QoS: Congestion Avoidance Configuration Guide, Cisco IOS XE Release 3S
Byte-Based Weighted Random Early Detection
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Byte-Based Weighted Random Early Detection

Byte-Based Weighted Random Early Detection

This module explains how to enable byte-based Weighted Random Early Detection (WRED), and set byte-based queue limits and WRED thresholds.

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.

Restrictions for Byte-Based Weighted Random Early Detection

  • WRED is only useful when the bulk of the traffic is TCP/IP traffic. With TCP, dropped packets indicate congestion, so the packet source will reduce its transmission rate. With other protocols, packet sources may not respond or may resend dropped packets at the same rate. Thus, dropping packets does not decrease congestion.
  • You cannot configure byte-based WRED on a class in which the queue-limit is configured in milliseconds or packets.

Information About Byte-Based Weighted Random Early Detection

Changes in functionality of WRED

This feature extends the functionality of WRED. In previous releases, you specified the WRED actions based on the number of packets. With the byte-based WRED, you can specify WRED actions based on the number of bytes.

Changes in Queue Limit and WRED Thresholds

In Cisco IOS XE Release 2.4, the Cisco ASR 1000 Series Aggregation Services Routers support the addition of bytes as a unit of configuration for both queue limits and WRED thresholds. Therefore, as of this release, packet-based and byte-based limits are configurable, with some restrictions.

How to Configure Byte-Based Weighted Random Early Detection

Configuring Byte-Based WRED

SUMMARY STEPS

    1.    enable

    2.    configure terminal

    3.    class-map class-map-name

    4.    match ip precedence ip-precedence-value

    5.    exit

    6.    policy-map policy-name

    7.    class class-name

    8.    random-detect

    9.    random-detect precedence precedence min-threshold bytes max-threshold bytes mark-prob-denominator


DETAILED STEPS
     Command or ActionPurpose
    Step 1 enable


    Example:
    Router> enable
     

    Enables privileged EXEC mode.

    • Enter your password if prompted.
     
    Step 2 configure terminal


    Example:
    Router# configure terminal
     

    Enters global configuration mode.

     
    Step 3 class-map class-map-name


    Example:
    Router(config)# class-map c1
     

    Specifies the user-defined name of the traffic class.

     
    Step 4 match ip precedence ip-precedence-value


    Example:
    Router(config-cmap)# match ip precedence 1
     

    Specifies up to eight IP Precedence values used as match criteria.

     
    Step 5 exit

    Example:
    Router(config-cmap)# exit
     

    Exits from class-map configuration mode.

     
    Step 6 policy-map policy-name


    Example:
    Router(config)# policy-map p1
     

    Specifies the name of the traffic policy to configure.

     
    Step 7 class class-name


    Example:
    Router(config-pmap)# class c1
     

    Specifies the name of a predefined traffic class, which was configured with the class-map command, used to classify traffic to the traffic policy.

    
    
     
    Step 8 random-detect


    Example:
    Router(config-pmap-c)# random-detect
     

    Enables WRED.

     
    Step 9 random-detect precedence precedence min-threshold bytes max-threshold bytes mark-prob-denominator


    Example:
    
    
            


    Example:
    Router(config-pmap-c)# random-detect precedence 1 2000 bytes 3000 bytes 200
     

    Configures the parameters for bytes with a specific IP precedence.

     

    Configuring the Queue Depth and WRED Thresholds

    Before You Begin

    Be sure that your configuration satisfies the following conditions when configuring the queue depth and WRED thresholds:

    • When configuring byte-based mode, the queue limit must be configured prior to the WRED threshold and before the service policy is applied.
    • When setting the queue depth and WRED thresholds in an enhanced QoS policies aggregation configuration, the limits are supported only for the default class at a subinterface policy map and for any classes at the main interface policy map.

    Note


    Consider the following restrictions when you configure the queue depth and WRED thresholds:

    • Do not configure the queue limit unit before you configure a queueing feature for a traffic class.
    • If you do not configure a queue limit, then the default mode is packets.
    • When you configure WRED thresholds, the following restrictions apply:
      • The WRED threshold must use the same unit as the queue limit. For example, if the queue limit is in packets, then the WRED thresholds also must be in packets.
      • If you do not configure a queue limit in bytes, then the default mode is packets and you must also configure the WRED threshold in packets.
      • The queue limit size must be greater than the WRED threshold.
    • The unit modes for either the queue limit or WRED thresholds cannot be changed dynamically after a service policy is applied.
    >
    SUMMARY STEPS

      1.    enable

      2.    configure terminal

      3.    policy-map policy-map-name

      4.    class class-name

      5.    qos-queueing-feature

      6.    queue-limit queue-limit-size [bytes | packets]

      7.    random-detect [dscp-based | prec-based]

      8.    Do one of the following:

      • random-detect dscp dscp-value {min-threshold max-threshold | min-threshold bytes max-threshold bytes} [max-probability-denominator]
      • random-detect precedence precedence {min-threshold max-threshold | min-threshold bytes max-threshold bytes} max-probability-denominator


    DETAILED STEPS
       Command or ActionPurpose
      Step 1 enable


      Example:
      Router> enable
       

      Enables privileged EXEC mode.

      • Enter your password if prompted.
       
      Step 2 configure terminal


      Example:
      Router# configure terminal
       

      Enters global configuration mode.

       
      Step 3 policy-map policy-map-name


      Example:
      Router(config)# policy-map main-interface
       

      Specifies the name of the traffic policy that you want to configure or modify and enters policy-map configuration mode.

       
      Step 4 class class-name


      Example:
      Router(config-pmap)# class AF1
       

      Specifies the name of the traffic class and enters policy-map class configuration mode.

       
      Step 5 qos-queueing-feature


      Example:
      Router(config-pmap-c)# bandwidth remaining ratio 90
       

      Enters a QoS configuration command. Some of the queueing features that are currently supported are bandwidth, priority, and shape.

      Note   

      Multiple QoS queueing commands can be entered at this step. However, due to dependencies between the queue limit and WRED thresholds, you should configure WRED after you configure the queue limit.

       
      Step 6 queue-limit queue-limit-size [bytes | packets]


      Example:
      Router(config-pmap-c)# queue-limit 547500 bytes
       

      Specifies the maximum number (from 1 to 8192000) of bytes or packets that the queue can hold for this class.

       
      Step 7 random-detect [dscp-based | prec-based]


      Example:
      Router(config-pmap-c)# random-detect dscp-based
       

      Enables WRED in either DSCP-based mode or precedence-based mode.

       
      Step 8Do one of the following:
      • random-detect dscp dscp-value {min-threshold max-threshold | min-threshold bytes max-threshold bytes} [max-probability-denominator]
      • random-detect precedence precedence {min-threshold max-threshold | min-threshold bytes max-threshold bytes} max-probability-denominator


      Example:
      Router(config-pmap-c)# random-detect precedence 8 750000 bytes 750000 bytes
       

      Configures WRED parameters for a particular DSCP value or IP precedence.

      Note   

      Use the min-threshold max-threshold arguments without the bytes keyword to configure packet-based thresholds, when the queue-limit unit is also packets (the default). Alternatively, use these arguments with the bytes keyword when the queue-limit unit is configured in bytes.

       

      Examples

      The following examples show both correct and invalid configurations to demonstrate some of the restrictions.

      The following example shows the correct usage of setting the queue limit in bytes mode after the bandwidth remaining ratio queueing feature has been configured for a traffic class:

      class AF1
      	bandwidth remaining ratio 90
      		queue-limit 750000 bytes

      The following example shows an invalid configuration for the queue limit in bytes mode before the bandwidth remaining ratio queueing feature has been configured for a traffic class:

      class AF1
      	queue-limit 750000 bytes
      		bandwidth remaining ratio 90

      The following example shows the correct usage of setting the queue limit in bytes mode after the bandwidth remaining ratio queueing feature has been configured for a traffic class, followed by the setting of the thresholds for WRED in compatible byte mode:

      class AF1
      	bandwidth remaining ratio 90
      		queue-limit 750000 bytes
      			random-detect dscp-based
      				random-detect dscp 8 750000 bytes 750000 bytes

      This example shows an invalid configuration of the WRED threshold in bytes without any queue limit configuration, which therefore defaults to a packet-based queue depth. Therefore, the WRED threshold must also be in packets:

      class AF1
      	bandwidth remaining ratio 90
      	random-detect dscp-based
      	random-detect dscp 8 750000 bytes 750000 bytes

      Changing the Queue Depth and WRED Threshold Unit Modes

      SUMMARY STEPS

        1.    enable

        2.    configure terminal

        3.    interface type number

        4.    no service-policy output policy-map-name

        5.    exit

        6.    policy-map policy-map-name

        7.    class class-name

        8.    queue-limit queue-limit-size [bytes | packets]

        9.    Do one of the following:

        • no random-detect dscp dscp-value {min-threshold max-threshold | min-threshold bytes max-threshold bytes} [max-probability-denominator]
        • no random-detect precedence precedence {min-threshold max-threshold | min-threshold bytes max-threshold bytes} max-probability-denominator

        10.    Do one of the following:

        • random-detect dscp dscp-value {min-threshold max-threshold | min-threshold bytes max-threshold bytes} [max-probability-denominator]
        • random-detect precedence precedence {min-threshold max-threshold| min-threshold bytes max-threshold bytes} max-probability-denominator


      DETAILED STEPS
         Command or ActionPurpose
        Step 1 enable


        Example:
        Router> enable
         

        Enables privileged EXEC mode.

        • Enter your password if prompted.
         
        Step 2 configure terminal


        Example:
        Router# configure terminal
         

        Enters global configuration mode.

         
        Step 3 interface type number


        Example:
        Router(config)# policy-map main-interface
         

        Specifies the interface where you want to remove a service policy, and enters interface configuration mode.

         
        Step 4 no service-policy output policy-map-name


        Example:
        Router(config-if)# no service-policy output main-interface-policy
         

        Removes a service policy applied to the specified interface.

         
        Step 5 exit


        Example:
        Router(config-if)# exit
         

        Exits interface configuration mode and returns you to global configuration mode.

         
        Step 6 policy-map policy-map-name


        Example:
        Router(config)# policy-map main-interface-policy
         

        Specifies the name of the Traffic policy that you want to modify and enters policy-map configuration mode.

         
        Step 7 class class-name


        Example:
        Router(config-pmap)# class AF1
         

        Specifies the name of the traffic class and enters policy-map class configuration mode.

         
        Step 8 queue-limit queue-limit-size [bytes | packets]


        Example:
        Router(config-pmap-c)# queue-limit 5000 packets
         

        Specifies the maximum number (from 1 to 8192000) of bytes or packets that the queue can hold for this class.

         
        Step 9Do one of the following:
        • no random-detect dscp dscp-value {min-threshold max-threshold | min-threshold bytes max-threshold bytes} [max-probability-denominator]
        • no random-detect precedence precedence {min-threshold max-threshold | min-threshold bytes max-threshold bytes} max-probability-denominator


        Example:
        Router(config-pmap-c)# no random-detect dscp 8 750000 bytes 750000 bytes
         

        Removes the previously configured WRED parameters for a particular DSCP value or IP precedence.

         
        Step 10Do one of the following:
        • random-detect dscp dscp-value {min-threshold max-threshold | min-threshold bytes max-threshold bytes} [max-probability-denominator]
        • random-detect precedence precedence {min-threshold max-threshold| min-threshold bytes max-threshold bytes} max-probability-denominator


        Example:
        Router(config-pmap-c)# random-detect dscp 8 4000 4000
         

        Configures WRED parameters for a particular DSCP value or IP precedence.

        Note   

        Use the min-threshold max-threshold arguments without the bytes keyword to configure packet-based thresholds, when the queue-limit unit is also packets (the default). Alternatively, use these arguments with the bytes keyword when the queue-limit unit is configured in bytes.

         

        Examples

        The following example shows how to change the queue depth and WRED thresholds to packet-based values once a service policy has been applied to an interface:

        interface GigabitEthernet1/2/0
        no service-policy output main-interface-policy
        end
        policy-map main-interface-policy
        class AF1
        	queue-limit 5000 packets
        	no random-detect dscp 8 750000 bytes 750000 bytes
        	random-detect dscp 8 4000 4000

        Verifying the Configuration for Byte-Based WRED

        SUMMARY STEPS

          1.    show policy-map

          2.    The show policy-map interface command shows output for an interface that is configured for byte-based WRED.


        DETAILED STEPS
          Step 1   show policy-map

          The show policy-map command shows the output for a service policy called pol1 that is configured for byte-based WRED.



          Example:
          Router# show policy-map
          Policy Map pol1
            Class class c1
          Bandwidth 10 (%)
          exponential weight 9
                  class   min-threshold(bytes)   max-threshold(bytes)  mark-probability
                  -------------------------------------------------------------------
           
                  0       -                       -                      1/10
                  1       20000                   30000                  1/10
                  2       -                       -                      1/10
                  3       -                       -                      1/10
                  4       -                       -                      1/10
                  5       -                       -                      1/10
                  6       -                       -                      1/10
                  7       -                       -                      1/10
                  rsvp    -                       -                      1/10
          
          Step 2   The show policy-map interface command shows output for an interface that is configured for byte-based WRED.

          Example:
          Router# show policy-map interface 
          serial3/1
          Service-policy output: pol
          Class-map: silver (match-all)
          366 packets, 87840 bytes
          30 second offered rate 15000 bps, drop rate 300 bps
          Match: ip precedence 1 
          Queueing
          Output Queue: Conversation 266 
          Bandwidth 10 (%)
          (pkts matched/bytes matched) 363/87120
          depth/total drops/no-buffer drops) 147/38/0
          exponential weight: 9
          mean queue depth: 25920
          class     Transmitted       Random drop      Tail drop     Minimum Maximum Mark
                    pkts/bytes        pkts/bytes       pkts/bytes    thresh  thresh  prob
                                                                     (bytes)  (bytes)
          0             0/0               0/0               0/0      20000    40000  1/10
          1           328/78720          38/9120            0/0      22000    40000  1/10
          2             0/0               0/0               0/0      24000    40000  1/10
          3             0/0               0/0               0/0      26000    40000  1/10
          4             0/0               0/0               0/0      28000    40000  1/10
          

          Configuration Examples for Byte-Based Weighted Random Early Detection

          Example Configuring Byte-Based WRED

          The following example shows a service policy called wred-policy that sets up byte-based WRED for a class called prec2 and for the default class. The policy is then applied to Fast Ethernet interface 0/0/1.

          policy wred-policy
           class prec2
             bandwidth 1000
             random-detect
             random-detect precedence 2 100 bytes 200 bytes 10
          class class-default
             random-detect
             random-detect precedence 4 150 bytes 300 bytes 15
             random-detect precedence 6 200 bytes 400 bytes 5
          interface fastethernet0/0/1
            service-policy output wred-policy
          

          The following example shows the byte-based WRED results for the service policy attached to Ethernet interface 0/0/1.

          Router# show policy-map interface
           Ethernet0/0/1
          Service-policy output: wred-policy (1177)
          Class-map: prec2 (match-all) (1178/10)
          0 packets, 0 bytes
          5 minute offered rate 0 bps, drop rate 0 bps
          Match: ip precedence 2  (1179)
          Queueing
          queue limit 62500 bytes
          (queue depth/total drops/no-buffer drops) 0/0/0
          (pkts queued/bytes queued) 0/0
          bandwidth 1000 (kbps)
          Exp-weight-constant: 9 (1/512)
          Mean queue depth: 0 bytes
          class     Transmitted       Random drop      Tail drop Minimum        Maximum     Mark
                    pkts/bytes        pkts/bytes       pkts/bytes thresh         thresh     prob
                                                                   bytes         bytes
          0             0/0               0/0              0/0     15625         31250     1/10
          1             0/0               0/0              0/0     17578         31250     1/10
          2             0/0               0/0              0/0       100           200     1/10
          3             0/0               0/0              0/0     21484         31250     1/10
          4             0/0               0/0              0/0     23437         31250     1/10
          5             0/0               0/0              0/0     25390         31250     1/10
          6             0/0               0/0              0/0     27343         31250     1/10
          7             0/0               0/0              0/0     29296         31250     1/10
          Class-map: class-default (match-any) (1182/0)
          0 packets, 0 bytes
          5 minute offered rate 0 bps, drop rate 0 bps
          Match: any  (1183)
          0 packets, 0 bytes
          5 minute rate 0 bps
          queue limit 562500 bytes
          (queue depth/total drops/no-buffer drops) 0/0/0
          (pkts queued/bytes queued) 0/0
          Exp-weight-constant: 9 (1/512)
          Mean queue depth: 0 bytes
          class     Transmitted       Random drop      Tail drop Minimum        Maximum     Mark
                    pkts/bytes        pkts/bytes       pkts/bytes thresh         thresh     prob
                                                                   bytes         bytes
          0             0/0               0/0              0/0     140625        281250     1/10
          1             0/0               0/0              0/0     158203        281250     1/10
          2             0/0               0/0              0/0     175781        281250     1/10
          3             0/0               0/0              0/0     193359        281250     1/10
          4             0/0               0/0              0/0        150           300     1/15
          5             0/0               0/0              0/0     228515        281250     1/10
          6             0/0               0/0              0/0        200           400     1/5
          7             0/0               0/0              0/0     263671        281250     1/10

          Additional References

          Related Documents

          Related Topic

          Document Title

          Cisco IOS commands

          Cisco IOS Master Commands List, All Releases

          QoS Commands

          Cisco IOS Quality of Service Solutions Command Reference

          Modular QoS CLI

          Modular Quality of Service Command-Line Interface module

          Standards

          Standard

          Title

          No new or modified standards are supported, and support for existing standards has not been modified.

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          MIBs

          MIB

          MIBs Link

          No new or modified MIBs are supported, and support for existing MIBs has not been modified.

          To locate and download MIBs for selected platforms, Cisco IOS XE software releases, and feature sets, use Cisco MIB Locator found at the following URL:

          http:/​/​www.cisco.com/​go/​mibs

          RFCs

          RFC

          Title

          No new or modified RFCs are supported, and support for existing RFCs has not been modified.

          --

          Technical Assistance

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

          Feature Information for Byte-Based Weighted Random Early Detection

          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 Byte-Based Weighted Random Early Detection

          Feature Name

          Releases

          Feature Information

          Byte-Based Weighted Random Early Detection

          Cisco IOS XE Release 2.4

          The Byte-Based Weighted Random Early Detection feature extends the functionality of WRED. In previous releases, you specified the WRED actions based on the number of packets. With the byte-based WRED, you can specify WRED actions based on the number of bytes.

          This feature was introduced on Cisco ASR 1000 Series Routers.

          The following commands were introduced or modified: random-detect, random-detect precedence, show policy-map, show policy-map interface.