14-2
Catalyst 4500 Series, Catalyst 2948G, Catalyst 2980G Switches Software Configuration Guide
—
Release 8.1
78-15486-01
Chapter 14 Configuring QoS
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
shows how QoS affects the traffic flow.
Figure 14-1 Traffic Flow Through the Switch with QoS Enabled—Catalyst 4500 Series, Catalyst 2948G,
and Catalyst 2980G Switches
Understanding QoS Terminology
The following QoS terminology is used in this chapter:
•
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 0 (low priority) and 7 (high priority).
•
Classification is the selection of traffic to be marked.
Yes
Incoming
802.1Q
frame?
1
Apply
switch
default
CoS value
1
From set qos default cos command
2
From set qos map command
2
Map
frame CoS
value to transmit
queue
Honor
current
CoS value
Drop
frame
Drop
frame
Write new
or original
CoS value
Address
lookup
and other
processing
No
Outgoing
802.1Q
frame?
No
Queue
full?
Queue
full?
No
No
Yes
Yes
Yes
Queue 2
Queue 1
Frame enters
switch
Transmit
frame
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