> # Extended BB_credits on Generation 1 Switching Modules

> The BB_credits feature allows you to configure up to 255 receive buffers on Generation 1 switching modules. To facilitate BB_credits for long haul links, you can configure up to 3,500 receive BB_credits on a Fibre Channel port on a Generation 1 switching module.

> To use this feature on Generation 1 switching modules, you must meet the following requirements:

> - Obtain the ENTERPRISE_PKG license. See the Cisco MDS 9000 Family NX-OS Licensing Guide.
> - Configure this feature in any port of the full-rate 4-port group in either the Cisco MDS 9216i Switch or in the MPS-14/2 module .

> The port groups that support extended credit configurations are as follows:

> - Any one port in ports 1 to 4 (identified as Group 1).
> - Any one port in ports 5 to 8 (identified as Group 2).
> - Any one port in ports 9 to 12 (identified as Group 3).

> Note The last two Fibre Channel ports (port 13 and port 14) and the two Gigabit Ethernet ports do not support the extended BB_credits feature.

> - Explicitly enable this feature in the required Cisco MDS switch.
> - Disable the remaining three ports in the 4-port group if you need to assign more than 2,400 BB_credits to the first port in the port group.
> - If you assign less than 2,400 extended BB_credits to any one port in a port group, the remaining three ports in that port group can retain up to 255 BB_credits based on the port mode.

> Note The receive BB_credit value for the remaining three ports depends on the port mode. The default value is 16 for the Fx mode and 255 for E or TE modes. The maximum value is 255 in all modes. This value can be changed as required without exceeding the maximum value of 255 BB_credits.

> - If you assign more than 2,400 (up to a maximum of 3,500) extended BB_credits to the port in a port group, you must disable the other three ports.
> - If you change the BB_credit value the port is disabled, and then reenabled.
> - Disable (explicitly) this feature if you need to nondisruptively downgrade to Cisco SAN-OS Release 1.3 or earlier. When you disable this feature, the existing extended BB_credit configuration is completely erased.

> Note The extended BB_credit configuration takes precedence over the receive BB_credit and performance buffer configurations.

> Copyright 2010-2013, Cisco Systems, Inc. All rights reserved.