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Cisco7600 Series Router Cisco IOS Software Configuration Guide, Release 12.2SX
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Chapter42 Configuring PFC QoS
Understanding How PFC QoS Works
Internal DSCP Values
During processing, PFC QoS represents the priority of all traffic (including non-IP traffic) with an
internal DSCP value.
Initial Internal DSCP Value
On the PFC, before any marking or policing takes place, PFC QoS derives the initial internal DSCP value
as follows:
•For untrusted traffic, when ignore port trust is not enabled, PFC QoS sets the initial internal DSCP
value to zero for both tagged and untagged untrusted traffic.
•For untrusted traffic, when ignore port trust is enabled, PFCQoS does the following:
–
For IP traffic, PFC QoS uses the received DSCP value as the initial internal DSCP value.
–
For traffic without a recognizable ToS byte, PFCQoS maps the port CoS value to the initial
internal DSCP value.
•For trust CoS traffic, when ignore port trust is enabled, PFC QoS does the following:
–
For IP traffic, PFC QoS uses the received DSCP value as the initial internal DSCP value.
Note For trust CoS traffic, when ignore port trust is enabled, PFCQoS does not use the
received CoS value in tagged IP traffic.
–
For tagged traffic without a recognizable ToS byte, PFC QoS maps the received CoS value to
the initial internal DSCP value.
–
For untagged traffic without a recognizable ToS byte, PFCQoS maps the port CoS value to the
initial internal DSCP value.
•For trust IP precedence traffic, PFC QoS does the following:
–
For IP traffic, PFC QoS maps the received IP precedence value to the initial internal DSCP
value.
–
For tagged traffic without a recognizable ToS byte, PFC QoS maps the received CoS value to
the initial internal DSCP value.
–
For untagged traffic without a recognizable ToS byte, PFCQoS maps the port CoS value to the
initial internal DSCP value.
•For trust DSCP traffic, PFC QoS, PFC QoS does the following:
–
For IP traffic, PFC QoS uses the received DSCP value as the initial internal DSCP value.
–
For tagged traffic without a recognizable ToS byte, PFC QoS maps the received CoS value to
the initial internal DSCP value.
–
For untagged traffic without a recognizable ToS byte, PFCQoS maps the port CoS value to the
initial internal DSCP value.
For trust CoS traffic and trust IP precedence traffic, PFCQoS uses configurable maps to derive the initial
internal 6-bit DSCP value from CoS or IP precedence, which are 3-bit values.
Final Internal DSCP Value
Policy marking and policing on the PFC can change the initial internal DSCP value to a final internal
DSCP value, which is then used for all subsequently applied QoS features.