Overview

Supported Platforms

Starting with Cisco NX-OS release 7.0(3)I7(1), use the Nexus Switch Platform Support Matrix to know from which Cisco NX-OS releases various Cisco Nexus 9000 and 3000 switches support a selected feature.

About QoS Features

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.

  • 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 can be modified.

Using QoS

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.

Classification

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

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

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.

Queuing and Scheduling

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 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.

Sequencing of QoS Actions

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.

Sequencing of Ingress Traffic Actions

The sequence of QoS actions on ingress traffic is as follows:

  1. Classification

  2. Marking

  3. Policing

Sequencing of Egress Traffic Actions

The sequencing of QoS actions on egress traffic is as follows:

  1. Queuing and scheduling

High Availability Requirements for QoS Features

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 3600 NX-OS High Availability and Redundancy Guide.

QoS Feature Configuration with MQC

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

Table 1. MQC Configuration Commands

MQC Command

Description

class-map

Defines a class map that represents a class of traffic.

policy-map

Defines a policy map that represents a set of policies to be applied to a set of class maps.

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:

Table 2. Interface Command to Attach a Policy Map to an Interface

Interface Command

Description

service-policy

Applies the specified policy map to input or output packets on the interface.

QoS Statistics

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.

Default QoS Behavior

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.


Note

There is also a default QoS policy that can be applied at the system level. It is inherited by all ports up to the point where the user applies a per-port policy.

The following table shows the default settings for various interface modes:

Table 3. Default Settings for Interface Modes

Trust DSCP/CoS by Default

Ingress

Egress (After Traffic is Routed)1

SVI

CoS

DSCP

Routed Interface

DSCP

DSCP

Layer 2 Interface

CoS22

DSCP

1

When traffic is routed, the DSCP value is used (by default) to derive the egress queue. If the egress interface is the trunk, the CoS is derived from the DSCP value of the routed packet.

2

When the Layer 2 Interface is an access port, it is considered as no CoS. CoS is set to 0 in the case when access to the trunk interface with bridged traffic, even if DSCP bits are set.

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

Virtual Device Contexts

Cisco NX-OS can segment operating system and hardware resources into virtual device contexts (VDCs) that emulate virtual devices. The Cisco Nexus 3600 device currently does not support multiple VDCs. All device resources are managed in the default VDC.

Notes for Enabling VLAN QoS

The VLAN QoS feature enables Layer 2 bridged database lookup for QoS with VLAN as the key instead of the port.

To enable VLAN QoS, you must decrease the TCAM size of another region and increase the TCAM size for the VLAN QoS region.

To configure the size of the VLAN QoS TCAM region:

  • Configure the IPv4 vqos to 640 entries.

  • Configure the IPv6 ipv6-vqos to 256 entries.

  • Decrease the IPv4 qos to 0 entries.

  • Decrease the IPv6 ipv6-qos to 0 entries. 


switch(config)# hardware access-list tcam region vqos 640
switch(config)# hardware access-list tcam region ipv6-vqos 256
switch(config)# hardware access-list tcam region qos 0
switch(config)# hardware access-list tcam region ipv6-qos 0

Note

After configuring the TCAM size for VLAN QOS, it is necessary to reload the line card.

Note

QoS has default TCAM sizes and these TCAM sizes must be non-zero on specific line cards to avoid line card failure during reload.

Nexus 3600 switches with the following line cards are affected:

  • N3K-C3636C-R

  • N3K-C36180YC-R