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Cisco 7600 Series Router Cisco IOS Software Configuration Guide—12.1E
78-14064-04
Chapter 32 Configuring PFC QoS
Understanding How PFC QoS Works
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Using standard receive-queue tail-drop threshold 3, the router drops incoming frames with
CoS 4 when the receive-queue buffer is 80 percent or more full.
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Using standard receive-queue tail-drop threshold 4, the router drops incoming frames with
CoS 6 or 7 when the receive-queue buffer is 100 percent full.
1p1q0t ingress LAN ports have this default drop-threshold configuration:
• Frames with CoS 5 go to the strict-priority receive queue (queue 2), where the router 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. The router drops incoming
frames when the receive-queue buffer is 100 percent full.
1p1q8t ports have this default drop-threshold configuration:
• 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, which uses WRED-drop
thresholds:
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Using standard receive-queue WRED-drop threshold 1 for incoming frames with CoS 0, the
switch starts to drop frames when the receive-queue buffer is 40 percent full and drops all
frames with CoS 0 when the receive-queue buffer is 70 percent or more full.
–
Using standard receive-queue WRED-drop threshold 2 for incoming frames with CoS 1, the
switch starts to drop frames when the receive-queue buffer is 40 percent full and drops all
frames with CoS 1 when the receive-queue buffer is 70 percent or more full.
–
Using standard receive-queue WRED-drop threshold 3 for incoming frames with CoS 2, the
switch starts to drop frames when the receive-queue buffer is 50 percent full and drops all
frames with CoS 2 when the receive-queue buffer is 80 percent or more full.
–
Using standard receive-queue WRED-drop threshold 4 for incoming frames with CoS 3, the
switch starts to drop frames when the receive-queue buffer is 50 percent full and drops all
frames with CoS 3 when the receive-queue buffer is 80 percent or more full.
–
Using standard receive-queue WRED-drop threshold 5 for incoming frames with CoS 4, the
switch starts to drop frames when the receive-queue buffer is 60 percent full and drops all
frames with CoS 4 when the receive-queue buffer is 90 percent or more full.
–
Using standard receive-queue WRED-drop threshold 6 for incoming frames with CoS 6, the
switch starts to drop frames when the receive-queue buffer is 60 percent full and drops all
frames with CoS 6 when the receive-queue buffer is 90 percent or more full.
–
Using standard receive-queue WRED-drop threshold 7 for incoming frames with CoS 7, the
switch starts to drop frames when the receive-queue buffer is 70 percent full and drops all
frames with CoS 7 when the receive-queue buffer is 100 percent or more full.
Note 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 traffic carrying CoS values mapped only to the queue. The
switch uses WRED-drop thresholds for traffic carrying CoS values mapped to the queue and
a threshold. See the “Configuring Standard-Queue Drop Threshold Percentages” section on
page 32-54.
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