Cisco Catalyst 4500 Series Configuration Guide

40-2
Software Configuration Guide—Release IOS XE 3.3.0SG and IOS 15.1(1)SG
OL-25340-01
Chapter 40 Configuring Quality of Service
Overview of QoS
This section contains the following subsections:
• Prioritization, page 40-2
• QoS Terminology, page 40-3
• Basic QoS Model, page 40-5
• Classification, page 40-6
• Policing and Marking, page 40-8
• Queueing and Scheduling, page 40-8
• Packet Modification, page 40-9
• Per Port Per VLAN QoS, page 40-10
• Flow-based QoS, page 40-10
• Using Metadata in QoS Policy, page 40-11
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 Figure 40-1:
• 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.

Cisco Catalyst 4500 Series Specifications

General

Series	Catalyst 4500 Series
Category	Switch
Layer Support	Layer 2, Layer 3
Form Factor	Modular chassis
Stackable	No
Chassis Slots	3, 6, 7, 10
Power Supply Options	AC, DC
Redundancy	Power supply, Supervisor engine
Network Management	Cisco IOS Software CLI, SNMP, Cisco Prime Infrastructure
Features	Security, QoS
Port Density	Up to 384 ports per chassis
Security Features	802.1X, ACLs, DHCP Snooping, Dynamic ARP Inspection, IP Source Guard
Supervisor Engine	8-E