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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
Egress LAN Port Features
PFC QoS supports egress LAN port scheduling and congestion avoidance using Layer 2 CoS values.
Egress LAN port marking sets Layer 2 CoS values and Layer 3 DSCP values. See the “LAN Egress
Port Features” section on page 31-21.
Egress OSM Port Features
Ingress PFC QoS sets Layer 3 DSCP values that can be used by the OSM egress QoS features.
MSFC Features
PFC QoS marks IP traffic transmitted to the MSFC with rewritten Layer 3 DSCP values. With PFC2,
CoS is equal to IP precedence in all traffic sent from the MSFC2 to egress ports; with PFC1, CoS is zero.
Note Traffic that is Layer 3 switched does not go through the MFSC and retains the CoS value assigned by
the PFC.
Ingress LAN Port Features
These sections describe ingress LAN port PFC QoS features:
• Ingress LAN Port Trust States, page 31-12
• Marking at Untrusted Ingress LAN Ports, page 31-13
• Marking at Trusted Ingress LAN Ports, page 31-13
• Ingress LAN Port Scheduling and Congestion Avoidance, page 31-13
Ingress LAN Port Trust States
The trust state of an ingress LAN port determines how the port marks, schedules, and classifies received
Layer 2 frames, and whether or not congestion avoidance is implemented. You can configure the trust
state of each ingress LAN port as follows:
• Untrusted (default)
• Trust IP precedence (not supported on 1q4t LAN ports except Gigabit Ethernet)
• Trust DSCP (not supported on 1q4t LAN ports except Gigabit Ethernet)
• Trust CoS (not supported on 1q4t LAN ports except Gigabit Ethernet)
See the “Configuring the Trust State of Ethernet LAN and OSM Ingress Ports” section on page 31-53.
PFC QoS implements ingress LAN port congestion avoidance only on LAN ports configured to trust
CoS.
Note Ingress LAN port marking, scheduling, and congestion avoidance use Layer 2 CoS values and does not
use or set Layer 3 IP precedence or DSCP values.
