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27-13
Software Configuration Guide—Release 12.2(25)SG
OL-7659-03
Chapter 27 Configuring Quality of Service
Overview of QoS
Internal DSCP Values
The following sections describe the internal DSCP values:
• Internal DSCP Sources, page 27-13
• Egress ToS and CoS Sources, page 27-13
Internal DSCP Sources
During processing, QoS represents the priority of all traffic (including non-IP traffic) with an internal
DSCP value. QoS derives the internal DSCP value from the following:
• For trust-CoS traffic, from received or ingress interface Layer 2 CoS values
• For trust-DSCP traffic, from received or ingress interface DSCP values
• For untrusted traffic, from ingress interface DSCP value
The trust state of traffic is the trust state of the ingress interface unless set otherwise by a policy action
for this traffic class.
QoS uses configurable mapping tables to derive the internal 6-bit DSCP value from CoS, which are 3-bit
values (see the“Configuring DSCP Maps” section on page 27-51).
Egress ToS and CoS Sources
For egress IP traffic, QoS creates a ToS byte from the internal DSCP value and sends it to the egress
interface 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 ToS value can mimic an IP precedence value (see Table 27-1 on page 27-4).
For all egress traffic, QoS uses a configurable mapping table to derive a CoS value from the internal ToS
value associated with traffic (see the “Configuring the DSCP-to-CoS Map” section on page 27-53). QoS
sends the CoS value to be written into ISL and 802.1Q frames.
For traffic received on an ingress interface configured to trust CoS using the qos trust cos command, the
transmit CoS is always the incoming packet CoS (or the ingress interface default CoS if the packet is
received untagged).
When the interface trust state is not configured to trust dscp using the qos trust dscp command, the
security and QoS ACL classification will always use the interface DSCP and not the incoming packet
DSCP.