Chapter 14 Configuring Qo; Qos Terminology - Cisco WS-C4003 - Catalyst 4000 Chassis Switch Software Configuration Manual

Understanding How QoS Works
QoS implements scheduling on supported egress ports with transmit queue drop thresholds and multiple
transmit queues that use the 802.1p CoS values to give preference to higher-priority traffic.
Figure 14-1
Figure 14-1 Traffic Flow Through the Switch with QoS Enabled—Catalyst 4000 Family,
Frame enters

QoS Terminology

The following QoS terminology is used in this chapter:
•
•
Software Configuration Guide—Catalyst 4000 Family, Catalyst 2948G, Catalyst 2980G, Releases 6.3 and 6.4
14-2
shows how QoS affects the traffic flow.
Catalyst 2948G, and Catalyst 2980G Switches
Incoming
802.1Q
switch
frame?
Yes
Honor
current
CoS value
Address
lookup
and other
processing
2
Map
frame CoS
value to transmit
queue
QoS labels are used to prioritize traffic:
Layer 2 CoS values—Layer 2 802.1Q frame headers have a 2-byte Tag Control Information field
–
that carries the CoS value in the three most significant bits (the User Priority bits). Other frame
types cannot carry CoS values. CoS values range between zero (low priority) and seven (high
priority).
Classification is the selection of traffic to be marked.
1
Apply
No
switch
default
CoS value
1
From set qos default cos command
2
From set qos map command
Yes
Queue 1
No
Queue
full?
Queue 2
No
Queue
full?
Yes
Chapter 14 Configuring QoS
Drop
frame
Write new
or original
CoS value
Yes
Outgoing
No
Transmit
802.1Q
frame
frame?
Drop
frame
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