Dell PowerSwitch S4820T Configuration Guide for the S4820T System 9.112.0P1 - Page 259

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. Hierarchical Scheduling in ETS Output Policies ETS supports up to three levels of hierarchical scheduling. For example, you can apply ETS output policies with the following configurations: Priority group 1 Priority group 2 Priority group 3 Assigns traffic to one priority queue with 20% of the link bandwidth and strict-priority scheduling. Assigns traffic to one priority queue with 30% of the link bandwidth. 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: Unused bandwidth usage: Normally, if there is no traffic or unused bandwidth for a priority group, the bandwidth allocated to the 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)%. Data Center Bridging (DCB) 259