Typically, customer markings placed on packets are not treated as trusted from an in-profile or out-of-profile perspective. This allows the use of the ingress buffering to absorb bursts over PIR from a customer and only perform marking as packets are scheduled out of the queue (as opposed to using a hard-policing function that operates on the received rate from the customer). The resulting profile (in or out) based on ingress scheduling into the switch fabric is used by network egress for tunnel marking and egress congestion management.
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 non-zero low drop tail setting, 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 ingress queue for the packet is above the low drop tail setting, the packet is 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. After the packet is scheduled out of the ingress queue, the packet is considered in-profile or out-of-profile based on the dynamic rate of the queue relative to the queue’s CIR parameter.
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 and FIR (where supported) does affect the scheduling priority between queues going to the same destination (either the switch fabric tap or egress port). See Queue scheduling for information about the strict operating priority for queues.
For access ingress, the CIR controls both dynamic scheduling priority and marking threshold (unless cir-non-profiling is configured). 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.