> # 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).

> Note If you use distance extension (buffer-to-buffer credit spoofing) on ISLs between switches, the BB_SCN parameter on both sides of the ISL needs to be disabled.

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