Basic Qos Model; Classification - Cisco 4500M Software Manual

Chapter 29 Configuring QoS

Basic QoS Model

Figure 29-2
traffic, policing, and marking:
•
•
•
Actions at the egress interface include queueing and scheduling:
•
•
Figure 29-2 Basic QoS Model
Generate DSCP

Classification

Inspect packet and
determine the DSCP
based on ACLs or
the configuration.
Map the Layer 2
CoS value to a
DSCP value.
Classification
Classification is the process of distinguishing one kind of traffic from another by examining the fields
in the packet. Classification is enabled only if QoS is globally enabled on the switch. By default, QoS is
globally disabled, so no classification occurs.
You specify which fields in the frame or packet that you want to use to classify incoming traffic.
Classification options are shown in
OL-6696-01
shows the basic QoS model. Actions at the ingress and egress interfaces include classifying
Classifying distinguishes one kind of traffic from another. The process generates an internal DSCP
for a packet, which identifies all the future QoS actions to be performed on this packet. For more
information, see the "Classification" section on page
Policing determines whether a packet is in or out of profile by comparing the traffic rate to the
configured policer, which limits the bandwidth consumed by a flow of traffic. The result of this
determination is passed to the marker. For more information, see the
on page 29-9.
Marking evaluates the policer configuration information regarding 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
Queueing evaluates the internal DSCP and determines which of the four egress queues in which to
place the packet.
Scheduling services the four egress (transmit) queues based on the sharing and shaping
configuration of the egress (transmit) port. Sharing and shaping configurations are described in the
"Queueing and Scheduling" section on page
Actions at ingress and egress
In profile or
out of profile
Policing
Compare traffic rate to
the configured policer
and determine if the
packet is in profile or
out of profile.
29-5.
29-9.
29-13.
Mark
Based on whether the
packet is in or out of
profile and the configured
parameters, determine
whether to pass through,
mark down, or drop the
packet. The DSCP and
CoS are marked or
changed accordingly.
Figure 29-3.
Software Configuration Guide—Release 12.2(25)EW
Overview of QoS
"Policing and Marking" section
Actions at egress
Queueing and
scheduling
Based on the marked
DSCP, determine into
which of the egress
queues to place the
packet. Then service
the queues according
to the configured
weights.
29-5