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26-7
Catalyst 2950 Desktop Switch Software Configuration Guide
78-14982-01
Chapter 26 Configuring QoS
Understanding QoS
• Only one policer can be applied to a packet in the input direction.
• Only the average rate and committed burst parameters are configurable.
• Policing occurs on the ingress interfaces:
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60 policers are supported on ingress Gigabit-capable Ethernet ports.
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6 policers are supported on ingress 10/100 Ethernet ports.
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Granularity for the average burst rate is 1 Mbps for 10/100 ports and 8 Mbps 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
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 one of the four egress queues.
The CoS-to-DSCP and DSCP-to-CoS maps have default values that might or might not be appropriate
for your network.
For configuration information, see the “Configuring CoS Maps” section on page 26-24.
Queueing and Scheduling
Note Both the EI and SI support this feature.
The switch provides QoS-based 802.1P CoS values. QoS uses classification and scheduling 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 802.1P CoS on a Catalyst 2950 switch that operates with the Catalyst 6000 family of
switches, refer to the Catalyst 6000 documentation. There are differences in the 802.1P implementation
that you should understand to ensure compatibility.