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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 PFC QoS Works
Figure 42-1 PFC QoS Feature Processing Overview
The PFC QoS features are applied in this order:
1. Ingress port PFC QoS features:
–
Port trust state—In PFC QoS, trust means to accept as valid and use as the basis of the initial
internal DSCP value. Ports are untrusted by default, which sets the initial internal DSCP value
to zero. You can configure ports to trust received CoS, IP precedence, or DSCP.
–
Layer 2 CoS remarking—PFC QoS applies Layer 2 CoS remarking, which marks the incoming
frame with the port CoS value, in these situations:
—If the traffic is not in an ISL, 802.1Q, or 802.1p frame.
—If a port is configured as untrusted.
On OSM ATM and POS ports, PFC QoS always sets ingress CoS equal to zero.
–
Congestion avoidance—If you configure an Ethernet LAN port to trust CoS or DSCP, QoS
classifies the traffic on the basis of its Layer 2 CoS value or its Layer 3 DSCP value and assigns
it to an ingress queue to provide congestion avoidance. Layer 3 DSCP-based queue mapping is
available only on WS-X6708-10GE ports.
2. PFC and DFC QoS features:
–
Internal DSCP—On the PFC and DFCs, QoS associates an internal DSCP value with all traffic
to classify it for processing through the system. There is an initial internal DSCP based on the
traffic trust state and a final internal DSCP. The final internal DSCP can be the same as the
initial value or an MQC policy map can set it to a different value.
–
MQC policy maps—MQC policy maps can do one or more of these operations:
—Change the trust state of the traffic (bases the internal DSCP value on a different QoS label)
—Set the initial internal DSCP value (only for traffic from untrusted ports)
—Mark the traffic
—Police the traffic
3. Egress Ethernet LAN port QoS features:
–
Layer 3 DSCP marking with the final internal DSCP (always with PFC2, optionally with PFC3)
–
Layer 2 CoS marking mapped from the final internal DSCP
–
Layer 2 CoS-based and Layer 3 DSCP-based congestion avoidance. (Layer 3 DSCP-based
queue mapping is available only on WS-X6708-10GE ports.)
MSFC
PFC
Switching
Module
1
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2
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3
3
Switching
Module
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