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Catalyst 6500 Series Switch Cisco IOS Software Configuration Guide—Release 12.1 E
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Chapter 31 Configuring PFC QoS
Understanding How PFC QoS Works

1p2q1t Ports

1p2q1t ports have a strict-priority queue and two standard transmit queues. The standard transmit queues
each have one WRED-drop threshold and one nonconfigurable tail-drop threshold.
• Frames with CoS 5 go to the strict-priority transmit queue (queue 3), where the switch drops frames
only when the buffer is 100 percent full.
• The standard transmit queues have WRED-drop thresholds:
–
Frames with CoS 0, 1, 2, or 3 go to the low-priority transmit queue (queue 1), where the switch
starts to drop frames when the low-priority transmit-queue buffer is 70 percent full and drops
all frames with CoS 0, 1, 2, or 3 when the buffer is 100 percent full.
–
Frames with CoS 4, 6, or 7 go to the high-priority transmit queue (queue 2), where the switch
starts to drop frames when the high-priority transmit-queue buffer is 70 percent full and drops
all frames with CoS 4, 6, or 7 when the buffer is 100 percent full.
Note You can configure each standard transmit queue to use both the tail-drop and the
WRED-drop threshold. See the “Configuring Standard-Queue Drop Threshold Percentages”
section on page 31-54.

Marking

When traffic is transmitted from the switch, PFC QoS writes the ToS byte into IP packets. On LAN ports,
PFC QoS also writes the CoS value that was used for scheduling and congestion avoidance into ISL and
802.1Q frames (see the “Egress CoS and ToS Values” section on page 31-21).
PFC QoS Statistics Data Export
Note Release 12.1(11b)E or later supports PFC QoS statistics data export.
The PFC QoS statistics data export feature generates per-LAN-port and per-aggregate policer utilization
information and forwards this information in UDP packets to traffic monitoring, planning, or accounting
applications. You can enable PFC QoS statistics data export on a per-LAN-port or on a per-aggregate
policer basis. The statistics data generated per port consists of counts of the input and output packets and
bytes. The aggregate policer statistics consist of counts of allowed packets and counts of packets
exceeding the policed rate.
The PFC QoS statistics data collection occurs periodically at a fixed interval, but you can configure the
interval at which the data is exported. PFC QoS statistics collection is enabled by default, and the data
export feature is disabled by default for all ports and all aggregate policers configured on the
Catalyst 6500 series switch.
Note The PFC QoS statistics data export feature is completely separate from NetFlow Data Export and does
not interact with it.