With or without QoS

Unanswered Question
May 25th, 2010


I'm trying to figure out how switch work with and without QoS. Let say that I would like to enable QoS  on 3750 in a way to speed up VoIP traffic. I enable priority queue, Q1, for VoIP traffic (Cos 5, DSCP  46). As you enable QoS (for egress) you have to deal with 4 queue (with 3750). For VoIP traffic no problem is on Priority Q1. But for all other traffic that supposed classified/marked to defualt to Cos=0 and DSCP=0 will be sent to Q2.

In term of performances and optimisation the fact I'm not using 2 queues (Q3, Q4) will mean that for non VoIP traffic I have performances/optimisation degradation vs the situation without QoS enabled ?

I have this problem too.
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Mohamed Sobair Tue, 05/25/2010 - 05:55


QoS on Switches are important because it minimizes Delay , Jitter on the Network which impacts for example VOIP traffic.

If you enable priority Queuing , be informed that whatever Marked traffic for that queue it will always be forwarde fiirst regardless if there is another marked traffic needs to be served or not. So be careful with that.

I hereby, recommend if you have another type of traffic that you want to make sure its getting served rather than dropped , delayed NOT to enable priority queueing but to enable them being served in a round robin fashion and based on thier bandwidth percentage.



helenio Tue, 05/25/2010 - 06:17

Thank's for reply .. my question is more related to traffic not in priority queue since as you enable it we have 4 queues. What about traffic other that VoIP (on Q1, PQ) since, by default DSCP=0, Cos=0 -> Q2 and in particula with Q3 & Q4 empty .. waste of switch resources ?? Normal traffic (not VoIP) will be penalized against the situation without QoS enabled ?

Mohamed Sobair Tue, 05/25/2010 - 06:23

Hi Helenio,

You can map those Cos values to any of the hardware Queues of the Switch, so its not a big problem.

But, as I told you, if you have Priority queuing enabled, and you have too much VOIP traffic and at the same time you have another important traffic, this will start up starving other queues as they will never get served. The best option in this case is to disable priority queuing in your Network and give VOIP traffic for example the highest bandwidth percentage.

Does this make sense?




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