Fair Queuing in Cisco IOS

Technology Resources » QoS Mechanisms » Fair Queuing in Cisco IOS

Fair queuing is enabled by default on all low-speed interfaces on Cisco IOS devices (high-speed interfaces use FIFO queuing as the default mechanism). It’s also used to implement the queuing actions (bandwidth and fair-queue) offered by the Modular QoS CLI (MQC).

The default queuing mechanism on a LAN interface of 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.

Service policy is configured on an Ethernet interface
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
Interface queuing strategy is changed to fair queuing
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).

Policing-only service policy applied to LAN interface
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
Policing requires no software queueing strategy
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.

Combine fair queueing with a policing service policy
interface FastEthernet0/0
 fair-queue
 service-policy output Simple
Service policy contains no queuing actions
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
Fair queuing is used even though the service policy doesn’t require it
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
This article written by Ivan Pepelnjak in early 2000s was originally published on CT3 wiki which became unreachable in 2019. The text was retrieved from an Internet Archive snapshot, updated, and republished on ipSpace.net.

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