Refer to – Brocade Fabric OS Administrators Guide (Supporting Fabric OS v7.3.0) User Manual
Page 121
14 = The number of buffer credits reserved for QoS. This number is static.
Using 50 km as the desired distance of the switch-to-switch connection and 2 Gbps as the speed of
the long-distance connection, insert the numbers into the appropriate formula. The formula should
read as follows:
(50 km * 2 Gbps / 2) + 6 = 5 6 buffers, which is the number of buffers reserved for distance.
The following examples use different speeds, all based on a distance of 50 km. The distances and
speeds are variables that can change depending on how your network is set up.
• If you have a distance of 50 km at 1 Gbps, then (50 km * 1 Gbps / 2) + 6 = 31 buffers.
• If you have a distance of 50 km at 2 Gbps, then (50 km * 2 Gbps / 2) + 6 = 56 buffers.
• If you have a distance of 50 km at 4 Gbps, then (50 km * 4 Gbps / 2) + 6 = 106 buffers.
• If you have a distance of 50 km at 8 Gbps, then (50 km * 8 Gbps / 2) + 6 = 206 buffers.
• If you have a distance of 50 km at 10 Gbps, then (50 km * 10 Gbps / 2) +6 = 256 buffers.
• If you have a distance of 50 km at 16 Gbps, then (50 km * 16 Gbps / 2) + 6 = 406 buffers.
Example
Consider the Brocade 300, which has a single 24-port port group and a total of 676 buffer credits for
that port group. The formulas use the following parameters:
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
The maximum remaining number of buffer credits for the port group, after each port reserves its 8
buffer credits, is obtained from the following formula:
676 - (24 * 8) = 484 unreserved buffer credits
492 buffers to a single port (484 + 8 [8 for the reserved buffers already allocated to that user 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
If you have a distance of 50 km at 8 Gbps, then 484 / (206 - 8) = 2 ports.
The following values are used in the example:
• 484 -- The total number of unreserved buffer credits
• 206 -- Buffer credits needed for 50 km at 8 Gbps
• 8 -- The number of reserved buffer credits already allocated to that port
The resulting number is rounded down to the next whole number 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 1,024 bytes, you must allocate twice the buffer
credits or configure twice the distance in the long-distance LS configuration mode. Refer to
Channel gigabit values reference definition
on page 119 for an approximation of the calculated number
of buffer credits.
1. Use the following formula to calculate the value for the desired_distance parameter needed for
Fabric OS to determine the number of buffer credits to allocate:
desired_distance = roundup [(real_estimated_distance * 2112) / average_payload_size]
Allocating buffer credits based on average-size frames
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