Port Channel Capacity Calculator
Size a LAG: aggregate bandwidth, capacity after a member fails and the single-flow ceiling hashing imposes.
Inputs
Aggregate Bandwidth
40,000Mbps
Bandwidth After Failure
30,000Mbps
Utilisation, All Members Up
55.00%
Utilisation After Failure
73.33%
Single-Flow Ceiling
10,000Mbps
Survives the Failure
Yes — 26.7% headroom remains
Step by step
Values used
Active member links = 4 links; Speed per member = 10,000 Mbps; Maximum members the platform bundles = 8 links; Members to survive losing = 1 links; Peak offered load = 22,000 Mbps
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 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
How it works
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
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.