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Hello ,


i have some question regarding fair-queue...

I am not clear whether fair-queue is same as CBWFQ.....


What is fair queue ? Can we configure fair queue on newer release...15 and later ? How to configure per-flow queue-limit and number of dynamic queues.





Fair queuing or Weighted Fair

Fair queuing or Weighted Fair queuing in Cisco is the queuing mechanism which define queue dynamically without user interaction on the basis of flows. But the CBWFQ are user defined queues on the basis of some selection criteria by defining the traffic classes.

Class-Based Weighted-Fair Queuing (CBWFQ) is a queuing algorithm that combines the ability to guarantee bandwidth with the ability to dynamically ensure fairness to other flows within a class of traffic.

In Cisco IOS release 15M&T , The fair-queueing behavior for the class-default class is flow-based. This is a change from the weighted fair queueing (WFQ) behavior in previous releases. With flow-based fair queueing, the flow queues in the class-default class are scheduled equally instead of by weight based on the IP Precedence bits(IPP)

So confirmed that the FQ pre-sorter included in the class-default and because do not include IPP values so it is not WFQ. When you do not explicitly configure the class-default class in a policy map, its default queueing behavior is FIFO. You can configure the bandwidth, fair-queue, or service-policy commands in the class-default class to achieve different queueing behaviors..



In newer IOS releases, Fair

In newer IOS releases, Fair-Queue in not limited to default-class only. Suppose you define a traffic class for mission critical data and allocate some bandwidth, then you can define Fair-queuing for this class also.

Router(config)# class-map match-any CRITICAL-DATA
Router(config-cmap)# match dscp af41
Then in the policy-map you allocate bandwidth for this class and assign it to the Fair-Queue.
Router(config-pmap-c)# class CRITICAL-DATA
Router(config-pmap-c)# bandwidth percent 35
Router(config-pmap-c)# fair-queue 128 (this defines how many dynamic queues)
I think this clears the concept. If need any more clarification let me know..
VIP Expert

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No, they are not the same, although they are similar.

What is FQ?  FQ insures each queue gets its fair share of available bandwidth.  Exactly what that means is probably better explained by searching the Internet.

CBWFQ (in theory, or in practice with HQF) is what it's named, i.e. Class Based Weighted Fair Queuing.  This too is probably better explained by searching the Internet.

Since HQF, which includes 15x code, FQ can be configured in CBWFQ classes (except LLQ classes).  (NB: pre-HQF, fair-queue could be configured in class-default, but it was actually WFQ, not FQ.)

How per-flow queue-limits and number of dynamic queues are defined, I believe, varies based on IOS version.  Sometimes you cannot explicitly configure them; defaults are often dependent on interface bandwidth  Sometimes there are subcommands that can set their values.  Check your IOS version's reference and usage guide manuals, and/or use a question mark while within the CBWQF policy.

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