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Cisco Systems IntelligentGigabit Ethernet Switch Modules for the IBM BladeCenter, Software Configuration Guide
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Chapter23 Configuring QoS
Understanding QoS
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60 policers are supported on ingress Gigabit-capable Ethernet ports.
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Granularity for the average burst rate is 8Mbps for Gigabit Ethernet ports.
•On an interface configured for QoS, all traffic received through the interface is classified, policed,
and marked according to the policy map attached to the interface. On a trunk interface configured
for QoS, traffic in all VLANs received through the interface is classified, policed, and marked
according to the policy map attached to the interface.
Note You cannot configure policers on the egress interfaces.
Mapping Tables
During classification, QoS uses a configurable CoS-to-DSCP map to derive an interna l 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 one of the four egress queues.
The CoS-to-DSCP and DSCP-to-CoS maps have default values that might or might n ot be appropriate
for your network.
For configuration information, see the “Configuring CoS Maps” section on page23-31.
Queueing and Scheduling
The switch gives QoS-based IEEE 802.1p CoS values. QoS uses classification and schedulin g to send
network traffic from the switch in a predictable manner. QoS classifies frames by assigning
priority-indexed CoS values to them and gives preference to higher-priority traffic such as telephone
calls.

How Class of Service Works

Before you set up IEEE 802.1p CoS on a Cisco Systems Intelligent Gigabit Ethernet Switch Module that
operates with the Catalyst6000 family of switches, see the Catalyst 6000 documentation. There are
differences in the IEEE 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 understood by each device before any
broadcasts or transmissions to other switches, routers, or end stations. When t he frame reaches the last
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 tag are called 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.