Cisco WS-X6066-SLB-APC - Content Switching Module Software Manual page 721

Chapter 41 Configuring QoS
Internal DSCP Values
These sections describe the internal DSCP values:
•
•
Internal DSCP Sources
During processing, the priority of all traffic (including non-IP traffic) is represented with an internal
DSCP value. QoS derives the internal DSCP value from the following:
•
•
•
•
The trust state of traffic is the trust state of the ingress port unless set otherwise by the matching ACE.
A trust-cos ACL cannot restore received CoS in traffic from untrusted ports. Traffic from untrusted
Note
ports always has the port CoS value.
QoS uses configurable mapping tables to derive the internal 6-bit DSCP value from CoS or IP
precedence, which are 3-bit values (see
section on page 41-55
on page
Egress DSCP and CoS Sources
For egress IP traffic, QoS creates a ToS byte from the internal DSCP value (which you can set equal to
an IP precedence 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 2 least-significant bits from the received ToS
byte.
For all egress traffic, QoS uses a configurable mapping table to derive a CoS value from the internal
DSCP value associated with traffic (see the
section on page
be written into ISL and 802.1Q frames.
ACLs
QoS uses ACLs that contain ACEs. The ACEs specify classification criteria, a marking rule, and
policing rules. QoS compares received traffic to the ACEs in ACLs until a match occurs. When the
traffic matches the classification criteria in an ACE, QoS marks and polices the packet as specified in
the ACE and makes no further comparisons.
There are three ACL types: IP and, with a Layer 3 switching engine, IPX and MAC. QoS compares
traffic of each type (IP, IPX, and MAC) only to the corresponding ACL type (see
78-13315-02
Internal DSCP Sources, page 41-15
Egress DSCP and CoS Sources, page 41-15
For trust-cos traffic, from received or port Layer 2 CoS values (traffic from an untrusted port has
the port CoS value and if traffic from an untrusted port matches a trust-cos ACL, QoS derives the
internal DSCP value from the port CoS value)
For trust-ipprec traffic, from received IP precedence values
For trust-dscp traffic, from received DSCP values
For untrusted traffic, from port CoS or configured DSCP values
or the "Mapping Received IP Precedence Values to Internal DSCP Values" section
41-56).
41-56). QoS sends the CoS value to Ethernet egress ports for use in scheduling and to
the"Mapping Received CoS Values to Internal DSCP Values"
"Mapping Internal DSCP Values to Egress CoS Values"
Catalyst 6000 Family Software Configuration Guide—Releases 6.3 and 6.4
Understanding How QoS Works
Table 41-2).
41-15