Configuring Qos; Understanding How Qos Works; Chapter 14 Configuring Qo; Understanding Qos Terminology - Cisco WS-C2948G-GE-TX 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 4500 Series, Catalyst 2948G,
Frame enters

Understanding QoS Terminology

The following QoS terminology is used in this chapter:
•
•
Catalyst 4500 Series, Catalyst 2948G, Catalyst 2948G-GE-TX, and Catalyst 2980G Switches Software Configuration Guide—Release 8.2GLX
14-2
shows how QoS affects the traffic flow.
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 0 (low priority) and 7 (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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