QoS Policies
Quality of Service Guide 27
Hi-Low Priority Only Buffers
Hi-low priority only buffers are defined on a queue and allow buffers to be reserved for traffic
classified as high or low priority. When the queue depth reaches a specified level, only high
and low priority traffic can be enqueued. The high/low priority only reservation for a queue
is defined as a percentage of the MBS value. This is available for service egress and egress
queue group queues. The high/low priority traffic for a given traffic class is traffic that has
been determined to be in-profile or out-of-profile.
Hi-low priority only buffers exist also for egress network queues in a network queue policy,
however, in these policies it is not configurable and has a default of an additional 10% of the
MBS value on top of the high priority only.
WRED Per Queue
Egress SAP, subscriber, and network queues by default use drop tails within the queues and
WRED slopes applied to the pools in which the queues reside in order to apply congestion
control to the traffic in those queues. An alternative to this is to apply the WRED slopes
directly to the egress queue using WRED per queue.
WRED per queue is supported for SAP egress QoS policy queues (and therefore to egress
SAP and subscriber queues) and for queues within an egress queue group template. There are
two modes available:
• Native, which is supported on FP3 queues.
• Pool per queue, which is supported on FP2 and higher based hardware.
Table 3: WRED Per Queue Congestion Control Summary
In profile
traffic
Out
profile
traffic
Exceed
profile
traffic
Wred-
queue
Mode
Slope
usage
Comments
FP3 queues Drop tail
(MBS)
used
Low slope
used
Exceed
slope used
native Exceed-
low
If slope shutdown,
MBS is used.
High-slope not
used.
Pool per queue High slope Low slope Exceed
slope
pool-per-
queue
(FP3/FP2)
Default If slope shutdown,
MBS is used.
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