> # 24-Port 4-Gbps Fibre Channel Module BB_Credit Buffers

> Table 14-6 lists the BB_credit buffer allocation for 24-port 4-Gbps Fibre Channel switching modules.

> Table 14-6 24-Port 4-Gbps Switching Module BB_Credit Buffer Allocation
> BB_Credit Buffer Allocation | BB_Credit Buffers Per Port
> --- | ---
> Dedicated Rate Mode 4-Gbps Speed | Shared Rate Mode 4-Gbps Speed
> ISL1 | Fx Port | Fx Port
> Default BB_credit buffers | 250 | 16 | 16
> Maximum BB_credit buffers | 250 | 250 | 16
> Total number of BB_credits buffers per module | 6000

> 1ISL = E port or TE port.

> The following considerations apply to BB_credit buffers on 24-port 4-Gbps Fibre Channel switching modules:

> - BB_credit buffers for ISL connections can be configured from a minimum of 2 buffers to a maximum of 250 buffers for dedicated rate mode or 16 buffers for shared rate mode.
> - BB_credit buffers for Fx port mode connections can be configured. The minimum is 2 buffers and the maximum of 250 buffers for dedicated rate mode or 16 buffers for shared rate mode.
> - Performance buffers are not supported on this module.

> Each port group on the 24-port 4-Gbps Fibre Channel switching module consists of six ports. The ports in shared rate mode have a bandwidth oversubscription of 2:1 by default. However, some configurations of the shared ports in a port group can have a maximum bandwidth oversubscription of 4:1 (considering that each port group has 12.8-Gbps bandwidth).

> The following example configurations are supported by the 24-port 4-Gbps Fibre Channel switching modules:

> - Six ports with shared rate mode and 4-Gbps speed (2:1 oversubscription) (default)
> - Two ports with dedicated rate mode and 4-Gbps speed plus
>   four ports with shared rate mode and 4-Gbps speed (with 4:1 oversubscription)
> - One port with dedicated rate mode and 4-Gbps speed plus
>   three ports with dedicated rate mode and 2-Gbps speed plus
>   two ports with shared rate mode and 4-Gbps speed (4:1 oversubscription)
> - Six ports with dedicated rate mode and 2-Gbps speed
> - Three ports with dedicated rate mode and 4-Gbps speed plus
>   three ports with shared rate mode and 1-Gbps speed

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