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Catalyst2950 and Catalyst2955 Switch Software Configuration Guide
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Chapter30 Co nf iguring QoS
Understanding QoS
Mapping Tables
Note This feature is available only if your switch is running the EI .
During classification, QoS uses a configurable CoS-to-DSCP map to derive an internal DSCP value from the
received CoS value. This DSCP value represents the priority of the traffic.
Before the traffic reaches the scheduling stage, QoS uses the configurable DSCP-to-CoS map to derive
a CoS value from the internal DSCP value. The CoS value is used to select o ne of th e f our e gress queu es.
The CoS-to-DSCP and DSCP-to-CoS maps have default values that might or might n ot be ap pro pria te
for your network.
For configuration information, see the “Configuring CoS Maps” section on page30-34.
Queueing and Scheduling
Note Both the SI and EI support this feature.
The switch gives QoS-based 802.1p CoS values. QoS uses classification and scheduling to send network
traffic from the switch in a predictable manner. Qo S classif ies f rames b y assigning priorit y-inde xed CoS
values to them and gives preference to higher-priority traffic such as telephone calls.

How Class of Service Works

Before you set up 802.1p CoS on a Catalyst 2950 or Catalyst 2955 switc h that ope rates wi th the
Catalyst 6000 family of switches, refer to the Catalyst 6000 documentation. There are dif feren ces in the
802.1p implementation that you should understand to ensure compatibility.

Port Priority

Frames received from users in the administratively defined VLANs are classified or tagged for
transmission to other devices. Based on rules that you define, a unique identifier (the tag) is inserted in
each frame header before it is forwarded. The tag is examined and under stood b y each devi ce befo re an y
broadcasts or transmissions to other switches, routers, or end stations. When t he f rame rea ches th e l ast
switch or router, the tag is removed before the frame is sent to the target end station. VLANs that are
assigned on trunk or access ports without identification or a ta g ar e ca lled native or untagged frames.
For IEEE 802.1Q frames with tag information, the priority value from the header frame is used. For native
frames, the default priority of the input port is used.

Port Scheduling

Each port on the switch has a single receive queue buffer (the ingress port) for incoming traffic. When
an untagged frame arrives, it is assigned the value of the port as its port default priority. You assign this
value by using the CLI or CMS. A tagged frame continues to use its assigned CoS value when it passes
through the ingress port.