Prioritization - Cisco Catalyst 4500 Series Software Configuration Manual

Overview of QoS
This section contains the following subsections:
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Prioritization

QoS implementation is based on the DiffServ architecture. This architecture specifies that each packet
is classified upon entry into the network. The classification is carried in the IP packet header, using 6
bits from the deprecated IP type of service (TOS) field to carry the classification (class) information.
Classification can also be carried in the Layer 2 frame. These special bits in the Layer 2 frame or
a Layer 3 packet are described here and shown in
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Catalyst 4500 Series Switch, Cisco IOS Software Configuration Guide - Cisco IOS XE 3.9.xE and IOS 15.2(5)Ex
44-2
Prioritization, page 44-2
QoS Terminology, page 44-3
Basic QoS Model, page 44-5
Classification, page 44-6
Policing and Marking, page 44-8
Queueing and Scheduling, page 44-8
Packet Modification, page 44-9
Per Port Per VLAN QoS, page 44-10
Flow-based QoS, page 44-10
Using Metadata in QoS Policy, page 44-11
Prioritization values in Layer 2 frames:
Layer 2 Inter-Switch Link (ISL) frame headers have a 1-byte User field that carries an IEEE 802.1p
class of service (CoS) value in the three least-significant bits. On interfaces configured as Layer 2
ISL trunks, all traffic is in ISL frames.
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. On interfaces configured
as Layer 2 802.1Q trunks, all traffic is in 802.1Q frames except for traffic in the native VLAN.
Other frame types cannot carry Layer 2 CoS values.
Layer 2 CoS values range from 0 for low priority to 7 for high priority.
Prioritization bits in Layer 3 packets:
Layer 3 IP packets can carry either an IP precedence value or a Differentiated Services Code Point
(DSCP) value. QoS supports the use of either value because DSCP values are backward-compatible
with IP precedence values.
IP precedence values range from 0 to 7.
DSCP values range from 0 to 63.
Chapter 44
Figure 44-1:
Configuring Quality of Service