Basic question about QoS

[Peter Coggle]

Hi Experts,

Need clarification on a quick point.

Let's say I have the following policy-map.

policy-map WAN
 class Voice
  priority percent 20
 class Mission_Critical
  bandwidth remaining percent 20
class class-default

fair-queue

I understand QoS come into play only when there is congestion. 

1. In case if there is no Voice or mission Critical traffic and if there is no congestion, can the traffic under class-default take up the entire 100% of the bandwidth? 

2. In case if there is no default or mission Critical traffic and if there is no congestion, can the traffic under Voice take up the entire 100% of the bandwidth? 

3. In case if there is no default or Voice traffic and if there is no congestion, can the traffic under mission critical take up the entire 100% of the bandwidth? 

I assume the answer to all the 3 questions above is yes. Can the experts please confirm?

4. let's say there is mission critical traffic equalling 150% of the configured link bandwidth (but no Voice or class-default traffic), can the mission critical traffic take 100% of the link bandwidth? Or will there be any changes for this scenario?

Many thanks for your time and attention.

Regards

Pete

[Joseph W. Doherty · Accepted solution]

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Posting

# 1, 2 and 3 - all yes

# 4 yes, and no

Non-LLQ classes, by default, can utilize otherwise unused bandwidth.

LLQ, if there's no LLQ queuing, can use 100% of link, but if there is LLQ queuing, the implicit policer engages.

[Peter Coggle]

Thanks a lot for taking time to reply.

Regarding your answer for the 4th question, can you please let me know if my understanding of your reply is correct?

If there is 100 % link utilization, voice (LLQ class) cannot take more than the alloted bandwidth. But if there is 100% link utilization, mission critical traffic and class-default traffic (non LLQ classes) can take more than their alloted bandwidth.

Regards

Pete

[Joseph W. Doherty]

Disclaimer

The Author of this posting offers the information contained within this posting without consideration and with the reader's understanding that there's no implied or expressed suitability or fitness for any purpose. Information provided is for informational purposes only and should not be construed as rendering professional advice of any kind. Usage of this posting's information is solely at reader's own risk.

Liability Disclaimer

In no event shall Author be liable for any damages whatsoever (including, without limitation, damages for loss of use, data or profit) arising out of the use or inability to use the posting's information even if Author has been advised of the possibility of such damage.

Posting

No, what I mean is, whenever LLQ traffic queues in the LLQ class-queue, the implicit policer (priority #) engages.  If LLQ traffic is not queued in its class queue, it can use all available bandwidth.

Some time back, Peter Paluch confirmed this, in his lab, for a similar question.