Compare a classic three-tier data center design with a spine-leaf design.
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Three tier is access, aggregation and core. It was built for north-south traffic, runs layer 2 up to the aggregation pair, and relies on STP, which blocks half the links. Spine-leaf is a two-stage Clos: every leaf connects to every spine, any leaf reaches any other leaf in exactly two hops (leaf, spine, leaf), and all links forward at once through ECMP routing. You scale it out by adding leaves for ports and spines for bandwidth.
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Three tier
- Access switches connect servers, aggregation holds the SVIs, the FHRP and often the services (firewalls, load balancers), core routes between aggregation blocks and to the WAN.
- VLANs stretch from access to aggregation, so STP is in charge of loop prevention and blocks redundant uplinks. Half the bandwidth you paid for sits idle.
- Server-to-server traffic in different blocks climbs to the core and back down. Latency and available bandwidth depend on where the two servers sit.
- Oversubscription compounds at every tier, and growth means buying a bigger aggregation chassis (scale up).
Spine-leaf
- Two layers. Servers, firewalls, routers and storage connect only to leaves. Spines only connect leaves.
- Leaf to spine links are routed, usually /31 point-to-point. No STP between switches.
- Every leaf has an equal-cost path through every spine, so ECMP uses all uplinks.
- Any two servers on different leaves are the same distance apart. Latency is predictable.
- Losing one spine out of four removes 25% of fabric bandwidth, not half of a block.
- Layer 2 between racks, when you need it, is rebuilt on top as an overlay (VXLAN with BGP EVPN).
| Three tier | Spine-leaf | |
|---|---|---|
| Built for | North-south | East-west |
| Loop control | STP, blocked links | Routing, all links active |
| Hops leaf to leaf | Varies | Always two |
| Growth | Scale up chassis | Scale out with more boxes |
| Layer 2 reach | Native VLANs | Overlay (VXLAN) |
They will ask next
Is three tier wrong today?
No. A small or mostly north-south site can run well on a collapsed core or classic design. The pain shows up with heavy east-west traffic, large layer 2 domains and growth beyond one aggregation pair.
Where does the name Clos come from?
Charles Clos described multistage switching networks for telephone exchanges in the 1950s. A spine-leaf fabric is a folded Clos network: many small switches give the capacity of one huge switch.
Traps
- Saying spine-leaf "needs VXLAN". A pure layer 3 fabric with no overlay is perfectly valid.
- Connecting servers or firewalls to spines. It breaks the uniform two-hop property and complicates the spine.