Understanding Classification And Marking - Cisco WS-SUP32-GE-3B - Supervisor Engine 32 Software Configuration Manual

Understanding How PFC QoS Works
After packets have been processed by ingress PFC QoS and any policing or marking changes have been
made, the packets are processed on the ingress interface by any configured Layer 3 features (for example,
ingress Cisco IOS ACLs, policy based routing (PBR), etc.) before being processed by egress PFC QoS.
The Layer 3 features configured on an interface where egress ACL support for remarked DSCP is
configured might redirect or drop the packets that have been processed by ingress PFC QoS, which
would prevent them from being processed by egress PFC QoS.

Understanding Classification and Marking

The following sections describe where and how classification and marking occur on the Catalyst 6500
series switches:
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Classification and Marking at Trusted and Untrusted Ingress Ports
The trust state of an ingress port determines how the port marks, schedules, and classifies received
Layer 2 frames, and whether or not congestion avoidance is implemented. These are the port trust states:
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In all releases, ingress LAN port classification, marking, and congestion avoidance can use Layer 2 CoS
values and do not set Layer 3 IP precedence or DSCP values.
Ingress LAN port classification, marking, and congestion avoidance use Layer 2 CoS values only.
The following sections describe classification and marking at trusted and untrusted ingress ports:
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Classification and Marking at Untrusted Ingress Ports
PFC QoS Layer 2 remarking marks all frames received through untrusted ports with the
(the default is zero).
To map the port CoS value that was applied to untrusted ingress traffic to the initial internal DSCP value,
configure a trust CoS policy map that matches the ingress traffic.
Ingress Classification and Marking at Trusted Ports
You should configure ports to trust only if they receive traffic that carries valid QoS labels. QoS uses the
received QoS labels as the basis of initial internal DSCP value. After the traffic enters the switch, you
can apply a different trust state to traffic with a policy map. For example, traffic can enter the switch
through a trust CoS port, and then you can use a policy map to trust IP precedence or DSCP, which uses
the trusted value as the basis of the initial internal DSCP value, instead of the QoS label that was trusted
at the port.
Catalyst Supervisor Engine 32 PISA Cisco IOS Software Configuration Guide, Release 12.2ZY
38-14
Classification and Marking at Trusted and Untrusted Ingress Ports, page 38-14
Classification and Marking on the PFC3B Using Service Policies and Policy Maps, page 38-15
Classification and Marking on the PISA, page 38-16
Untrusted (default)
Trust IP precedence
Trust DSCP
Trust CoS
Classification and Marking at Untrusted Ingress Ports, page 38-14
Ingress Classification and Marking at Trusted Ports, page 38-14
Chapter 38 Configuring PFC QoS
port CoS value
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