Traditional and Modern Network Architectures

Cisco's own reference designs migrated from three-tier campus (access/distribution/core) to collapsed core starting around 2010. Starting around 2016, fabric-based SD-Access became the recommended pattern for new campus deployments. Each shift reduced the number of tiers, centralized more control, and increased software-defined behavior. Traditional designs aren't obsolete, but they're no longer what a new deployment should look like.

#Traditional Three-Tier Campus#link

The classic design. Access layer connects users, distribution layer does routing and policy, core layer moves packets between distribution blocks. Clean separation of concerns, but expensive — the core layer is dedicated hardware that only does high-speed transit.

#Collapsed Core#link

Modern campuses collapse distribution and core into one pair of switches. The core hardware is eliminated; routing happens in the collapsed-core pair. This is what most enterprises deploy today. Same logical separation (access for endpoints, collapsed core for routing and policy) but fewer physical devices.

#Spine-Leaf (Data Center)#link

For data centers, spine-leaf replaced three-tier. Every leaf connects to every spine. No leaf connects to another leaf. Traffic between any two leaves always goes leaf→spine→leaf — exactly two hops, predictable latency. Every link is used with Layer 3 ECMP routing. This is the modern data center standard.

#Comparison#link

#Architecture Decision#link