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Cisco 7600 Series Router Cisco IOS Software Configuration Guide, Release 12.2SX
OL-4266-08
Chapter 42 Configuring PFC QoS
Understanding How PFCQoS Works
Port Trust
In PFC QoS, trust means to accept as valid and use as the basis of the initial internal DSCP value. You
can configure ports as untrusted or you can configure them to trust these QoS values:
• Layer 2 CoS
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A port configured to trust CoS is called a trust CoS port.
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Traffic received through a trust CoS port or configured by a policy map to trust CoS is called
trust CoS traffic.
Note Not all traffic carries a CoS value. Only ISL, 802.1Q, and 802.1P traffic carries a CoS value.
PFC QoS applies the port CoS value to any traffic that does not carry a CoS value. On
untrusted ports, PFC QoS applies the port CoS value to all traffic, overwriting any received
CoS value.
• IP precedence
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A port configured to trust IP precedence is called a trust IP precedence port.
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Traffic received through a trust IP precedence port or configured by a policy map to trust
IP precedence is called trust IP precedence traffic.
• DSCP
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A port configured to trust DSCP is called a trust DSCP port.
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Traffic received through a trust DSCP port or configured by a policy map to trust DSCP is called
trust DSCP traffic.
Traffic received through an untrusted port is called untrusted traffic.
Ingress Congestion Avoidance
PFC QoS implements congestion avoidance on trust CoS ports. On a trust CoS port, QoS classifies the
traffic on the basis of its Layer 2 CoS value and assigns it to an ingress queue to provide congestion
avoidance. In Release 12.2(18)SXF5 and later releases, you can configure WS-X6708-10GE trust DSCP
ports to use received DSCP values for congestion avoidance. See the “Ingress Classification and
Marking at Trust CoS LAN Ports” section on page 42-17 for more information about ingress congestion
avoidance.
PFC and DFC QoS Features
These sections describe PFCs and DFCs as they relate to QoS:
• Supported Policy Feature Cards, page 42-9
• Supported Distributed Forwarding Cards, page 42-9
• PFC and DFC QoS Feature List and Flowchart, page 42-9
• Internal DSCP Values, page 42-11