Buffer-To-Buffer Credit Recovery - Cisco MDS 9000 Series Configuration Manual

Configuring Interface Buffers
Note The ENTERPRISE_PKG license is required to use extended buffer-to-buffer credits on 4-Gbps, 8-Gbps,
advanced 8-Gbps, 16-Gbps, and 32-Gbps switching modules. Also, extended buffer-to-buffer credits are not
supported by ports in shared rate mode.
All ports on the 4-Gbps, 8-Gbps, 16-Gbps, and 32-Gbps switching modules support extended buffer-to-buffer
credits. There are no limitations for how many extended buffer-to-buffer credits you can assign to a port
(except for the maximum and minimum limits). If necessary, you can configure interfaces to use minimum
credits to make more extended buffer-to-buffer credits available to other ports.

Buffer-to-Buffer Credit Recovery

Although Fibre Channel standards require low bit and frame error rates, there is a likelihood of errors occuring.
When these errors affect certain Fibre Channel primitives, credit loss might occur. When credits are lost,
performance degradation might occur. When all credits are lost, transmission of frames in that direction stops.
The Fibre Channel standards introduces a feature for two attached ports to detect and correct such scenarios
nondisruptively. This feature is called buffer-to-buffer credit recovery.
A credit can be lost in either of these scenarios:
• An error corrupts the start-of-frame (SoF) delimiter of a frame. The receiving port fails to recognize the
• An error corrupts an R_RDY primitive. The receiving port fails to recognize the R_RDY and does not
The Buffer-to-Buffer Credit Recovery feature can help recover from the two specified scenarios. It is a per-hop
feature and is negotiated between two directly attached peer ports when the link comes up, by exchanging
parameters. Buffer-to-buffer credit recovery is enabled when a receiver acknowledges a nonzero buffer-to-buffer
state change number (BB_SC_N).
Buffer-to-buffer credit recovery functions as follows:
1. The local port and peer port agree to send checkpoint primitives to each other for frames and R_RDYs,
starting from the time the link comes up.
2. If a port detects frame loss, it sends the corresponding number of R_RDYs to replenish the lost credits at
the peer port.
3. If a port detects R_RDY loss, the port internally replenishes the lost credits to the interface buffer pool.
Buffer-to-buffer credit recovery implementation is as follows:
1. Buffer-to-buffer state change SOF (BB_SCs) primitives are transmitted every 2
sent. This enables an attached port to determine if any frames are lost. If frames loss is detected, the
receiver of the BB_SCs transmits the appropriate number of R_RDYs to compensate for the lost frames.
2. Buffer-to-buffer state change R_RDY (BB_SCr) primitives are transmitted every 2
R_RDY primitives sent. This enables an attached port to determine if any R_RDY primitives are lost. If
R_RDY primitive loss is detected, the receiver of the BB_SCr increments the number of transmit credits
by the appropriate number to compensate for the lost R_RDYs.
frame and subsequently does not send a corresponding receiver ready (R_RDY) primitive to the sender.
The sending port does not replenish the credit to the receiving port.
replenish the corresponding credit to the sending port.
Cisco MDS 9000 Series Interfaces Configuration Guide, Release 8.x
Buffer-to-Buffer Credit Recovery
BB_SC_N
number of frames
BB_SC_N
number of
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