QoS: Congestion Management Configuration Guide, Cisco IOS XE Release 3S
Multi-Level Priority Queues
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Multi-Level Priority Queues

Multi-Level Priority Queues

The Multi-Level Priority Queues (MPQ) feature allows you to configure multiple priority queues for multiple traffic classes by specifying a different priority level for each of the traffic classes in a single service policy map. You can configure multiple service policy maps per device. Having multiple priority queues enables the device to place delay-sensitive traffic (for example, voice) on the outbound link before delay-insensitive traffic. As a result, high-priority traffic receives the lowest latency possible on the device.

Finding Feature Information

Your software release may not support all the features documented in this module. For the latest caveats and feature information, see Bug Search Tool and the release notes for your platform and software release. To find information about the features documented in this module, and to see a list of the releases in which each feature is supported, see the feature information table at the end of this module.

Use Cisco Feature Navigator to find information about platform support and Cisco software image support. To access Cisco Feature Navigator, go to www.cisco.com/​go/​cfn. An account on Cisco.com is not required.

Prerequisites for Multi-Level Priority Queues

You must configure traffic classes using the class-map command.

Restrictions for Multi-Level Priority Queues

  • You cannot configure both the priority command and the priority level command for two different classes in the same policy map. For example, the device does not accept the following configuration:
    Device> enable
    Device# configure terminal
    Device(config)# policy-map Map1
    Device(config-pmap)# class Bronze
    Device(config-pmap-c)# priority level 1
    Device(config-pmap-c)# exit
    Device(config-pmap)# class Gold
    Device(config-pmap-c)# priority rate 1000
    Device(config-pmap-c)# end
    
  • You cannot specify the same priority level for two different classes in the same policy map. For example, the device does not accept the following configuration:
    Device> enable
    Device# configure terminal
    Device(config)# policy-map Map1
    Device(config-pmap)# class Bronze
    Device(config-pmap-c)# priority level 1
    Device(config-pmap-c)# police cir percent 30
    Device(config-pmap-c)# exit
    Device(config-pmap)# class Gold
    Device(config-pmap-c)# priority level 1
    Device(config-pmap-c)# police cir 10000
    Device(config-pmap-c)# end
    
  • You cannot configure the default queue as a priority queue at any level.
  • You cannot configure the bandwidth command and multi-level priority queues on the same class. For example, the device rejects the following configuration:
    policy-map P1
     class C1
      priority level 1
      bandwidth 200
    
  • You cannot configure the shape command and multi-level priority queues on the same class. For example, the device rejects the following configuration:
    policy-map P1
     class C1
      priority level 1
      shape average 56000
    
  • To convert a one-level (flat) service policy with multiple priority queues configured to a hierarchical multi-level priority queues service policy, you must first detach the flat service policy from the interface using the no service-policy command and then add a child policy map to it.
  • The sum of the police committed information rate (cir), which can be specified as in kilobits per second (kbps) or as a percentage, configured at the two priority levels cannot exceed the interface bandwidth. Any policy that has a police cir sum greater than the interface bandwidth will be rejected. For example, the following policy will be rejected by a 1-gigabit (Gb) interface because the sum of the police cir (600 Mb + 700 Mb) is greater than 1 Gb.
    policy-map new
     class prec1
      priority level 1
      police cir 600000000
     class prec2
       priority level 2
       police cir 700000000
  • On Cisco ASR 1000 Series Aggregation Services Routers, the use of a conditional priority rate limiter, such as bandwidth-kbps or percentage, is not supported in the lowest level (that is, grandchild or leaf) of a three-layer policy map configuration. At the lowest level of a three level policy, the conditional limiter will not be applied. However, priority with a strict policer is supported at this level of the hierarchy. This restriction does not apply to flat or two-level hierarchical policy maps.

Information About Multi-Level Priority Queues

Benefits of Multi-Level Priority Queues

The MPQ feature allows you to configure multiple priority queues for multiple traffic classes by specifying a different priority level for each of the traffic classes in a single service policy map. You can configure multiple service policy maps per device.

Previously, devices could have only one strict priority queue per policy map for all delay-sensitive traffic—the device associated all priority traffic with this one single priority queue. However, having only one priority queue can cause significant delay in delivering traffic, especially if the device sends high-priority traffic (for example, voice) behind low-priority traffic (for example, video). Using class-based weighted fair queueing (CBWFQ) to reduce delay by heavily weighting one queue can affect the granularity of bandwidth allocations to the other queues. The MPQ feature addresses these issues and improves latency.

Functionality of Multi-Level Priority Queues

The priority command is used to specify that a class of traffic has latency requirements with respect to other classes. For multiple priority queues, you can use the priority level command to configure a level of priority service on a class in a policy map. Currently, the device supports two priority levels: level 1 (high) and level 2 (low). The device places traffic with a high-priority level on the outbound link ahead of traffic with a low-priority level. High-priority packets, therefore, are not delayed behind low-priority packets.

The device services the high-level priority queues until empty before servicing the next-level priority queues and non-priority queues. While the device services a queue, the service rate is as fast as possible and is constrained only by the rate of the underlying link or parent node in a hierarchy. If a rate is configured and the device determines that a traffic stream has exceeded the configured rate, the device drops the exceeding packets during periods of congestion. If the link is currently not congested, the device places the exceeding packets onto the outbound link.

When configuring MPQ on different traffic classes in a policy map, you must specify different priority levels for the traffic classes. For example, configure one traffic class to have priority level 2 and another class to have priority level 1.


Note


In a hierarchical MPQ configuration in which all traffic is sent through the level-2 priority queue only, the traffic sent through the level-2 priority queue receives the same treatment as the traffic sent through the level-1 priority queue.


If high-priority traffic is not policed appropriately, bandwidth starvation of low-priority traffic can occur. Therefore, though not required, we recommend that you configure a policer for high-priority traffic using the police command. If you configure the police command for priority queues, the traffic rate is policed to the police rate for each of the priority queues.

You cannot configure the priority command and the priority level command on different classes in the same policy map.

Traffic Policing and Multi-Level Priority Queues

Bandwidth guarantees can be given to other classes only if traffic policing is enabled on the priority queue.

Using the priority and police commands, multi-level priority queues can be configured to police traffic in one of the following ways:

  • Conditional traffic policing, for example:
    policy-map my_policy
      class voice
       priority 400000   <<< Priority queue conditionally policed to 400M
      class gold
       bandwidth 400000 <<< 400M minimum guaranteed to class gold
    
    With conditional traffic policing on the queue, you run the risk of sudden degradation in priority service when an interface becomes congested. You can go from an instance of a priority class using the entire link to suddenly traffic being policed to the configured value. You need to know the available bandwidth and use some form of admission control to ensure that your offered loads do not exceed the available bandwidth.

    Note


    With the conditional policing, traffic policing does not engage unless the interface is congested.


  • Unconditional traffic policing, for example:
    policy-map my_policy
      class voice
       priority           <<< Indicates priority scheduling
       police 400000000   <<< Traffic policed to 400M
      class gold
       bandwidth 400000  <<<400M minimum guaranteed to class gold
    
    The priority class is configured with an “always on” (unconditional) policer. The priority class is always policed to the configured value regardless of whether the interface is congested. The advantage of an unconditional policer is that you always know how much priority traffic will be offered to the downstream devices, thus making your bandwidth planning much simpler. This is the recommended choice.
  • Absolute priority queue (no traffic policing) If traffic policing is not configured, the priority traffic may consume the entire interface bandwidth.

How to Configure Multi-Level Priority Queues

Configuring Multi-Level Priority Queues in a Policy Map

Before You Begin

The traffic classes, class maps, and policy maps must exist.

SUMMARY STEPS

    1.    enable

    2.    configure terminal

    3.    policy-map policy-name

    4.    class class-name

    5.    priority level level

    6.    police cir bps

    7.    police cir percent percent

    8.    end


DETAILED STEPS
     Command or ActionPurpose
    Step 1 enable


    Example:
    Device> enable
     

    Enables privileged EXEC mode.

    • Enter your password if prompted.
     
    Step 2 configure terminal


    Example:
    Device# configure terminal
     

    Enters global configuration mode.

     
    Step 3 policy-map policy-name


    Example:
    Device(config)# policy-map Premium
     

    Creates or modifies a policy map and enters policy-map configuration mode.

    • Enter the name of the policy map.
     
    Step 4 class class-name


    Example:
    Device(config-pmap)# class business
     

    Specifies a traffic class and enters policy-map class configuration mode.

    • Enter the name of a previously configured traffic class.
     
    Step 5 priority level level


    Example:
    Device(config-pmap-c)# priority level 2
     

    Assigns priority to a traffic class at the priority level specified.

    • Enter the level of priority assigned to the priority class. Valid values are 1 (high priority) and 2 (low priority). The default is 1.
    Note   

    Do not specify the same priority level for two different classes in the same policy map.

     
    Step 6 police cir bps


    Example:
    Device(config-pmap-c)# police cir 8000 
     

    (Optional) Configures traffic policing based on a bits per second (bps) rate.

    • cir is the committed information rate and is based on the interface shape rate. This keyword indicates an average rate at which the policer meters traffic.
    • bps specifies the average rate in bits per second (bps). Valid values are from 8000 to 2488320000 bps.
     
    Step 7 police cir percent percent


    Example:
    Device(config-pmap-c)# police cir percent 20 
     

    (Optional) Configures traffic policing based on a percentage of bandwidth available on the interface.

    • cir is the committed information rate and is based on the interface shape rate. This keyword indicates an average rate at which the policer meters traffic.
    • percent percent indicates to use the percentage of available bandwidth specified in percent to calculate the CIR. Valid values are from 1 to 100.
     
    Step 8 end


    Example:
    Device(config-pmap-c)# end
     

    (Optional) Exits policy-map class mode.

     

    Verifying Multi-Level Priority Queues

    SUMMARY STEPS

      1.    enable

      2.    show policy-map interface type number

      3.    exit


    DETAILED STEPS
       Command or ActionPurpose
      Step 1 enable


      Example:
      Device> enable
       

      Enables privileged EXEC mode.

      • Enter your password if prompted.
       
      Step 2 show policy-map interface type number


      Example:
      Device# show policy-map interface serial4/0/0
       

      Displays the packet statistics of all classes that are configured for all service policies either on the specified interface or subinterface or on a specific PVC on the interface.

      • Enter the interface type and number.
       
      Step 3 exit


      Example:
      Device# exit
       

      (Optional) Exits privileged EXEC mode.

       

      Configuration Examples for Multi-Level Priority Queues

      Example: Configuring Multi-Level Priority Queues

      The following example shows how to configure multiple priority queues. The policy map named Business has two traffic classes: Bronze and Gold. Bronze traffic has a level 2 (low) priority, whereas Gold traffic has a level 1 (high) priority. To prevent bandwidth starvation of Bronze traffic, the Gold traffic is policed at 30 percent of the interface bandwidth.

      Device> enable
      Device# configure terminal
      Device(config)# policy-map Business
      Device(config-pmap)# class Bronze
      Device(config-pmap-c)# priority level 2
      Device(config-pmap-c)# police cir 1000
      Device(config-pmap-c)# exit
      Device(config-pmap)# class Gold
      Device(config-pmap-c)# priority level 1
      Device(config-pmap-c)# police cir percent 30
      Device(config-pmap-c)# end
      

      Note


      Although a policer is not required, configure policing for priority traffic to prevent bandwidth starvation of low-priority traffic. When policing is configured, the traffic rate is policed at the police rate for each of the priority queues.


      Example: Verifying Multi-Level Priority Queues

      The following is partial sample output from the show policy-map interface command.

      Device# show policy-map interface serial2/1/0
      
      Serial2/1/0
      Service-policy output: P1
      Queue statistics for all priority classes:
      .
      .
      .
      Class-map: Gold (match-all)
      0 packets, 0 bytes				/*Updated for each priority level configured.*/
      5 minute offered rate 0 bps, drop rate 0 bps
      Match: ip precedence 2
      Priority: 0 kbps, burst bytes 1500, b/w exceed drops: 0
      Priority Level 2:
      0 packets, 0 bytes

      Additional References for Multi-Level Priority Queues

      Related Documents

      Related Topic

      Document Title

      QoS commands: complete command syntax, command modes, command history, defaults, usage guidelines, and examples

      Cisco IOS Quality of Service Solutions Command Reference

      Priority queues, creating classes, class maps, and policy maps

      “Applying QoS Features Using the MQC” module

      Technical Assistance

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      Link

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      http:/​/​www.cisco.com/​cisco/​web/​support/​index.html

      Feature Information for Multi-Level Priority Queues

      The following table provides release information about the feature or features described in this module. This table lists only the software release that introduced support for a given feature in a given software release train. Unless noted otherwise, subsequent releases of that software release train also support that feature.

      Use Cisco Feature Navigator to find information about platform support and Cisco software image support. To access Cisco Feature Navigator, go to www.cisco.com/​go/​cfn. An account on Cisco.com is not required.

      Table 1 Feature Information for Multi-Level Priority Queues

      Feature Name

      Releases

      Feature Information

      Multi-Level Priority Queues

      Cisco IOS Release XE 2.1

      Cisco IOS Release XE 3.7S

      The MPQ feature allows you to configure multiple priority queues for multiple traffic classes by specifying a different priority level for each of the traffic classes in a single service policy map. You can configure multiple service policy maps per device. Having multiple priority queues enables the device to place delay-sensitive traffic (for example, voice) on the outbound link before delay-insensitive traffic. As a result, high-priority traffic receives the lowest latency possible on the device.

      in Cisco IOS XE Release 2.1, this feature was implemented on the Cisco ASR 1000 Series Routers.

      In Cisco IOS XE Release 3.7S, support was added for the Cisco ASR 903 Router.

      The following commands were introduced or modified: priority level, show policy-map interface.