Port Channel Capacity Calculator
Size a LAG: aggregate bandwidth, capacity after a member fails and the single-flow ceiling hashing imposes.
About Port Channel Capacity Calculator
A LAG adds bandwidth but not per-flow speed: the load-balancing hash sends every packet of a flow down the same member to preserve ordering, so a single TCP session can never exceed one link. Sizing therefore needs both the aggregate figure and the post-failure figure. Bundles are routinely run above 50 percent, which looks fine until one member drops and the remainder tail-drops — the post-failure utilisation is the number that decides whether a fibre cut is an incident.
Formula
Port Channel Capacity
Aggregate = active members × member speed; after a failure it is (members − failures) × member speed, while any single flow is still capped at one member's speed.
- aggregate
- Sum of member link speeds — LACP bundles up to 8 active members by default
- single-flow ceiling
- One member's speed, because a hash pins a flow to one link
- survivors
- Members still up after the failures you plan for
Worked example
With the default values below, Port Channel Capacity Calculator returns Aggregate Bandwidth of 40,000. Change any input above and every figure here is recalculated in your browser.
Inputs used
- Active member linksaccepts 1–32 links
- 4 links
- Speed per memberminimum 1 Mbps
- 10,000 Mbps
- Maximum members the platform bundlesaccepts 1–32 links
- 8 links
- Members to survive losingaccepts 0–31 links
- 1 links
- Peak offered loadminimum 0 Mbps
- 22,000 Mbps
Results
- Aggregate Bandwidth
- 40,000 Mbps
- Bandwidth After Failure
- 30,000 Mbps
- Utilisation, All Members Up
- 55.00%
- Utilisation After Failure
- 73.33%
- Single-Flow Ceiling
- 10,000 Mbps
- Survives the Failure
- Yes — 26.7% headroom remains
Step by step
- 1Values used
- 2Port Channel Capacity
- 3Aggregate Bandwidth= 40,000 Mbps
- 4Bandwidth After Failure= 30,000 Mbps
- 5Utilisation, All Members Up= 55.00
- 6Utilisation After Failure= 73.33
- 7Single-Flow Ceiling= 10,000 Mbps
- 8Survives the Failure= Yes — 26.7% headroom remains
Assumptions and limits
- The result is derived only from the 5 inputs listed above. Anything not among them is not part of the calculation.
- In particular, this calculator does not take inflation, tax or compounding frequency into account.
- Figures are rounded for display to 2 decimal places, 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 Port Channel Capacity calculated?
Aggregate = active members × member speed; after a failure it is (members − failures) × member speed, while any single flow is still capped at one member's speed. A LAG adds bandwidth but not per-flow speed: the load-balancing hash sends every packet of a flow down the same member to preserve ordering, so a single TCP session can never exceed one link. Sizing therefore needs both the aggregate figure and the post-failure figure.
Why does Port Channel Capacity matter?
Bundles are routinely run above 50 percent, which looks fine until one member drops and the remainder tail-drops — the post-failure utilisation is the number that decides whether a fibre cut is an incident.
What values do I need to enter?
This calculator takes 5 inputs: Active member links, Speed per member, Maximum members the platform bundles, Members to survive losing, Peak offered load. The pre-filled defaults are a realistic starting point — replace them with figures from your own environment for a result you can act on.
Why is my 4×10G bundle not giving one server 40 Gbps?
Because hashing is per-flow. A single flow is capped at 10 Gbps; to fill the bundle you need at least as many flows as members, and ideally many more so the hash distributes evenly.