. See the hierarchical model.
- Collapsed core (two-tier): one pair of switches acts as both core and distribution. Ideal for a single building or small campus: fewer devices, lower cost, simpler to run.
- Routed access: routing extends down to the access switches, removing spanning tree from uplinks and giving faster recovery.
Why data centres moved to spine-leaf
Traditional three-tier data centres relied on spanning tree, which blocks redundant links, and had paths of uneven length. Virtualisation and microservices multiplied east-west traffic, so a flatter design took over.
- Leaf switches connect servers, storage and firewalls. Spine switches connect only to leaves.
- Links are routed (Layer 3) and all active, using ECMP (equal-cost multipath) instead of spanning tree.
- To grow, add leaves for more ports or spines for more bandwidth, without redesigning.
Oversubscription and overlays
Oversubscription compares the bandwidth servers could send with the uplink capacity. A leaf with 48 × 25 Gbps server ports (1,200 Gbps) and 6 × 100 Gbps uplinks (600 Gbps) is 2:1 oversubscribed. Around 3:1 is common for general workloads; storage and AI clusters aim for 1:1.
Applications sometimes still need Layer 2 between racks. An overlay such as VXLAN wraps Layer 2 frames in UDP so they cross the routed underlay; EVPN (using BGP) distributes MAC and IP reachability so it scales. VXLAN offers about 16 million segment IDs, compared with 4,094 VLANs.
Choosing an architecture
| Situation | Good fit |
|---|---|
| Small office or single building | Collapsed core |
| Large multi-building campus | Three-tier, often with routed access |
| Server room growing beyond a few racks | Small spine-leaf (2 spines, a few leaves) |
| Large or multi-tenant data centre | Spine-leaf with VXLAN EVPN |
Real-world example: one company, two architectures
A fictional insurance company runs a four-storey head office and a small private data centre in the basement. The office uses a collapsed core design: access switches on each floor connect to a pair of core switches that also route between VLANs, because only about 400 users work there. The data centre is different. Its servers run databases and virtual machines that talk to each other all day, so the team built a small spine-leaf fabric with two spine switches and four leaf switches. Every server is two hops from any other, and adding a rack only means adding a leaf pair. A firewall pair sits between the office core and the data centre fabric.
Practice
What kind of traffic dominates a modern data centre?
A leaf has 40 x 10 Gbps server ports and 4 x 40 Gbps uplinks. What is the oversubscription?
Why does spine-leaf not need spanning tree on its links?
Key takeaways
- Campus networks serve users (north-south); data centres serve servers (east-west).
- Use a collapsed core for small sites and three-tier for large campuses.
- Spine-leaf: every leaf connects to every spine, with routed ECMP links.
- Oversubscription = server bandwidth divided by uplink bandwidth.
- VXLAN overlays carry Layer 2 over a routed underlay; EVPN makes it scale.
Original content written by Mohamed Fayas for FM TecHub. © 2026 FM TecHub. You're welcome to share a link to this page; please don't copy the text elsewhere without permission.