BB Credit Calculator
Calculate Fibre Channel BB credits for ISL links based on speed and distance
How to use BB Credit Calculator
- 1Enter the link line rate in Gbps (e.g., 16, 32, 64).
- 2Enter the round-trip time between the two switches in milliseconds.
- 3Read the recommended BB_Credit buffer-to-buffer value.
Fibre Channel BB_Credit sizing
BB_Credit ≈ RTT(ms) × lineRate(Gbps) / 10BB_Credit is the number of buffer-to-buffer credits a Fibre Channel port advertises so the link stays full without waiting for ACKs.
Too few credits throttles long-distance links; the formula sizes credits so the in-flight data fills the pipe during one round trip.
Frequently asked questions
What happens with too few credits?
The sender stalls waiting for ACK, leaving the link underutilized on long distances.
Is more always better?
Excess credits waste switch buffer memory; size to the actual RTT, not the maximum.
Does this apply to TCP too?
TCP uses a similar window/BDP concept; FC BB_Credit is the storage-network equivalent.
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