From CT3

Fair queuing is enabled by default on all low-speed interfaces (high-speed interfaces use FIFO queuing as the default mechanism). It's also used to implement the queuing (bandwidth and fair-queue) actions offered by the Modular QoS CLI (MQC) (class maps, policy maps and service policies).

The default queuing mechanism on a Fast Ethernet interface on a Cisco 2811 router (the platform used to generate the following printouts) is thus FIFO queuing. However, as soon as a simple queuing policy is applied to the interface with the service-policy interface configuration command, the queuing mechanism changes to fair queuing.

a1#show run | section Simple|Fast
policy-map Simple
 class class-default
  bandwidth 20000
interface FastEthernet0/0
 ip address 10.0.0.5 255.255.255.0
 service-policy output Simple

a1#show queueing interface FastEthernet 0/0
Interface FastEthernet0/0 queueing strategy: fair
  Input queue: 0/75/55887/0 (size/max/drops/flushes); Total output drops: 0
  Queueing strategy: Class-based queueing
  Output queue: 0/1000/64/0 (size/max total/threshold/drops)
     Conversations  0/1/256 (active/max active/max total)
     Reserved Conversations 1/1 (allocated/max allocated)
     Available Bandwidth 55000 kilobits/sec

If the policy-map applied as an outbound service policy does not include queuing actions, the interface queuing model reverts back to the default (FIFO for high-speed interfaces, fair queuing for low-speed ones).

a1#show run | section Simple|Fast
policy-map Simple
 class class-default
  police rate 50000000 bps
    exceed-action drop
interface FastEthernet0/0
 ip address 10.0.0.5 255.255.255.0
 service-policy output Simple

a1#show queueing interface FastEthernet 0/0
Interface FastEthernet0/0 queueing strategy: none

You could also combine non-queuing service policies with the fair-queue interface configuration command to change the default interface queuing mechanism.

interface FastEthernet0/0
 fair-queue
 service-policy output Simple

a1#show queueing interface FastEthernet 0/0
Interface FastEthernet0/0 queueing strategy: fair
  Input queue: 0/75/55887/0 (size/max/drops/flushes); Total output drops: 0
  Queueing strategy: weighted fair
  Output queue: 0/1000/64/0 (size/max total/threshold/drops)
     Conversations  0/1/256 (active/max active/max total)
     Reserved Conversations 0/0 (allocated/max allocated)
     Available Bandwidth 75000 kilobits/sec

a1#show policy-map interface FastEthernet 0/0
 FastEthernet0/0

  Service-policy output: Simple

    Class-map: class-default (match-any)
      576 packets, 40292 bytes
      5 minute offered rate 0 bps, drop rate 0 bps
      Match: any
      police:
          rate 50000000 bps, burst 1562500 bytes
        conformed 0 packets, 0 bytes; actions:
          transmit
        exceeded 0 packets, 0 bytes; actions:
          drop
        conformed 0 bps, exceed 0 bps