HP StoreFabric SN6500B Administrator's Manual page 559

Fabric os administrator's guide, 7.1.0 (53-1002745-02, march 2013)
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Below are additional examples using 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:
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 maximum remaining number of buffer credits for the port group, after each
port reserves its eight buffer credits, is as follows:
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:
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 as follows:
Fabric OS Administrator's Guide
53-1002745-02
If QoS is not enabled:
(Reserved Buffer for Distance Y) = (X * LinkSpeed / 2) + 6
where
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.
With the answers provided in steps 1 and 2, 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.
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
676 – (24 * 8) = 484 unreserved buffer credits
where
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
Maximum Distance X (in km) = (BufferCredits + 6) * 2 / LinkSpeed
498 km = (492 + 6 buffers for Fabric Services) * 2 / 2 Gbps
484 — the total number of unreserved buffer credits
206 — buffer credits needed for 50 km @ 8 Gbps (as calculated previously
Buffer credit management
Table 83
on page 563
23
559

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