Check whether new leaves fit the existing spines, and count the spine ports, cables and bandwidth they add.
Each new leaf consumes one port on every spine for each parallel uplink, so spine port count — not rack space — is what caps a two-tier fabric. When it runs out the options are a super-spine tier, fewer uplinks per leaf, or faster uplinks. Discovering the spines are full during a build weekend is the classic data-centre stall; this is the arithmetic that should be done at design review.
Fabric Expansion
ports used per spine = total leaves × uplinks per spine; added bandwidth = new leaves × spines × uplinks per spine × uplink speed.
ports used per spine = total leaves × uplinks per spine; added bandwidth = new leaves × spines × uplinks per spine × uplink speed. Each new leaf consumes one port on every spine for each parallel uplink, so spine port count — not rack space — is what caps a two-tier fabric. When it runs out the options are a super-spine tier, fewer uplinks per leaf, or faster uplinks.
Discovering the spines are full during a build weekend is the classic data-centre stall; this is the arithmetic that should be done at design review.
This calculator takes 6 inputs: Leaves today, Leaves being added, Spine switches, Uplinks from each leaf to each spine, Fabric ports available per spine, Uplink 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.