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Cisco MDS 9000 Family NX-OS Interfaces Configuration Guide
OL-29284-01, Release 6.x
Chapter 4 Configuring Interface Buffers
Information About Interface Buffers
Buffer-to-Buffer Credit Recovery
Although the Fibre Channel standards require low bit error rates, bit errors do occur. Over time, the
corruption of receiver-ready messages, known as R_RDY primitives, can lead to a loss of credits, which
can eventually cause a link to stop transmitting in one direction. The Fibre Channel standards provide a
feature for two attached ports to detect and correct this situation. This feature is called buffer-to-buffer
credit recovery.
Buffer-to-buffer credit recovery functions as follows: the sender and the receiver agree to send
checkpoint primitives to each other, starting from the time that the link comes up. The sender sends a
checkpoint every time it has sent the specified number of frames, and the receiver sends a checkpoint
every time it has sent the specified number of R_RDY primitives. If the receiver detects lost credits, it
can retransmit them and restore the credit count on the sender.
The buffer-to-buffer credit recovery feature can be used on any non arbitrated loop link. This feature is
most useful on unreliable links, such as MANs or WANs, but can also help on shorter, high-loss links,
such as a link with a faulty fiber connection.
Note The buffer-to-buffer credit recovery feature is not compatible with the distance extension (DE) feature,
also known as buffer-to-buffer credit spoofing. If you use intermediate optical equipment, such as
DWDM transceivers or Fibre Channel bridges, on ISLs between switches that use DE, then
buffer-to-buffer credit recovery on both sides of the ISL needs to be disabled.
Buffer-to-Buffer State Change Number
The BB_SC_N field (word 1, bits 15-12) specifies the buffer-to-buffer state change (BB_SC) number.
The BB_SC_N field indicates that the sender of the port login (PLOGI), fabric login (FLOGI), or ISLs
(E or TE ports) frame is requesting 2^SC_BB_N number of frames to be sent between two consecutive
BB_SC send primitives, and twice the number of R_RDY primitives to be sent between two consecutive
BB_SC receive primitives.
For Generation 2 and Generation 3 modules, the BB_SCN on ISLs (E or TE ports) is enabled by default.
This can fail the ISLs if used with optical equipment using distance extension (DE), also known as
buffer-to-buffer credit spoofing.
On a Generation-2 module, one port will not come up for the following configuration for all ports:
• Port Mode: auto or E for all the ports
• Rate Mode: dedicated
• Buffer Credits: default value
On a Generation-3 module, one or two ports will not come up for the following configuration for the first
half of the ports, the second half of the ports, or all ports:
• Port Mode: auto or E for the first half of the ports, the second half of the ports, or for all of the ports
• Rate Mode: dedicated
• Buffer Credits: default value
When you configure port mode to auto or E and rate-mode to dedicated for all ports in the global buffer
pool, you need to reconfigure buffer credits on one or more ports (other than default).
