Table of Contents
Advertisement
Understanding How QoS Works
QoS sets Layer 2 and Layer 3 values in network traffic to a configured value or to a value based on
received Layer 2 or Layer 3 values. IP traffic retains the Layer 3 value when it leaves the switch.
These sections describe QoS:
•
•
•
•
•
•
•
•
Definitions
This section defines some QoS terminology:
•
•
•
•
•
Catalyst 6000 Family Software Configuration Guide—Releases 6.3 and 6.4
41-2
Definitions, page 41-2
Flowcharts, page 41-3
QoS Feature Set Summary, page 41-8
Ethernet Ingress Port Marking, Scheduling, Congestion Avoidance, and Classification, page 41-10
Classification, Marking, and Policing with a Layer 3 Switching Engine, page 41-14
Classification and Marking with a Layer 2 Switching Engine, page 41-24
Ethernet Egress Port Scheduling, Congestion Avoidance, and Marking, page 41-24
QoS Statistics Data Export, page 41-27
Packets carry traffic at Layer 3.
Frames carry traffic at Layer 2. Layer 2 frames carry Layer 3 packets.
Labels are prioritization values carried in packets and frames:
–
Layer 2 class of service (CoS) values range between zero for low priority and seven for high
priority:
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.
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.
Other frame types cannot carry CoS values.
On ports configured as ISL trunks, all traffic is in ISL frames. On ports configured as
Note
802.1Q trunks, all traffic is in 802.1Q frames except for 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 IP precedence. 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 represented by a particular DSCP value is configurable. DSCP values range between 0
and 63 (for more information, see the
Layer 3 IP packets can carry either an IP precedence value or a DSCP value. QoS
Note
supports the use of either value, because DSCP values can be set equal to IP precedence
values.
Classification is the selection of traffic to be marked.
Marking, according to RFC 2475, is the process of setting a Layer 3 DSCP value in a packet; in this
publication, the definition of marking is extended to include setting Layer 2 CoS values.
"Configuring DSCP Value Maps" section on page
Chapter 41
Configuring QoS
41-55).
78-13315-02
Hide quick links:
Advertisement
Table of Contents
Troubleshooting
