Dell PowerSwitch S4820T 9.50.1 Configuration Guide for the S4820T System - Page 306

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 Assigns traffic to one priority queue with 20% of the link bandwidth and strictpriority scheduling. Priority group 2 Assigns traffic to one priority queue with 30% of the link bandwidth. Priority group 3 Assigns traffic to two priority queues with 50% of the link bandwidth and strictpriority 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)%. 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. 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). Configuring DCB Maps and its Attributes This topic contains the following sections that describe how to configure a DCB map, apply the configured DCB map to a port, configure PFC without a DCB map, and configure lossless queues. This functionality is supported on the S4820T platform. DCB Map: Configuration Procedure A DCB map consists of PFC and ETS parameters. By default, PFC is not enabled on any 802.1p priority and ETS allocates equal bandwidth to each priority. To configure user-defined PFC and ETS settings, you must create a DCB map. 306 Data Center Bridging (DCB)