Cisco 6500 Series Software Configuration Manual page 404

Understanding How PFC QoS Works
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Strict-priority queues are queues that are serviced in preference to other queues. PFC QoS services
traffic in a strict-priority queue before servicing the standard queue. When PFC QoS services the
standard queue, after receiving a packet, it checks for traffic in the strict-priority queue. If PFC QoS
detects traffic in the strict-priority queue, it suspends its service of the standard queue and completes
service of all traffic in the strict-priority queue before returning to the standard queue.
Scheduling
PFC QoS schedules traffic through the receive queues based on Layer 2 CoS values. In the 1p1q4t,
1p1q0t and 1p1q8t default configurations, PFC QoS assigns all traffic with CoS 5 to the strict-priority
queue; PFC QoS assigns all other traffic to the standard queue. In the 1q4t default configuration, PFC
QoS assigns all traffic to the standard queue.
Congestion Avoidance
If an ingress LAN port is configured to trust CoS, PFC QoS implements Layer 2 CoS-value-based
receive-queue drop thresholds to avoid congestion in received traffic.
1q2t ingress LAN ports have this default drop-threshold configuration:
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1q4t ingress LAN ports have this default drop-threshold configuration:
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1p1q4t ingress LAN ports have this default drop-threshold configuration:
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Catalyst 6500 Series Switch Cisco IOS Software Configuration Guide—Release 12.1 E
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1p1q0t indicates one strict-priority queue and one standard queue with no configurable threshold
(effectively a tail-drop threshold at 100 percent).
1p1q8t indicates one strict-priority queue and one standard queue with eight thresholds, each
configurable as either WRED-drop or tail-drop, and one non-configurable (100 percent) tail-drop
threshold.
Frames with CoS 0, 1, 2, 3, or 4 go to tail-drop threshold 1, where the switch drops incoming frames
when the standard receive-queue buffer is 80 percent full.
Frames with CoS 5, 6, or 7 go to tail-drop threshold 2, where the switch drops incoming frames when
the standard receive-queue buffer is 100 percent full.
Using receive-queue tail-drop threshold 1, the switch drops incoming frames with CoS 0 or 1 when
the receive-queue buffer is 50 percent or more full.
Using receive-queue tail-drop threshold 2, the switch drops incoming frames with CoS 2 or 3 when
the receive-queue buffer is 60 percent or more full.
Using receive-queue tail-drop threshold 3, the switch drops incoming frames with CoS 4 or 5 when
the receive-queue buffer is 80 percent or more full.
Using receive-queue tail-drop threshold 4, the switch drops incoming frames with CoS 6 or 7 when
the receive-queue buffer is 100 percent full.
Frames with CoS 5 go to the strict-priority receive queue (queue 2), where the switch drops
incoming frames only when the strict-priority receive-queue buffer is 100 percent full.
Frames with CoS 0, 1, 2, 3, 4, 6, or 7 go to the standard receive queue.
– Using standard receive-queue tail-drop threshold 1, the switch drops incoming frames with
CoS 0 or 1 when the receive-queue buffer is 50 percent or more full.
Using standard receive-queue tail-drop threshold 2, the switch drops incoming frames with
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CoS 2 or 3 when the receive-queue buffer is 60 percent or more full.
Chapter 31 Configuring PFC QoS
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