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I have some HP ProCurve series Switch. There is a QoS option for prioritize traffic by TCP/UDP port. I set two ports: one with highest priority and another with lowest. Then I performed tests to ensure that prioritizing really works. I run two instances (per computer) of Network Benchmark application on two computers being in the LAN. One computer acts like server and other like client. In benchmark applications I set two ports configured earlier on the switch. Then I run tests, so that server sends some amount of data to client showing current transmission speed. I assumed that, since one port has higher priority then other, the transmission speed will not be equal, and the traffic going throughout the tcp port with higher priority will be significant faster. Unfortunately there is no difference. The speeds are nearly equal (half of bandwidth) and non prioritizing seems to be applied.

Can anyone explain me why this is not working. I suppose that I misunderstand some QoS basics and this is a reason, but I will be grateful if someone clarify this issue for me.

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3 Answers

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Priority on the Switch will have no effect in your test because the sending computer's NIC is the bottleneck. The switch is seeing 1Gb (or 100Mb) of input, sees a path at that same speed and sends the data along happily. If it has to choose what data to send, then it would prioritize.

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As I mentioned under Oliver's reply, I've tried to limit my client bandwidth using NetLimiter (www.netlimiter.com) without any effect. Transmission speed was still divided equally for both clients apps, despite client's bandwith was much tighter than server's one. –  anth May 21 '12 at 19:15
    
The switch would still see two ports of the same speed. The switch doesn't magically know what the software on the computers is doing. It only shovels from one port to another, and if there's bandwidth on those physical ports then QoS is never going to be factored. –  Chris S May 21 '12 at 19:23
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The priority setting only comes into play when a resource is limited - so if both 1G servers were transmitting at full speed to another 1G host there would be twice as much traffic as the destination link could handle. The switch would then, in theory, reference the relative priority of the frames to determine which would be transmitted and which would be dropped.

Note that there may be multiple mechanisms that you can select to perform evaluation and drops and that each may yield different results (not very familiar with HP's network gear) - for example a 2:1 ratio of high to low priority packets (vs all high and no low)

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As I mentioned under Oliver's reply, I've tried to limit my client bandwidth using NetLimiter (www.netlimiter.com) without any effect. Transmission speed was still divided equally for both clients apps, despite client's bandwith was much tighter than server's one. –  anth May 21 '12 at 19:15
    
The switch can handle full bandwidth and will hand off full bandwidth regardless of whether you're limiting inbound traffic. The switch port itself needs to be overloaded to see any effect. Bring a third machine into the mix and send 2G -> 1G and you'll see the queueing work. –  rnxrx May 22 '12 at 1:15
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You might be able to verify if your QoS configuration works by configuring the speed of the ports where the sender and the receiver is to 100 Mbps (sender) and 10 Mbps (receiver). That way, provided that your sender is able to create more than 10 Mbps of traffic, you should overload your switch port and force the switch to prioritize.

If your sender is fast enough, you can also do that experiment at 1 Gbps and 100 Mbps.

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How can I configure the speed? I've tried to limit my client bandwidth using NetLimiter (www.netlimiter.com), without any effect. Transmission speed was still divided equally for both clients apps. –  anth May 21 '12 at 19:10
    
I'm not sure if your switch allows to do this, but usually this is in the ports configuration. You usually can configure duplex and speed settings. Hope that helps. –  Oliver May 21 '12 at 19:11
    
Forget NetLimiter if you want your switch to prioritize. Your switch is designed to deliver the packets at wire speed. –  Oliver May 21 '12 at 19:24
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