Dell PowerSwitch S4820T Configuration Guide for the S4820T System 9.100.0 - Page 247

• Use the ETS configuration associated with 802.1p priority traffic in a DCB map in DCBx negotiation with ETS peers. • Traffic in priority groups is assigned to strict-queue or weighted round-robin (WRR) scheduling in an ETS configuration and is managed using the ETS bandwidth-assignment algorithm. Dell Networking OS de-queues all frames of strict-priority traffic before servicing any other queues. A queue with strict-priority traffic can starve other queues in the same port. • ETS-assigned bandwidth allocation and strict-priority scheduling apply only to data queues, not to control queues. • Dell Networking OS supports hierarchical scheduling on an interface. The control traffic on Dell Networking OS is redirected to control queues as higher priority traffic with strict priority scheduling. After the control queues drain out, the remaining data traffic is scheduled to queues according to the bandwidth and scheduler configuration in the DCB map. The available bandwidth calculated by the ETS algorithm is equal to the link bandwidth after scheduling non-ETS higher-priority traffic. • The configuration of bandwidth allocation and strict-queue scheduling is not supported at the same time for a priority group. • Bandwidth assignment: By default, equal bandwidth is assigned to each dot1p priority in a priority group. To configure the bandwidth assigned to the port queues associated with dot1p priorities in a priority group, use the bandwidth percentage parameter. The sum of the bandwidth allocated to all priority groups in a DCB map must be 100% of the bandwidth on the link. You must allocate at least 1% of the total bandwidth to each priority group. • Scheduling of priority traffic: dot1p priority traffic on the switch is scheduled to the current queue mapping. dot1p priorities within the same queue must have the same traffic properties and scheduling method. • ETS configuration error: If an error occurs in an ETS configuration, the configuration is ignored and the scheduler and bandwidth allocation settings are reset to the ETS default value: 100% of available bandwidth is allocated to priority group 0 and the bandwidth is equally assigned to each dot1p priority. If an error occurs when a port receives a peer's ETS configuration, the port's configuration resets to the ETS configuration in the previously configured DCB map. If no DCB map was previously applied, the port resets to the default ETS parameters. ETS Prerequisites and Restrictions On a switch, ETS is enabled by default on Ethernet ports with equal bandwidth assigned to each 802.1p priority. You can change the default ETS configuration only by using a DCB map. The following prerequisites and restrictions apply when you configure ETS bandwidth allocation or strict-priority queuing in a DCB map: • When allocating bandwidth or configuring strict-priority queuing for dot1p priorities in a priority group on a DCBx CIN interface, take into account the CIN bandwidth allocation (see Configuring Bandwidth Allocation for DCBx CIN) and dot1p-queue mapping. • Because all the priorities mapped to a priority group is scheduled using a single queue, the priorities are treated with first come first served basis. • Although ETS bandwidth allocation or strict-priority queuing does not support weighted random early detection (WRED), explicit congestion notification (ECN), rate shaping, and rate limiting because these parameters are not negotiated by DCBx with peer devices, you can apply a QoS output policy with WRED and/or rate shaping on a DCBx CIN-enabled interface. In this case, the WRED or rate shaping configuration in the QoS output policy must take into account the bandwidth allocation or queue scheduler configured in the DCB map. Priority-Group Configuration Notes When you configure priority groups in a DCB map: • A priority group consists of 802.1p priority values that are grouped together for similar bandwidth allocation and scheduling, and that share the same latency and loss requirements. All 802.1p priorities mapped to the same queue must be in the same priority group. • In a DCB map, each 802.1p priority must map to a priority group. • The maximum number of priority groups supported in a DCB map on an interface is equal to the number of data queues (4) on the port. Each priority group can support more than one data queue. • You can enable PFC on a maximum of two priority queues on an interface. • If you configure more than one priority group as strict priority, the higher numbered priority queue is given preference when scheduling data traffic. Data Center Bridging (DCB) 247