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Fabric OS Administrator's Guide v6.4.0 (53-1001763-01, June 2010)

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Fabric OS Administrator’s Guide 67
53-1001763-01
Inter-switch links
4
There are non-fabric parameters that must match as well, such as zoning. Some fabric services,
such as Management Server must match. If it is enabled in the fabric, then the switch you are
introducing into the fabric must also have it enabled. If you experience a segmented fabric, refer to
the Fabric OS Troubleshooting and Diagnostics Guide to fix the problem.
Buffer credits
In order to prevent the dropping of frames in the fabric, a device can never send frames without the
receiving device being able to receive them, so an end-to-end flow control is used on the switch.
Flow control in Fibre Channel uses buffer-to-buffer credits which are distributed by the switch.
When all buffer-to-buffer credits are utilized, a device will wait for a VC_RDY or an R_RDY primitive
from the destination switch before resuming I/O. The primitive is dependent on whether you have
R_RDYs enabled on your switch using the portCfgISLMode command. When a device logs into a
fabric, it typically requests anywhere from two to sixteen buffer credits from the switch, depending
on device type, driver version, and configuration. This determines the maximum number of frames
the port may transmit before receiving an acknowledgement from the receiving device.
For more information on how to set the buffer-to-buffer credits on an extended link, refer to Chapter
20, “Managing Long Distance Fabrics”.
Virtual Channels
Virtual channels create multiple logical data paths across a single physical link or connection. They
are allocated their own network resources such as queues and buffer-to-buffer credits. Virtual
channel technology is the fundamental building block used to construct Adaptive Networking
services. For more information on Adaptive Networking services, refer to Chapter 18, “Optimizing
Fabric Behavior”.
Virtual channels are divided into three priority groups. P1 is the highest which is used for Class F,
F_RJT, and ACK traffic. P2 is the next highest which is used for data frames. The data virtual
channels can be further prioritized to provide higher levels of Quality of Service. P3 is the lowest
and is used for broadcast and multicast traffic.