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Leaf-Spine Capacity Calculator

Size a leaf-spine fabric: total fabric bandwidth, per-leaf uplink capacity and the resulting oversubscription.

About Leaf-Spine Capacity Calculator

In a two-tier Clos every leaf connects to every spine, so leaf uplink capacity is spines multiplied by link speed multiplied by links per spine, and the fabric total is that figure across all leaves. Comparing it with the access ports on a leaf gives the oversubscription the design accepts. Fabric bandwidth and oversubscription decide whether east-west traffic congests under load, and spine port count is the hard limit on how many leaves the fabric can ever hold.

Formula

Leaf-Spine Capacity

fabric bandwidth = leaves × uplinks per leaf × uplink speed; oversubscription = access capacity per leaf ÷ uplink capacity per leaf.

uplinks per leaf
Spines × links to each spine
oversubscription
Access bandwidth divided by uplink bandwidth, quoted as x:1

Worked example

With the default values below, Leaf-Spine Capacity Calculator returns Total Fabric Bandwidth of 12,800. Change any input above and every figure here is recalculated in your browser.

Inputs used

Leaf switchesaccepts 1–512 switches
16 switches
Spine switchesaccepts 1–64 switches
4 switches
Uplinks from each leaf to each spineaccepts 1–16 links
2 links
Uplink speedaccepts 1–800 Gbps
100 Gbps
Access ports per leafaccepts 1–512 ports
48 ports
Access port speedaccepts 1–800 Gbps
25 Gbps

Results

Total Fabric Bandwidth
12,800 Gbps
Uplink Capacity per Leaf
800 Gbps
Oversubscription Ratio
1.5:1
Total Access Capacity
19,200 Gbps
Ports Used on Each Spine
32

Step by step

  1. 1Values used
  2. 2Leaf-Spine Capacity
  3. 3Total Fabric Bandwidth= 12,800 Gbps
  4. 4Uplink Capacity per Leaf= 800 Gbps
  5. 5Oversubscription Ratio= 1.5:1
  6. 6Total Access Capacity= 19,200 Gbps
  7. 7Ports Used on Each Spine= 32

Assumptions and limits

  • The result is derived only from the 6 inputs listed above. Anything not among them is not part of the calculation.
  • In particular, this calculator does not take inflation, tax, fees or charges or compounding frequency into account.
  • Figures are rounded for display to whole numbers, so totals built from several rounded parts can differ slightly from the unrounded arithmetic.

How these calculators are built and tested is described on the Methodology page. If a figure looks wrong, please report it.

Frequently Asked Questions

How is Leaf-Spine Capacity calculated?

fabric bandwidth = leaves × uplinks per leaf × uplink speed; oversubscription = access capacity per leaf ÷ uplink capacity per leaf. In a two-tier Clos every leaf connects to every spine, so leaf uplink capacity is spines multiplied by link speed multiplied by links per spine, and the fabric total is that figure across all leaves. Comparing it with the access ports on a leaf gives the oversubscription the design accepts.

Why does Leaf-Spine Capacity matter?

Fabric bandwidth and oversubscription decide whether east-west traffic congests under load, and spine port count is the hard limit on how many leaves the fabric can ever hold.

What values do I need to enter?

This calculator takes 6 inputs: Leaf switches, Spine switches, Uplinks from each leaf to each spine, Uplink speed, Access ports per leaf, Access port speed. The pre-filled defaults are a realistic starting point — replace them with figures from your own environment for a result you can act on.

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