Brocade Fabric OS Administrators Guide (Supporting Fabric OS v7.3.0) User Manual
Page 120
Fibre Channel data frames (Continued)
TABLE 16
Fibre Channel frame fields
Field size
Final frame size
End of frame
4 bytes
32 bits
Total (number bits/frame)
36-2,148 bytes
288-7,184 bits
Allocating buffer credits based on full-sized frames
You can allocate buffer credits based on distance using the portCfgLongDistance command. The
long-distance link modes allow you to select the dynamic mode (LD) or the static mode (LS) to
calculate the buffer credits.
For LD, the estimated distance in kilometers is the smaller of the distance measured during port
initialization versus the desired_distance parameter, which is required when a port is configured as an
LD or an LS mode link. A best practice is to use LS over LD. The assumption that Fibre Channel
payloads are consistently 2,112 bytes is not realistic in practice. To gain the proper number of buffer
credits with the LS mode, there must be enough buffer credits available in the pool, because Fabric
OS will check before accepting a value.
NOTE
The desired_distance parameter of the portCfgLongDistance command’s is the upper limit of the link
distance and is used to calculate buffer availability for other ports in the same port group. When the
measured distance exceeds the value of desired_distance, this value is used to allocate the buffers. In
this case, the port operates in degraded mode instead of being disabled as a result of insufficient
buffer availability. In LS mode, the actual link distance is not measured; instead, the desired_distance
value is used to allocate the buffers required for the port.
Refer to the data in
on page 125 and
on page 126 to get the total ports in a switch
or blade, the number of user ports in a port group, and the unreserved buffer credits available per port
group. The values reflect an estimate, and may differ from the supported values in
on page
126.
Calculating the number of buffers required based on full-size frames
Use the following procedure to calculate the number of buffers required for a long-distance connection:
1. Determine the desired distance in kilometers of the switch-to-switch connection.
2. Determine the speed that you will use for the long-distance connection.
3. Use one of the following formulas to calculate the reserved buffers for distance:
• If QoS is enabled:
(Reserved_Buffer_for_Distance_Y) = (X * LinkSpeed / 2) + 6 + 14
• If QoS is not enabled:
(Reserved_Buffer_for_Distance_Y) = (X * LinkSpeed / 2) + 6
The formulas use the following parameters:
X = The distance determined in step 1 (in km).
LinkSpeed = The speed of the link determined in step 2.
6 = The number of buffer credits reserved for fabric services, multicast, and broadcast traffic. This
number is static.
Allocating buffer credits based on full-sized frames
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Fabric OS Administrators Guide
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