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Catalyst 3550 Multilayer Switch Software Configuration Guide
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Chapter20 Co nf iguring QoS
Configuring QoS
Configuring QoS
Before configuring QoS, you must have a thorough understanding o f th ese i te ms:
The types of applications used and the traffic patterns on y our ne tw ork .
Traffic characteristics and needs of your network. Is the traffic bursty? Do you need to reserve
bandwidth for voice and video streams?
Bandwidth requirements and speed of the network.
Location of congestion points in the network.
This section describes how to configure QoS on your switch. It contains this configu ration information:
Default QoS Configuration, page 20-18
Enabling QoS Globally, page 20-21
Configuring Classification Using Port Trust States, page 20-21
Configuring a QoS Policy, page 20-26
Configuring DSCP Maps, page 20-39
Configuring Egress Queues on Gigabit-Capable Ethernet Ports, page 20-44
Configuring Egress Queues on 10/100 Ethernet Ports, page 20-51

Default QoS Configuration

Table20-1 show s the de faul t Q oS co nfi gura tion wh en Q oS is di sable d.
When QoS is disabled, there is no concept of trusted or untrusted ports because the packets are not
modified (the CoS, DSCP, and IP precedence values in the packet are not changed).
Table20-2 show s the defa ult Q oS para met ers w it hout an y fur t her c on figur ation whe n QoS i s en abl ed.
Table20-1 Default QoS Parameters when QoS is Disabled
Port
Type QoS
State
Egress traffic
(DSCP and CoS
Value) Queue Queue
Weights Tail-drop
Thresholds CoS Mapping
to Queue
Gigabit-capable
Ethernet ports Disabled Pass through. All of the queue
RAM is allocated to
queue 1 (no expedite
queue).
100%, 100%
WRED is
disabled.
All CoS
values map to
queue 1.
10/100 Ethernet
ports Disabled Pass through. Each of the eight
minimum-reserve
levels have a buffer
size of 100 packets.
The queue selects
the level.
––All CoS
values map to
queue 1.