Cisco 7604 Configuration Manual page 618

Understanding How PFC QoS Works
On interfaces where egress ACL support for remarked DSCP is configured, the PFC3 processes each
QoS-filtered IP packet twice: once to apply ingress PFC QoS and once to apply egress PFC QoS.
If the router is operating in PFC3A mode with egress ACL support for remarked DSCP configured, when
Caution
the PFC3 processes traffic to apply ingress PFC QoS, it applies ingress PFC QoS filtering and ingress
PFC QoS, and incorrectly applies any egress QoS filtering and egress PFC QoS configured on the
ingress interface, which results in unexpected behavior if QoS filtering is configured on an interface
where egress ACL support for remarked DSCP is enabled. This problem does not occur in other
PFC3 modes.
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.
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.
Marking on Egress OSM Ports
Ingress PFC QoS sets DSCP values that can be used by the OSM egress QoS features (see
Figure 41-8 Egress OSM Port Marking
Cisco 7600 Series Router Cisco IOS Software Configuration Guide, Release 12.2SX
41-16
From PFC or
MSFC
IP traffic
Yes
from PFC?
No
OSM
QoS
Features
Transmit
OSM traffic
OSM switching module marking
PFC3
only
Write ToS
DSCP
Yes
byte into
rewrite
packet
enabled?
No
Chapter 41 Configuring PFC QoS
Figure 41-8).
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