52-16
Catalyst 6500 Series Switch Software Configuration Guide—Release 8.7
OL-8978-04
Chapter 52 Configuring QoS
Understanding How QoS Works
These sections describe classification, marking, and policing with a Layer 3 switching engine:
• Internal DSCP Values, page 52-16
• ACLs, page 52-17
• Named ACLs, page 52-17
• Default ACLs, page 52-22
• Marking Rules, page 52-23
• Policers, page 52-24
• PFC2 Policing Decisions, page 52-25
• PFC3 Policing Decisions, page 52-26
• Attaching ACLs, page 52-26
• PFC3 Egress DSCP Mutation, page 52-27
• Final Layer 3 Switching Engine CoS and ToS Values, page 52-27
Note Classification with a Layer 3 switching engine uses the Layer 2, 3, and 4 values. Marking with a Layer 3
switching engine uses the Layer 2 CoS values and the Layer 3 IP precedence or DSCP values.
Internal DSCP Values
These sections describe the internal DSCP values:
• Internal DSCP Sources, page 52-16
• Egress DSCP and CoS Sources, page 52-17
Internal DSCP Sources
During processing, the priority of all traffic (including non-IP traffic) is represented with an internal
DSCP value. QoS derives the internal DSCP value from the following:
• For the trust-cos traffic, from the received or port Layer 2 CoS values (the traffic from an untrusted
port has the port CoS value and if the traffic from an untrusted port matches a trust-cos ACL, QoS
derives the internal DSCP value from the port CoS value)
• For the trust-ipprec traffic, from the received IP precedence values
• For the trust-dscp traffic, from the received DSCP values
• For the untrusted traffic, from the port CoS or configured DSCP values
The trust state of the traffic is the trust state of the ingress port unless set otherwise by the matching ACE.
Note A trust-cos ACL cannot restore the received CoS in the traffic from the untrusted ports. The traffic from
the untrusted ports always has the port CoS value.
QoS uses the configurable mapping tables to derive the internal 6-bit DSCP value from the CoS or IP
precedence, which are 3-bit values (see the“Mapping the Received CoS Values to the Internal DSCP
Values” section on page 52-73 or the “Mapping the Received IP Precedence Values to the Internal DSCP
Values” section on page 52-74).
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