Data Center Bridging (DCB) | 351Hierarchical Scheduling in ETS Output PoliciesETS supports up to three levels of hierarchical scheduling. For example, you can apply ETS output policieswith the following configurations:• Priority group 1 assigns traffic to one priority queue with 20% of the link bandwidth and strict-priorityscheduling.• 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 andstrict-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, thebandwidth allocated to the group is distributed to the other priority groups according to the bandwidthpercentage allocated to each group. However, when three priority groups with different bandwidthallocations are used on an interface:• If priority group 3 has free bandwidth, it is distributed as follows: 20% of the free bandwidth topriority 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 topriority 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 thepriority group with the higher priority-queue number is scheduled first. However, when three prioritygroups 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 prioritygroup whose traffic is mapped to two queues.Therefore, in the example, scheduling traffic to priority group 1 (mapped to one strict-priority queue) takesprecedence over scheduling traffic to priority group 3 (mapped to two strict-priority queues).
