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22-19
Cisco MDS 9000 Family Fabric Manager Configuration Guide
OL-17256-03, Cisco MDS NX-OS Release 4.x
Chapter 22 Configuring Generation 2 and Generation 3 Switching Modules
Buffer Credit Allocation
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 (see Figure 22-10)
Figure 22-10 Example Speed and Rate Configuration on a 24-Port 4-Gbps Switching Module
18-Port Fibre Channel/4-Port Gigabit Ethernet Multiservice Module BB_Credit Buffers
Table 22-12 lists the BB_credit buffer allocation for 18-port 4-Gbps multiservice modules.
Maximum BB_credit buffers 250 250 16
Total number of BB_credits buffers per module 6000
1. ISL = E port or TE port.
Table 22-12 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
ISL
1
Fx Port Fx Port
4-Gbps 4-Gbps 4-Gbps 1-Gbps 1-Gbps 1-Gbps
Dedicated Dedicated Dedicated
Shared
Shared
Shared
1
23 456
144857
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