Dell PowerConnect Brocade 300 Fabric OS Administrator's Guide v7.1.0 - Page 115

Inter-switch links 4 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 waits 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 in to a fabric, it typically requests from two to sixteen buffer credits from the switch, depending on the device type, driver version, and configuration. This determines the maximum number of frames the port can 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 23, "Managing Long-Distance Fabrics". Congestions versus over-subscription Congestions occurs when a channel is bottlenecked and fully utilized. This kind of bottleneck is a congestion bottleneck. You should be aware that "over-subscription" does not have the same meaning as "congestion". Over-subscription refers only to the potential for congestion; an over-subscribed link may go through a lifetime of normal operation and never be congested. The term over-subscription is not to be used in place of congestion, which is the actual contention for bandwidth by devices through an ISL. 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 21, "Optimizing Fabric Behavior". Virtual channels are divided into three priority groups. P1 is the highest priority, which is used for Class F, F_RJT, and ACK traffic. P2 is the next highest priority, 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 priority and is used for broadcast and multicast traffic. This example is illustrated in Figure 7. Quality of Service (QoS) is a licensed traffic shaping feature available in Fabric OS. QoS allows the prioritization of data traffic based on the SID and DID of each frame. Through the use of QoS zones, traffic can be divided into three priorities: high, medium, and low, as shown in Figure 7. The seven data virtual channels (VC8 through VC14) are used to multiplex data frames based upon QoS zones when congestion occurs. For more information on QoS zones, refer to Chapter 21, "Optimizing Fabric Behavior". Fabric OS Administrator's Guide 115 53-1002745-02