Step Task Command Command Mode1,3 or pfc no-drop queues 2-3 Default: No lossless queuesare configured.Priority-Based Flow Control Using Dynamic BufferMethodIn a data center network, priority-based flow control (PFC) manages large bursts of one traffic type in multiprotocol links so that it does notaffect other traffic types and no frames are lost due to congestion. When PFC detects congestion on a queue for a specified priority, itsends a pause frame for the 802.1p priority traffic to the transmitting device.Pause and Resume of TrafficThe pause message is used by the sending device to inform the receiving device about a congested, heavily-loaded traffic state that hasbeen identified. When the interface of a sending device transmits a pause frame, the recipient acknowledges this frame by temporarilyhalting the transmission of data packets. The sending device requests the recipient to restart the transmission of data traffic when thecongestion eases and reduces. The time period that is specified in the pause frame defines the duration for which the flow of data packetsis halted. When the time period elapses, the transmission restarts.When a device sends a pause frame to another device, the time for which the sending of packets from the other device must be stopped iscontained in the pause frame. The device that sent the pause frame empties the buffer to be less than the threshold value and restarts theacceptance of data packets.Dynamic ingress buffering enables the sending of pause frames at different thresholds based on the number of ports that experiencecongestion at a time. This behavior impacts the total buffer size used by a particular lossless priority on an interface. The pause and resumethresholds can also be configured dynamically. You can configure a buffer size, pause threshold, ingress shared threshold weight, andresume threshold to control and manage the total amount of buffers that are to be used in your network environment.Buffer Sizes for Lossless or PFC PacketsYou can configure up to a maximum of 4 lossless (PFC) queues. By configuring 4 lossless queues, you can configure 4 different prioritiesand assign a particular priority to each application that your network is used to process. For example, you can assign a higher priority fortime-sensitive applications and a lower priority for other services, such as file transfers. You can configure the amount of buffer space to beallocated for each priority and the pause or resume thresholds for the buffer. This method of configuration enables you to effectivelymanage and administer the behavior of lossless queues.Although the system contains of space for shared buffers, a minimum guaranteed buffer is provided to all the internal and external ports inthe system for both unicast and multicast traffic. This minimum guaranteed buffer reduces the total available shared buffer to . This sharedbuffer can be used for lossy and lossless traffic.The default behavior causes up to a maximum of 6.6 MB to be used for PFC-related traffic. The remaining approximate space of 1 MB canbe used by lossy traffic. You can allocate all the remaining 1 MB to lossless PFC queues. If you allocate in such a way, the performance oflossy traffic is reduced and degraded. Although you can allocate a maximum buffer size, it is used only if a PFC priority is configured andapplied on the interface.The number of lossless queues supported on the system is dependent on the availability of total buffers for PFC. The default configurationin the system guarantees a minimum of 52 KB per queue if all the 128 queues are congested. However, modifying the buffer allocation perqueue impacts this default behavior.264 Data Center Bridging (DCB)
