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).
At egress, the profile of a packet is only important for egress queue buffering decisions and egress marking decisions, not for scheduling priority. The egress queue’s CIR will determine the dynamic scheduling priority, but will not affect the packet’s ingress determined profile.