Heterogeneous networks include different protocols used by applications, giving rise to the need to prioritize traffic in order to satisfy time-critical applications while still addressing the needs of less time-dependent applications, such as file transfer. If your network is designed to support different traffic types that share a single data path between devices in a network, implementing congestion avoidance mechanisms ensures fair treatment across the various traffic types and avoids congestion at common network bottlenecks. Congestion avoidance mechanism is achieved through packet dropping.

Random Early Detection (RED) is a commonly used congestion avoidance mechanism in a network.

Tail drop treats all traffic equally and does not differentiate within a class of service. When the output queue is full and tail drop is in effect, packets are dropped until the congestion is eliminated and the queue is no longer full.

The RED mechanism takes advantage of the congestion control mechanism of TCP. Packets are randomly dropped prior to periods of high congestion. Assuming the packet source uses TCP, it decreases its transmission rate until all the packets reach their destination, indicating that the congestion is cleared. You can use RED as a way to cause TCP to slow down transmission of packets. TCP not only pauses, but also restarts quickly and adapts its transmission rate to the rate that the network can support.

WRED is the Cisco implementation of RED. It combines the capabilities of RED algorithm with IP Precedence or Differentiated Services Code Point (DSCP) or Class of Service (COS) values.

To configure AFD based WRED feature, specify the policy map and add the class. Use the random-detect command to specify the method (using the dscp-based / cos-based / precedence-based arguments) that you want WRED to use to calculate the drop probability.

Note	You can modify the policy on the fly. The AFD weights are automatically recalculated.

WRED can be configured for any kind of traffic like IPv4/IPv6, Multicast, and so on.

Consider the following points when you are configuring WRED with random-detect command:

• With dscp-based argument, WRED uses the DSCP value to calculate drop probability.

• With cos-based argument, WRED uses the COS value to calculate drop probability.

• By default, WRED uses the IP Precedence value to calculate drop probability. precedence-based argument is the default and it is not displayed in the CLI.

  Note	show run policy-map policy-map command does not display “precedence” though precedence is configured with random-detect command.

• The dscp-based and precedence-based arguments are mutually exclusive.

• Each of the eight physical queues can be configured with different WRED profiles.

Hierarchical QoS allows you to specify QoS behavior at multiple policy levels, which provides a high degree of granularity in traffic management.

For HQoS, WRED is allowed only on the child policy and not on the parent policy. You can have the shaping or bandwidth configured on the parent policy and WRED on the child.

The following example configures the parent policy pwred-parent with traffic shaped on the basis of 10 percent of the bandwidth, that applies to its child, pwred-child configured for DSCP-based WRED.

policy-map PWRED-CHILD
 class CWRED
    bandwidth percent 10
    random-detect dscp-based
    random-detect dscp 1 percent 10 20
    random-detect dscp 10 percent 20 30
 
policy-map PWRED-PARENT
  class class-default
  shape average percent 10
  service-policy PWRED-CHILD

The following show command verifies the HQoS WRED configuration:

device# show policy-map PWRED-PARENT
  policy Map PWRED-PARENT
    class class-default
      average Rate Traffic Shaping
      cir 30%   
      service-policy PWRED-CHILD
policy-map PWRED-CHILD
 class CWRED
  bandwidth percent 10
  random-detect dscp-based
  random-detect dscp 1 percent 10 20
  random-detect dscp 10 percent 20 30
policy-map PWRED-PARENT

• Support for three WRED configuration pairs

  Each WRED Physical Queue (AFD Queue) can support three WRED configuation pairs, with unique WRED threshold pair configuration.

  Policy-map P1
      Class CS
          Random-detect dscp-based
          Random-detect dscp CS1 percent 10 20      // WRED pair 1
          Random-detect dscp CS2  percent 20 30      // WRED pair 2
          Random-detect dscp CS3 percent 30 40      // WRED pair 3
  Class-map match-any CS
          match cs1
          match cs2
          match cs3
  

• Appending WRED configuration pairs

  You can add overlapping threshold pairs into the WRED configuration pairs.

  Policy-map P1
      Class CS 
         Random-detect dscp-based
         Random-detect dscp CS1 percent 10 20       // WRED pair 1
         Random-detect dscp CS2 percent 20 30       // WRED pair 2
         Random-detect dscp CS3 percent 30 40       // WRED pair 3
         Random-detect dscp CS4 percent  30 40     ==> belongs to WRED pair 3
         Random-detect dscp CS5 percent 20 30     ==> belongs to WRED pair 2
  Class-map match-any CS
         match cs1
         match cs2
         match cs3
         match cs4  >>
         match cs5  >>
  

• Default WRED pairs

  If less than three WRED pairs are configured, any class-map filter participating WRED gets assigned to the third default WRED pair with maximum threshold (100, 100).

  Policy-map P1
      Class CS
          Random-detect dscp-based
          Random-detect dscp CS1 percent 10 20   // WRED pair 1
          Random-detect dscp CS2 percent 20 30   // WRED pair 2
  Class-map match-any CS
          match CS1
          match CS2
          match CS3
          match CS4
  

  In this case, classes CS3 and CS4 are mapped to WRED pair 3 with threshold (100, 100).

• Rejection of Mismatched Configuration

  If you configure random-detect without matching filters in a class-map, the policy installation is rejected.

  Class-map match-any CS
         match CS1
         match CS2
         match CS5
  Policy-map P1
      Class CS
          Shape average percent 10
          Random-detect dscp-based
          Random-detect dscp CS1 percent 10 20   // WRED pair 1
          Random-detect dscp CS2 percent 20 30   // WRED pair 2
          Random-detect dscp CS3 percent 30 40   // WRED pair 3  ===> Mismatched sub-class.
  

  When this policy is applied to the interface on the egress side, the policy fails during installation as the class-map values are incorrect:

  device(config)# int Fo1/0/5
  device(config-if)# service-policy output P1
  device(config-if)#
  *Feb 20 17:33:16.964: %IOSXE-5-PLATFORM: Switch 1 R0/0: fed: WRED POLICY INSTALL FAILURE.Invalid WRED filter mark: 24 in class-map: CS
  *Feb 20 17:33:16.965: %FED_QOS_ERRMSG-3-LABEL_2_QUEUE_MAPPING_HW_ERROR: Switch 1 R0/0: fed: Failed to detach queue-map for FortyGigabitEthernet1/0/5: code 2