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Catalyst 2950 and Catalyst 2955 Switch Software Configuration Guide
78-11380-12
Chapter 29 Configuring QoS
Understanding QoS
Basic QoS Model
Figure 29-2 shows the basic QoS model. Actions at the ingress interface include classifying traffic,
policing, and marking:
Note If you have the SI installed on your switch, only the queueing and scheduling features are available.
• Classifying distinguishes one kind of traffic from another. For more information, see the
“Classification” section on page 29-5.
• Policing determines whether a packet is in or out of profile according to the configured policer, and
the policer limits the bandwidth consumed by a flow of traffic. The result of this determination is
passed to the marker. For more information, see the “Policing and Marking” section on page 29-7.
• Marking evaluates the policer and configuration information for the action to be taken when a packet
is out of profile and decides what to do with the packet (pass through a packet without modification,
mark down the DSCP value in the packet, or drop the packet). For more information, see the
“Policing and Marking” section on page 29-7.
Actions at the egress interface include queueing and scheduling:
• Queueing evaluates the CoS value and determines which of the four egress queues in which to place
the packet.
• Scheduling services the four egress queues based on their configured weighted round robin (WRR)
weights.
Figure 29-2 Basic QoS Model
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Classification Policing
Actions at ingress Actions at egress
Mark
In profile or
out of profile
Classifies the packet
based on the ACL.
Determines if the
packet is in profile or
out of profile based
on the policer
associated with the
filter.
Based on whether
the packet is in or
out of profile and the
configured
parameters,
determines whether
to pass through,
mark down, or drop
the packet. The
DSCP and CoS are
marked or changed
accordingly.
Queuing and
scheduling
Based on the CoS,
determines into
which of the egress
queues to place the
packet, then
services the queues
according to the
configured weights.
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