FC switch port Buffer Credit requirements for long distance calculations
You can calculate how many ports can be configured for long distance on all switch modules or ASICs
except Bloom-based switches. For information on the port, speed and distance for Bloom-based ASICs, see
Table
82. Following are the considerations for the calculation:
•
Each user port reserves eight buffers when they are not online.
•
Remaining buffers can be reserved by any port in the port group.
•
When QoS is enabled and the port is online, an additional 14 buffers are allocated.
•
Condor-based switches have a limitation of 255 buffers maximum, which can be allocated by a port;
this limits the distance of ~500km at 1G.
•
For LD, distance in km is the smaller of the distance measured during port initialization versus the
desired distance value.
•
For LS, distance in km is always the desired distance value.
Before you can calculate the buffer requirement, note the following Fibre Channel gigabit values reference
definition:
• 1.0625 for 1 Gbps
• 2.125 for 2 Gbps
• 4.25 for 4 Gbps
• 8.5 for 8 Gbps
Determining how many ports can be configured for long distance
NOTE:
The following formula is used to determine the number of buffer credits:
buffer credits = [(distance in km) * (data rate) * 1000] / 21 12, the maximum frame size
1.
Determine the distance in kilometers between switch-to-switch. For this example we will use 50km.
2.
Determine the speed that you will use for the long distance connection. For this example, we will use 2
Gbps.
3.
Use the following formula to get the reserved buffers for distance:
Reserved Buffer for Distance Y = (X * LinkSpeed / 2 ) + 6
Where:
X = the distance determined in step 1.
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 is a
static number.
Based on the answers provided in steps 1 and 2, plug the numbers into the formula. The formula should
read as follows:
(50km * 2 Gbps / 2 ) + 6 = 56 buffers, which is the number of buffers reserved for distance
Below are additional examples using different speeds all base on a distance of 50km. The distances and
speeds are variables which can change based on how your network is set up.
If you have a distance of 50km at 1 Gbps then, (50km * 1 Gbps / 2) + 6 = 31 buffers
If you have a distance of 50km at 2 Gbps then, (50km * 2 Gbps / 2) + 6 = 56 buffers
If you have a distance of 50km at 4Gbps then, (50km * 4 Gbps/ 2) + 6 = 106 buffers
If you have a distance of 50km at 8 Gbps then, (50km * 8 Gbps/ 2) + 6 = 206 buffers
Fabric OS 6.x administrator guide 365