52-2
Catalyst 6500 Series Switch Software Configuration Guide—Release 8.7
OL-8978-04
Chapter 52 Configuring QoS
Understanding How QoS Works
QoS sets the Layer 2 and Layer 3 values in the network traffic to a configured value or to a value that is
based on the received Layer 2 or Layer 3 values. The IP traffic retains the Layer 3 value when it leaves
the switch.
With PFC3, you can configure QoS for both the ingress and egress traffic. You can configure QoS
per-port or per-VLAN for the ingress traffic. You can configure QoS only per VLAN for the egress
traffic.
With other hardware, you can configure QoS per port or per VLAN for the ingress traffic.
These sections describe QoS:
•
QoS Terminology, page 52-2
•
Flowcharts, page 52-3
•
QoS Feature Set Summary, page 52-10
•
Ethernet Ingress Port Marking, Scheduling, Congestion Avoidance, and Classification, page 52-12
•
Classification, Marking, and Policing with a Layer 3 Switching Engine, page 52-15
•
Classification and Marking on a Supervisor Engine 1 with a Layer 2 Switching Engine, page 52-28
•
Ethernet Egress Port Scheduling, Congestion Avoidance, and Marking, page 52-28
•
QoS Statistics Data Export, page 52-29
QoS Terminology
This section defines some QoS terminology:
•
Packets
carry traffic at Layer 3.
•
Frames
carry traffic at Layer 2. The Layer 2 frames carry the Layer 3 packets.
•
Labels
are prioritization values that are carried in the packets and the frames:
–
Layer 2 class of service (CoS) values range between zero for low priority and seven for high
priority:
The Layer 2 Inter-Switch Link (ISL) frame headers have a 1-byte User field that carries an
IEEE 802.1p CoS value in the three least significant bits.
The Layer 2 802.1Q frame headers have a 2-byte Tag Control Information field that carries the
CoS value in the three most significant bits, which are called the User Priority bits.
The other frame types cannot carry the CoS values.
Note
On the ports that are configured as ISL trunks, all traffic is in ISL frames. On the ports
that are configured as 802.1Q trunks, all traffic is in the 802.1Q frames except for the
traffic in the native VLAN.
–
Layer 3 IP precedence values—The IP version 4 specification defines the three most significant
bits of the 1-byte Type of Service (ToS) field as the IP precedence. The IP precedence values
range between zero for low priority and seven for high priority.
–
Layer 3 differentiated services code point (DSCP) values—The Internet Engineering Task
Force (IETF) defines the six most significant bits of the 1-byte ToS field as the DSCP. The
priority that is represented by a particular DSCP value is configurable. The DSCP values range
between 0 and 63 (for more information, see the
“Configuring the DSCP Value Maps” section
on page 52-73
).
