Cisco WS-X6K-SUP1A-PFC= Software Guide - Page 721

Chapter 41 Configuring QoS Understanding How QoS Works Internal DSCP Values These sections describe the internal DSCP values: • Internal DSCP Sources, page 41-15 • Egress DSCP and CoS Sources, page 41-15 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: • 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 The trust state of traffic is the trust state of the ingress port unless set otherwise by the matching ACE. Note A trust-cos ACL cannot restore received CoS in traffic from untrusted ports. Traffic from untrusted 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 the"Mapping Received CoS Values to Internal DSCP Values" section on page 41-55 or the "Mapping Received IP Precedence Values to Internal DSCP Values" section on page 41-56). 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 "Mapping Internal DSCP Values to Egress CoS Values" section on page 41-56). QoS sends the CoS value to Ethernet egress ports for use in scheduling and to 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 Table 41-2). 78-13315-02 Catalyst 6000 Family Software Configuration Guide-Releases 6.3 and 6.4 41-15