You use the QoS features to provide the most desirable flow of traffic through a network. QoS allows you to classify the network traffic, police and prioritize the traffic flow, and help avoid traffic congestion in a network. The control of traffic is based on the fields in the packets that flow through the system. You use the Modular QoS (MQC) CLI to create the traffic classes and policies of the QoS features.

QoS features are applied using QoS and queuing policies as follows:

• QoS policies include classification and marking features.

• QoS policies include policing features.

• QoS policies include shaping, weighted random early detection (WRED), and explicit congestion notification (ECN) features.

• Queuing policies use the queuing and scheduling features.

Note	The system-defined QoS features and values that are discussed in the “Using Modular QoS CLI” section apply globally to the entire device and cannot be modified.

Traffic is processed based on how you classify it and the policies that you create and apply to traffic classes.

To configure QoS features, you use the following steps:

1. Create traffic classes by classifying the incoming packets that match criteria such as IP address or QoS fields.

2. Create policies by specifying actions to take on the traffic classes, such as policing, marking, or dropping packets.

3. Apply policies to a port, port channel, or subinterface.

You use MQC to create the traffic classes and policies of the QoS features.

Note	The queuing and scheduling operations of the overall QoS feature are applicable to both IPv4 and IPv6.

Note	IP tunnels do not support access control lists (ACLs) or QoS policies.

You use classification to partition traffic into classes. You classify the traffic based on the port characteristics or the packet header fields that include IP precedence, differentiated services code point (DSCP), Layer 3 to Layer 4 parameters, and the packet length.

The values used to classify traffic are called match criteria. When you define a traffic class, you can specify multiple match criteria, you can choose to not match on a particular criterion, or you can determine the traffic class by matching any or all criteria.

Traffic that fails to match any class is assigned to a default class of traffic called class-default.

Marking is the setting of QoS information that is related to a packet. You can set the value of a standard QoS field for COS, IP precedence and DSCP, and internal labels (such as QoS groups) that can be used in subsequent actions. Marking QoS groups is used to identify the traffic type for queuing and scheduling traffic.

Policing is the monitoring of data rates for a particular class of traffic. The device can also monitor associated burst sizes.

Single-rate policers monitor the specified committed information rate (CIR) of traffic. Dual-rate policers monitor both CIR and peak information rate (PIR) of traffic.

The queuing and scheduling process allows you to control the bandwidth allocated to traffic classes so that you achieve the desired trade-off between throughput and latency.

You can apply weighted random early detection (WRED) to a class of traffic, which allows packets to be dropped based on the QoS group. The WRED algorithm allows you to perform proactive queue management to avoid traffic congestion.

You can shape traffic by imposing a maximum data rate on a class of traffic so that excess packets are retained in a queue to smooth (constrain) the output rate. In addition, minimum bandwidth shaping can be configured to provide a minimum guaranteed bandwidth for a class of traffic.

You can limit the size of the queues for a particular class of traffic by applying either static or dynamic limits.

ECN can be enabled along with WRED on a particular class of traffic to mark the congestion state instead of dropping the packets.

The following are the three types of policies:

• network qos—Defines the characteristics of QoS properties network wide.

• qos—Defines MQC objects that you can use for marking and policing.

• queuing—Defines MQC objects that you can use for queuing and scheduling.

Note	The default type of policy is qos.

The system performs actions for QoS policies only if you define them under the type qos service policies.

The Cisco NX-OS QoS software recovers its previous state after a software restart, and it is capable of a switchover from the active supervisor to the standby supervisor without a loss of state.

Note	For complete information on high availability, see the Cisco Nexus 9000 Series NX-OS High Availability and Redundancy Guide.

You use MQC to configure QoS features. The MQC configuration commands are shown in the following table:

You can modify or delete MQC objects, except system-defined objects, when the objects are not associated with any interfaces.

After a QoS policy is defined, you can attach the policy map to an interface by using the interface configuration command shown in the following table:

Statistics are maintained for each policy, class action, and match criteria per interface. You can enable or disable the collection of statistics, you can display statistics using the show policy-map interface command, and you can clear statistics based on an interface or policy map with the clear qos statistics command. Statistics are enabled by default and can be disabled globally.

The QoS queuing features are enabled by default. Specific QoS-type features, such as policing and marking, are enabled only when a policy is attached to an interface. Specific policies are enabled when that policy is attached to an interface.

By default, the device always enables a system default queuing policy, or system-defined queuing policy map, on each port and port channel. When you configure a queuing policy and apply the new queuing policy to specified interfaces, the new queuing policy replaces the default queuing policy, and those rules now apply.

The device enables other QoS features, policing and marking, only when you apply a policy map to an interface.

Cisco NX-OS can segment operating system and hardware resources into virtual device contexts (VDCs) that emulate virtual devices. The Cisco Nexus 9000 Series device currently does not support multiple VDCs. All device resources are managed in the default VDC.

Note	The VDC feature is not supported on the Cisco Nexus 9508 switch (NX-OS 7.0(3)F3(3)).