Cisco WS-SUP32-GE-3B - Supervisor Engine 32 Software Configuration Manual page 547

Chapter 38 Configuring PFC QoS
Egress ToS Byte
Except when
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
DSCP Values" section on page
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.
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Egress ACL Support for Remarked DSCP
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Egress ACL support for remarked DSCP 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 PFC3B 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).
On interfaces where egress ACL support for remarked DSCP is configured, the PFC3B processes each
QoS-filtered IP packet twice: once to apply ingress PFC QoS and once to apply egress PFC QoS.
After packets have been processed by ingress PFC QoS and any policing or marking changes have been
made, the packets are processed again on the ingress interface by any configured Layer 2 features (for
example, VACLs) before being processed by egress PFC QoS.
On an interface where egress ACL support for remarked DSCP is configured, if a Layer 2 feature
matches the ingress-QoS-modified IP precedence or DSCP value, the Layer 2 feature might redirect or
drop the matched packets, which prevents them from being processed by egress QoS.
OL-11439-03
DSCP transparency
Output policies do not support microflow policing.
With a PFC3B, you cannot apply microflow policing to ARP traffic.
You cannot set a trust state in an output policy.
Egress PFC QoS is not supported on interfaces where you configure PISA-accelerated features.
Egress ACL support for remarked DSCP is also known as packet recirculation.
Egress ACL support for remarked DSCP is not supported on interfaces where you configure
PISA-accelerated features.
Catalyst Supervisor Engine 32 PISA Cisco IOS Software Configuration Guide, Release 12.2ZY
is enabled, PFC QoS creates a ToS byte for egress IP traffic from the
38-44).
Understanding How PFC QoS Works
"IP Precedence and
38-13