Cisco WS-C4003 Software Guide - Page 224
QoS Terminology, Traffic Flow Through the Switch with QoS Enabled-Catalyst 4000 Family
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Understanding How QoS Works Chapter 14 Configuring QoS 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 4000 Family, Catalyst 2948G, and Catalyst 2980G Switches Frame enters switch Incoming No 802.1Q frame? Yes Honor current CoS value 1Apply switch default CoS value 1From set qos default cos command 2From set qos map command Address lookup and other processing 2Map frame CoS Queue 1 value to transmit queue Yes Queue No full? Queue 2 Queue No full? Yes Drop frame Yes Outgoing No 802.1Q frame? Drop frame Write new or original CoS value Transmit frame 26705 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 zero (low priority) and seven (high priority). • Classification is the selection of traffic to be marked. 14-2 Software Configuration Guide-Catalyst 4000 Family, Catalyst 2948G, Catalyst 2980G, Releases 6.3 and 6.4 78-12647-02
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