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Software Configuration Guide—Release 15.0(2)SG
OL-23818-01
Chapter 37 Configuring Quality of Service About QoS
For non-IP traffic, you have the following classification options:
• Use the port default. If the packet is a non-IP packet, assign the default port DSCP value to the
incoming packet.
• Trust the CoS value in the incoming frame (configure the port to trust CoS). Next, use the
configurable CoS-to-DSCP map to generate the internal DSCP value. Layer 2 ISL frame headers
carry the CoS value in the three least-significant bits of the 1-byte User field. Layer 2 802.1Q frame
headers carry the CoS value in the three most-significant bits of the Tag Control Information field.
CoS values range from 0 for low priority to 7 for high priority. If the frame does not contain a CoS
value, assign the default port CoS to the incoming frame.
The trust DSCP configuration is meaningless for non-IP traffic. If you configure a port with trust
DSCP and non-IP traffic is received, the switch assigns the default port DSCP.
Note On a Catalyst 4948-10GE, Supervisor V-10GE, and Supervisor V, when you send non-IP traffic
(such as IPX) from port 1 with .1Q tag and a Pri=x value, for all x values, the transmit CoS at
the output interface varies as srcMac changes.
"trust dscp" in a policy map only works for IP packets. If a non-IP packet is matched by class
with the “trust dscp" action, it could be transmitted with a random CoS value rather than treating
the "trust dscp" as no-OP as assumed otherwise.
Workaround: Make the classification criteria in you class-maps more granular so that "trust
dscp" is applied to a class that won't match non-IP packet. Separate the class-map matching both
IP and non-IP packets into a set of class-maps so that "trust dscp" is applied to a class that
matches only IPv4 traffic. For class-maps match non-IPv4 traffic, "trust cos" can be used.
For IP traffic, you have the following classification options:
• Trust the IP DSCP in the incoming packet (configure the port to trust DSCP), and assign the same
DSCP to the packet for internal use. The IETF defines the six most-significant bits of the 1-byte type
of service (ToS) field as the DSCP. The priority represented by a particular DSCP value is
configurable. DSCP values range from 0 to 63.
• Trust the CoS value (if present) in the incoming packet, and generate the DSCP by using the
CoS-to-DSCP map.
• Perform the classification based on a configured IP standard or extended ACL, which examines
various fields in the IP header. If no ACL is configured, the packet is assigned the default DSCP
based on the trust state of the ingress port; otherwise, the policy map specifies the DSCP to assign
to the incoming frame.
Note It is not possible to classify traffic based on the markings performed by an input QoS policy. In the
Catalyst 4500 platform, the input and output QoS lookup happen in parallel, and therefore, input marked
DSCP value cannot be used to classify traffic in the output QoS policy.
Note It is not possible to classify traffic based on internal DSCP. The internal DSCP is purely an internal
classification mechanism used for all packets to determine transmit queue and transmit CoS values only.
For information on the maps described in this section, see the “Mapping Tables” section on page 37-14.
For configuration information on port trust states, see the “Configuring the Trust State of Interfaces”
section on page 37-48.