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Cisco7600 Series Router Cisco IOS Software Configuration Guide, Release 12.2SX
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Chapter42 Configuring PFC QoS
Understanding How PFC QoS Works
These sections describe classification and marking at trusted ingress ports:
•Ingress Classification and Marking at Trust CoS LAN Ports, page 42-17
•Ingress Classification and Marking at Trust IP Precedence Ports, page 42-17
•Ingress Classification and Marking at Trust DSCP Ports, page 42-17
Ingress Classification and Marking at Trust CoS LAN Ports
You should configure LAN ports to trust CoS only if they receive traffic that carries valid Layer 2 CoS.
When an ISL frame enters the router through a trusted ingress LAN port, PFC QoS accepts the three least
significant bits in the User field as a CoS value. When an 802.1Q frame enters the router through a
trusted ingress LAN port, PFC QoS accepts the User Priority bits as a CoS value. PFC QoS Layer 2
remarking marks all traffic received in untagged frames with the ingress port CoS value.
On ports configured to trust CoS, PFC QoS does the following:
•PFC QoS maps the received CoS value in tagged trust CoS traffic to the initial internal DSCP value.
•PFC QoS maps the ingress port CoS value applied to untagged trusted traffic to the initial internal
DSCP value.
•PFC QoS enables the CoS-based ingress queues and thresholds to provide congestion avoidance.
See the “Understanding Port-Based Queue Types” section on page42-22 for more information about
ingress queues and thresholds.
Ingress Classification and Marking at Trust IP Precedence Ports
You should configure ports to trust IP precedence only if they receive traffic that carries valid Layer 3
IP precedence. For traffic from trust IP precedence ports, PFC QoS maps the received IP precedence
value to the initial internal DSCP value. Because the ingress port queues and thresholds use Layer 2 CoS,
PFC QoS does not implement ingress port congestion avoidance on ports configured to trust IP
precedence. PFC does not mark any traffic on ingress ports configured to trust IP precedence.
Ingress Classification and Marking at Trust DSCP Ports
You should configure ports to trust DSCP only if they receive traffic that carries valid Layer 3 DSCP.
In Release 12.2(18)SXF5 and later releases, you can enable DSCP-based ingress queues and thresholds
on WS-X6708-10GE ports to provide congestion avoidance (see the “Configuring DSCP-Based Queue
Mapping” section on page 42-98).
In releases earlier than Release 12.2(18)SXF5, the ingress port queues and thresholds use only Layer2
CoS, and PFC QoS does not implement ingress port congestion avoidance on ports configured to trust
DSCP.
For traffic from trust DSCP ports, PFCQoS uses the received DSCP value as the initial internal DSCP
value. PFC QoS does not mark any traffic on ingress ports configured to trust received DSCP.
Classification and Marking at Ingress OSM Ports
PFC QoS associates CoS zero with all traffic received through ingress OSM ports. You can configure
ingress OSM port trust states that can be used by the PFC to set IP precedence or DSCP values and the
CoS value. You can configure the trust state of each ingress OSM port as follows:
•Untrusted (default)
•Trust IP precedence
•Trust DS CP