Users Guide

Fabric OS Administrator’s Guide 139
53-1002920-02
Buffer credit management
5
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.
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 = 56 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 Table 15 on page 143
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.