31-17
Catalyst 6500 Series Switch Cisco IOS Software Configuration Guide—Release 12.1 E
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Chapter 31 Configuring PFC QoS
Understanding How PFC QoS Works
• Policers, page 31-19
• Attaching Policy Maps, page 31-21
• Egress CoS and ToS Values, page 31-21
Note Filtering for PFC QoS can use Layer 2, 3, and 4 values. Marking uses Layer 2 CoS values and Layer 3
IP precedence or DSCP values.
Internal DSCP Values
These sections describe the internal DSCP values:
• Internal DSCP Sources, page 31-17
• Egress DSCP and CoS Sources, page 31-17

Internal DSCP Sources

During processing, PFC QoS represents the priority of all traffic (including non-IP traffic) with an
internal DSCP value. PFC QoS derives the internal DSCP value from the following:
• For trust-cos traffic, from received or ingress port Layer 2 CoS values
Note Traffic from an untrusted ingress LAN port has the ingress port CoS value and if traffic from
an untrusted ingress Ethernet port matches a trust-cos policer, PFC QoS derives the internal
DSCP value from the ingress port CoS value.
• For trust-ipprec traffic, from received IP precedence values
• For trust-dscp traffic, from received DSCP values
• For untrusted traffic, from ingress port CoS or configured DSCP values
The trust state of traffic is the trust state of the ingress LAN port unless set otherwise by a matching ACE.
Note A trust-cos policer cannot restore received CoS in traffic from untrusted ingress LAN ports. Traffic from
untrusted ingress LAN ports always has the ingress port CoS value.
PFC QoS uses configurable mapping tables to derive the internal 6-bit DSCP value from CoS or IP
precedence, which are 3-bit values (see the“Mapping Received CoS Values to Internal DSCP Values”
section on page 31-66 or the “Mapping Received IP Precedence Values to Internal DSCP Values” section
on page 31-67).

Egress DSCP and CoS Sources

For egress IP traffic, PFC QoS creates a ToS byte from the internal DSCP value and sends it to the egress
port to be written into IP packets. For trust-dscp and untrusted IP traffic, the ToS byte includes the
original 2 least-significant bits from the received ToS byte.
Note The internal DSCP value can mimic an IP precedence value (see Table 31-1 on page 31-5).