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Buffer credit management

20

24 = the number of user ports in a port group retrieved from 

8 = the number of reserved credits for each user port.

676 = the number of buffer credits available in the port group.

If you allocate the entire 484 + 8 (8 for the reserved buffers already allocated to that user port) 
= 492 buffers to a single port, you can calculate the maximum single port extended distance 
supported:

[Maximum Distance X in km] = (BufferCredits + 6) * 2 / LinkSpeed

498 km = (492 + 6 buffers for Fabric Services) * 2 / 2 Gbps

How many 50 km ports can you configure?

If you have a distance of 50 km at 8 Gbps then, 484 / (206 – 8) = 2 ports

The numbers used are: 484, which equals the total number of unreserved buffer credits, 206, 
which equals buffer credits needed for 50 km @ 8 Gbps (calculated previously), and 8, which 
equals number of reserved buffer credits already allocated to that port. The floor of the 
resulting number is taken because fractions of a port are not allowed.

If you have a distance of 50 km at 1 Gbps then, 484 / (31 – 8) = 21 ports

Allocating buffer credits based on average-size frames

In cases where the frame size is average, for example 1024 bytes, you must allocate twice the 
buffer credits or give twice the distance in the long-distance LS configuration mode. Refer to the 

 to get an approximation of the calculated number 

of buffer credits.

1. Use the following formula to calculate value for the desired_distance needed for Fabric OS to 

determine the number of BB credits to allocate:

Where average_payload_size = 1024 bytes

This example uses 100 km for the real estimated distance.

desired_distance = roundup [(100 * 2112) / 1024] = 207

When configuring the LS mode with the portCfgLongDistance command, enter a 
desired_distance value of 207 for an actual 100 km link connected to an 8 Gbps E_Port. This 
causes Fabric OS to allocate the correct number of BB credits.

2. Determine the speed that you will use for the long-distance connection. This example uses 8 

Gbps.

3. Look up the data_rate value for the speed of the connection. See 

 on page 447 to determine the data_rate value.

For 8 Gbps, the data_rate is 8.5

4. Use the following formula to calculate the number of buffer-to-buffer credits to allocate:

Using the values for desired_distance and data_rate from 

, the value for BB 

credits is calculated as follows: