QoS and QoS Policies
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Quality of Service Guide
3HE 11014 AAAC TQZZA Edition: 01
The high-priority-only buffers for network queues are defined within network queue
policies based on the forwarding class. High-priority-only traffic for a specific traffic
class is marked as in-profile either on ingress classification or based on interpretation
of the QoS markings.
3.7.6.8 High and Low Enqueuing Thresholds
The high/low priority feature allows a provider to offer a customer the ability to have
some packets treated with a higher priority when buffered to the ingress queue. If the
queue is configured with a high-prio-only setting (which set the high-priority MBS
threshold higher than the queue’s low-priority MBS threshold), then a portion of the
ingress queue’s allowed buffers are reserved for high-priority traffic. An access
ingress packet must hit an ingress QoS action in order for the ingress forwarding
plane to treat the packet as high priority (the default is low priority).
If the packet’s ingress queue is above the low-priority MBS, the packet will be
discarded unless it has been classified as high priority. The priority of the packet is
not retained after the packet is placed into the ingress queue. Once the packet is
scheduled out of the ingress queue, the packet will be considered in-profile or
out-of-profile based on the dynamic rate of the queue relative to the queue’s CIR
parameter.
If an ingress queue is not configured with a high-prio-only parameter (the parameter
is set to 0%), the low-priority and high-priority MBS thresholds are the same. There
is no difference in high-priority and low-priority packet handling. At access ingress,
the priority of a packet has no effect on which packets are scheduled first. Only the
first buffering decision is affected. At ingress and egress, the current dynamic rate of
the queue relative to the queue’s CIR does affect the scheduling priority between
queues going to the same destination (egress port).
From highest to lowest, the strict operating priority for queues is:
• expedited queues within the CIR (conform)
• best effort queues within the CIR (conform)
• expedited queues above the CIR (exceed)
• best effort queues above the CIR (exceed)
For access ingress, the CIR controls both dynamic scheduling priority and the
marking threshold. At network ingress, the queue’s CIR affects the scheduling
priority but does not provide a profile marking function (as the network ingress policy
trusts the received marking of the packet based on the network QoS policy).
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