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Systems & Data Center · Advanced

Buffer-to-Buffer Credits (BB credits)

The Fibre Channel flow control mechanism where a sender may transmit only as many frames as the receiver has advertised buffers for.

Definition

Each received frame returns a credit (R_RDY). Long-distance links need more credits to keep the pipe full, and credit starvation from slow-drain devices can congest a fabric.

Example

Increasing BB credits on a 10 km ISL maintains full throughput.

Exam tip

Slow-drain devices that hold credits are a classic FC fabric congestion cause.

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