52-14
Catalyst 6500 Series Switch Software Configuration Guide—Release 8.7
OL-8978-04
Chapter 52 Configuring QoS
Understanding How QoS Works
Ingress Scheduling
QoS schedules the traffic through the receive queues based on the CoS values. In the default
configuration, PFC QoS assigns all traffic with CoS 5 to the strict-priority queue (if present); PFC QoS
assigns all other traffic to the standard queue. In the absence of a strict-priority queue, PFC QoS assigns
all traffic to the standard queues.
Ingress Congestion Avoidance
If a port is configured with the
trust-cos
keyword, QoS implements CoS-value-based receive-queue drop
thresholds to avoid congestion in the received traffic. See the
“QoS Default Configuration” section on
page 52-30
for the default CoS-to-threshold mapping.
Note
For some port types, you can configure the standard receive queue to use both a tail-drop and a
WRED-drop threshold by mapping a CoS value to the queue or to the queue and a threshold. The switch
uses the tail-drop threshold for the traffic carrying the CoS values that are mapped only to the queue.
The switch uses the WRED-drop thresholds for the traffic carrying the CoS values that are mapped to
the queue and a threshold. For more information, see the
“1p1q8t Receive Queues” section on
page 52-71
.
Figure 52-10
shows the drop thresholds for a
1q4t
port. The drop thresholds in the other configurations
function similarly.
Figure 52-10
Receive-Queue Drop Thresholds
CoS
0
an
d
1
CoS
2
an
d
3
Co
S
4
an
d
5
C
oS
6
a
nd
7
Traffic is dropped
Drop threshold 4: 100%
Drop threshold 3: 80%
Drop threshold 2: 60%
Drop threshold 1: 50%
Available for
traffic with any
CoS value
Reserved for
CoS 6 and 7
Reserved for
CoS 4 and higher
Reserved for
CoS 2 and higher
Receive queue
(Default values shown)
100% available for CoS 6 and 7
80% available for CoS 4 and 5
60% available for CoS 2 and 3
50% available for CoS 0 and 1
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