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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 PFCQoS Works
Egress DSCP Mutation with a PFC3
With a PFC3, you can configure 15 egress DSCP mutation maps to mutate the internal DSCP value
before it is written in the egress ToS byte. You can attach egress DSCP mutation maps to any interface
that PFC QoS supports.
Note • If you configure egress DSCP mutation, PFC QoS does not derive the egress CoS value from the
mutated DSCP value.
• The PFC2 does not support egress DSCP mutation.
Egress ToS Byte
Except when DSCP transparency is enabled, PFC QoS creates a ToS byte for egress IP traffic from the
final internal or mutated 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 two least-significant bits from the
received ToS byte.
The internal or mutated DSCP value can mimic an IP precedence value (see the “IP Precedence and
DSCP Values” section on page 42-55).
Egress PFC QoS Interfaces
You can attach an output policy map to a Layer 3 interface (either a LAN port configured as a Layer 3
interface or a VLAN interface) to apply a policy map to egress traffic.
Note • Output policies do not support microflow policing.
• With a PFC3, you cannot apply microflow policing to ARP traffic.
• You cannot set a trust state in an output policy.
Egress ACL Support for Remarked DSCP
Note Egress ACL support for remarked DSCP is also known as packet recirculation.
With a PFC3, Release 12.2(18)SXE and later releases support egress ACL support for remarked DSCP,
which enables IP precedence-based or DSCP-based egress QoS filtering to use any IP precedence or
DSCP policing or marking changes made by ingress PFC QoS.
Without egress ACL support for remarked DSCP, egress QoS filtering uses received IP precedence or
DSCP values; it does not use any IP precedence or DSCP changes made by ingress PFC QoS as the result
of policing or marking.
The PFC3 provides egress PFC QoS only for Layer 3-switched and routed traffic on egress Layer 3
interfaces (either LAN ports configured as Layer 3 interfaces or VLAN interfaces).
You configure egress ACL support for remarked DSCP on ingress Layer 3 interfaces (either LAN ports
configured as Layer 3 interfaces or VLAN interfaces).