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QoS - Fundamental question

Hi All,

I am trying to understand cisco QoS techniques. I came to know that queueing techniques is used(or comes alive) only when you experience conjestion on your network. Here I am a little confused with a few scenarios in front of me. I am trying to post them in the best way I can describe.

Suppose you have a 30 Mbps link. I have defined 3 classes of traffic

a. Voice - 50 % Bandwidth - Priority

b. Applications : 25% Bandwidth

c. Best Effort : remaining with WFQ

Case 1: Is queing techniques applies to the buffer of my interface here ? If so what is the size of the buffer - 30 Mbps ?

Case 2 : You are not expereincing conjestion at all. eg :-  We are using only 15 Mbps of the link.

              Do we have any sort queuing happening at this moment ?

Case 3 : You are expereincing congestion.   Cause of the congestion is application traffic.  As soon as the buffers start filling up, do queing limit my applications traffic to just 20 % of my link though there is no other traffic is alive at this moment ?

Please answer. If some one can share a good url to understand these as well is very much appreciated.

Thank you


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Re: QoS - Fundamental question


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Case 1 - depends on the device and how it's configured.

Case 2 - depends on "how" you're using 15 Mbps.  What "using" often means is a measured capacity over some time period.  Your 15 Mbps could be the result of severe congestion to no congestion.

Case 3 - depends on configuration of QoS features

The above probably doesn't help you much, as actual instances depends so much on a device's QoS capabilities/features and how it has been configured.

It might help you to realize congestion is anytime a frame/packet transmission cannot be immediately started.

QoS, logically, most often is how you manage any congestion.  When managing congestion, your policies generally address frame/packet prioritization and/or frame/policy drop management.  NB: Some QoS policies, such as policing, can impact traffic even when there's no congestion.  Some QoS policies, such as shaping, can create artificial congestion.

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