Basic Qos Model - Cisco Catalyst 3550 series Software Configuration Manual

Understanding QoS

Basic QoS Model

Figure 29-2
policing, and marking:
•
•
•
Actions at the egress interface include queueing and scheduling:
•
•
Policing and marking also can occur on egress interfaces.
Note
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.
Catalyst 3550 Multilayer Switch Software Configuration Guide
29-4
shows the basic QoS model. Actions at the ingress interface include classifying traffic,
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 internal DSCP to the
configured policer. 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
on page 29-8.
Marking evaluates the policer and the 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
Queueing evaluates the internal DSCP and determines which of the four egress queues in which to
place the packet. The DSCP value is mapped to a CoS value, which selects one of the queues. For
more information, see the
Scheduling services the four egress queues based on their configured weighted round robin (WRR)
weights and thresholds. One of the queues can be the expedite queue, which is serviced until empty
before the other queues are serviced. Congestion avoidance techniques include tail drop and
Weighted Random Early Detection (WRED) on Gigabit-capable Ethernet ports and tail drop (with
only one threshold) on 10/100 Ethernet ports. For more information, see the
Scheduling" section on page
Actions at ingress
In profile or
out of profile
Policing
Compare DSCP to
the configured
policer and
determine if the
packet is in profile or
out of profile.
29-8.
"Mapping Tables" section on page
29-11.
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.
Chapter 29
29-5.
"Policing and Marking" section
29-10.
"Queueing and
Actions at egress
Queueing and
scheduling
Based on the CoS,
determine into which
of the egress
queues to place the
packet. Then service
the queues
according to the
configured weights.
Configuring QoS
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