shaping the real time traffic

[from88]

Hello,

As i know from practice it's best to police the real time traffic (e.g voip).

But do it's possible to use shaping for this type of traffic class ? do the delay from shaping would impact the voice call quality ? Thanks..

[Carlos Villagran]

Hi!

Yes it is possible and of course, this is totally dependent on your traffic throughput. You will want to have the less delay/jitter you can for voice traffic, so the shaping will have to be somewhat consistent or you will start having drops. 

The thing is that you should give it priority over any other class of traffic, if the voice payload passes enough time waiting in the queue then you will be having bad voice performance, so according to your needs, shape as needed.

You do not want 100mbps of voice trying to go out from an interface with the voice class shaped to 512kbps. 

Hope it helps, best regards!

JC

[from88]

thank you, guys.

1) So it's safe to say that even for real time it's suitable to use a shaper(with lower tc value) instead of policer  ?

2) for example if one user is configured to shape its real time traffic at ~1mbps, and other user is cofnigured to shape the real time ~50mbps. The TC (bc) value should be calculated uniquely accordingly to its shaped rate ? I guess it would be not good to use same tc(bc) value for different rates?

thanks..

[Joseph W. Doherty]

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Posting

#1 - Again, depends on what you're trying to accomplish.  A shaper is a more risky than a policer, because it can introduce latency and jitter.

#2 - On Cisco platforms, Tc is set indirectly by setting the Bc.  You cannot use the same Bc value, at different data rates to obtain the same Tc value.  You can, though, use the same Bc percentage of the CIR to obtain the same Tc.

[from88]

Thank you, by the way - the shaping uses buffers. If we want that the shaper would somehow work like a policer - shouldn't we lower to minimum queue depth(limit) size also ?

[Joseph W. Doherty · Accepted solution]

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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.

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In no event shall Author be liable for any damages wha2tsoever (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

Yes, you could do that, i.e. make a shaper behave more like a policer.  However, that sort of defeats the purpose of using a shaper rather than a policer and your only "option" would be to drop where with a policer you could drop or remark.

Additionally, often a shaper is used to control all your trafffic, to create a bottleneck, and with the shaper you can also prioritize traffic where a policer cannot.

[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 wha2tsoever (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

As i know from practice it's best to police the real time traffic (e.g voip).

BTW, much would depend on exactly how you police.

But do it's possible to use shaping for this type of traffic class ? do the delay from shaping would impact the voice call quality ? Thanks..

As also noted by Carlos, yes you can shape something like VoIP traffic.  As also noted by Carlos, you need to take care in how you shape.  Remember something like VoIP has overall delay and jitter requirements, if you don't exceed them, you should be okay.

To help avoid a shaper being adverse to something like VoIP, you often want to decrease the Tc value.