QoS Policies
Quality of Service Guide 35
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 hi-prio-only setting (setting the high priority MBS threshold higher than
the queue’s low priority MBS threshold) 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 hi-prio-only parameter, the low priority and high
priority MBS thresholds will be the same. There will be no difference in high priority and low
priority packet handling. At access ingress, the priority of a packet has no affect 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 (either the switch fabric tap or egress
port). The strict operating priority for queues are (from highest to lowest):
• Expedited queues within the CIR (conform)
• Best Effort queues within the CIR (conform)
• Expedited and Best Effort queues above the CIR (exceed)
For access ingress, the CIR controls both dynamic scheduling priority and 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 determines the
dynamic scheduling priority, but does not affect the packet’s ingress determined profile.
Queue Counters
The router maintains counters for queues within the system for granular billing and
accounting. Each queue maintains the following counters:
• Counters for packets and octets accepted into the queue
• Counters for packets and octets rejected at the queue
• Counters for packets and octets transmitted in-profile
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