Dell C1048P Port Extender Networking Configuration Guide for the C9000 Series - Page 282

Priority group 3 Assigns traffic to two priority queues with 50% of the link bandwidth and strict-priority scheduling. In this example, the configured ETS bandwidth allocation and scheduler behavior is as follows: Therefore, in this example, scheduling traffic to priority group 1 (mapped to one strict-priority queue) takes precedence over scheduling traffic to priority group 3 (mapped to two strict-priority queues). Unused bandwidth Normally, if there is no traffic or unused bandwidth for a priority group, the bandwidth allocated to the usage: group is distributed to the other priority groups according to the bandwidth percentage allocated to each group. However, when three priority groups with different bandwidth allocations are used on an interface: • If priority group 3 has free bandwidth, it is distributed as follows: 20% of the free bandwidth to priority group 1 and 30% of the free bandwidth to priority group 2. • If priority group 1 or 2 has free bandwidth, (20 + 30)% of the free bandwidth is distributed to priority group 3. Priority groups 1 and 2 retain whatever free bandwidth remains up to the (20+ 30)%. Strict-priority groups: If two priority groups have strict-priority scheduling, traffic assigned from the priority group with the higher priority-queue number is scheduled first. However, when three priority groups are used and two groups have strict-priority scheduling (such as groups 1 and 3 in the example), the strict priority group whose traffic is mapped to one queue takes precedence over the strict priority group whose traffic is mapped to two queues. Priority-Based Flow Control Using Dynamic Buffer Method Priority-based flow control using dynamic buffer spaces is supported on the switch. In a data center network, priority-based flow control (PFC) manages large bursts of one traffic type in multiprotocol links so that it does not affect other traffic types and no frames are lost due to congestion. When PFC detects congestion on a queue for a specified priority, it sends a pause frame for the 802.1p priority traffic to the transmitting device. Pause and Resume of Traffic The pause message is used by the sending device to inform the receiving device about a congested, heavily-loaded traffic state that has been identified. When the interface of a sending device transmits a pause frame, the recipient acknowledges this frame by temporarily halting the transmission of data packets. The sending device requests the recipient to restart the transmission of data traffic when the congestion eases and reduces. The time period that is specified in the pause frame defines the duration for which the flow of data packets is 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 is contained in the pause frame. The device that sent the pause frame empties the buffer to be less than the threshold value and restarts the acceptance of data packets. Dynamic ingress buffering enables the sending of pause frames at different thresholds based on the number of ports that experience congestion at a time. This behavior impacts the total buffer size used by a particular lossless priority on an interface. The pause and resume thresholds can also be configured dynamically. You can configure a buffer size, pause threshold, ingress shared threshold weight, and resume threshold to control and manage the total amount of buffers that are to be used in your network environment. Data Center Bridging (DCB) 282