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This chapter describes the Quality of Service (QoS) features that are used to prevent traffic congestion.
QoS lets you classify network traffic so that it can be policed and prioritized in a way that prevents congestion. Traffic is processed based on how you classify it and the QoS policies that you put in place.
You can implement a QoS policy using the following steps:
Step 1 Define a traffic class by using the class-map command.
Step 2 Create a traffic policy by using the policy-map command.
A traffic policy defines how specific traffic is to be acted upon to improve the quality of service.
Step 3 Attach the traffic policy to an interface or port profile by using the service-policy command.
Traffic classification and traffic marking are used together to sort and modify traffic for the best quality of service. Table 1-1 describes these processes.
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Classifying network traffic allows you to organize traffic (that is, packets) into traffic classes or categories on the basis of whether the traffic matches specific criteria. Classifying network traffic (used in conjunction with marking network traffic) is the foundation for applying QoS actions. For more information, see Chapter2, “Configuring QoS Classification”
Marking is the process of assigning a priority and involves changing the fields, such as class of service or DSCP, in a packet. The traffic is then marked accordingly as it comes into the device on an ingress interface. The markings are used to treat the traffic as it leaves the device on the egress interface. For more information about configuring marking, see Chapter3, “Configuring QoS Marking Policies”
Policing is the monitoring of data rates and burst sizes for a particular class of traffic. QoS policing on a network determines whether network traffic is within a specified profile (contract). This may cause out-of-profile traffic to drop or to be marked down to another differentiated services code point (DSCP) value to enforce a contracted service level. DSCP is a measure of the QoS level of the frame. For more information about configuring policing, see Chapter4, “Configuring QoS Policing”
QoS configuration commands are shown in Table 1-2 .
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Port Profile Configuration Command |
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Defines a table map that represents a mapping from one set of field values to another set of field values. You can reference a table map from a policy map. |
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Defines a policy map that represents a set of policies to be applied to a set of class maps. Policy maps can reference table maps. |
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Applies a specified policy map to input or output packets on interfaces configured as follows:
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1.For information about port profiles see the Cisco Nexus 1000V Port Profile Configuration Guide, Release 4.0(4)SV1(1). |
Statistics are maintained for each policy, class action, and match criteria per interface. For information about monitoring QoS statistics, see Chapter5, “Monitoring QoS Statistics”
There is no default QoS behavior. Policing and prioritization of traffic are only implemented when you apply a policy map to an interface.
However, when designing your QoS and ACL policies, keep in mind that ACLs referenced within a QoS policy are processed as follows as part of the QoS policy:
The following RFCs are supported by QoS.
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Management Information Base for the Differentiated Services Architecture |
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QoS recovers its previous state after a software restart, and it is able to switch over from the active supervisor to the standby supervisor without a loss of state.