Service Manual

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 Assigns traffic to one priority queue with 20% of the link bandwidth and strict-
priority 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 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)%.
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).
Data Center Bridging (DCB)
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