Dell PowerConnect Brocade 300 Fabric OS Administrator's Guide v7.1.0 - Page 558

23 Buffer credit management TABLE 82 Fibre Channel data frames Fibre Channel frame fields Field size Start of frame 4 bytes Standard frame header 24 bytes Data (payload) 0-2,112 bytes CRC 4 bytes End of frame 4 bytes Total (number bits/frame) 36-2,148 bytes 32 bits 192 bits 0-16,896 bits 32 bits 32 bits 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. It is best practice 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 portCfgLongDistance command's desired_distance parameter 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 asa result of insufficient buffers. 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 Table 83 on page 563 and Table 84 on page 564 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 Table 84. 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. This example uses 50 km. 2. Determine the speed that you will use for the long-distance connection. This example uses 2 Gbps. 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 + 14 558 Fabric OS Administrator's Guide 53-1002745-02