QoS Questions for 3750 Switches

Unanswered Question
Sep 19th, 2010

2x3750 switches are stacked and we are trying to simulate traffic congestion at the UTP ports by using Smartbit 6000C. The objective of the test is to see if the QoS setting works in reality even though we see from Wireshark that the packets are marked with DSCP for voice traffic.

Setup is as follows :

Smartbit<->Avaya IP Phone<->3750 switches<->6509 switch.

Please note that the configuration is set on the 3750 switch port as well as trusted on the Cat 6 switch port. However, when I set to continuous traffic with byte size of 64. Even though its a 100Mbps port, the Avaya IP Phone is already acting weird with hanged symptom. Just side note is that performing "show mls qos inter gi2/0/7 statistic" shows that data and voice traffic are marked on the different priority which seemed correct.

1) Is this the right way to test? If not, what should be the correct way?

2) The port that's connected to Smartbit is configured and it seemed that with the continuous traffic, even other IP Phones are hanged even though I have set Smartbit to hit on the IP address of the CAT6 Switch port. This is not normal right as this is supposed to be unicast traffic. Any idea what could be the reason?

I have this problem too.
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Calin Chiorean Fri, 10/01/2010 - 01:43

Hello Brandon,

I understand your concern and how you want to test, but with the VoIP services you need to understand that there are 2 points (telephones if you want) involved. Your local one, where you might have taken all the necessary steps to protect and prioritize your voice traffic, and the oposite end which also need to have the voice packets prioritized.

Now, from your description, I understand that the packets (voice and data) marked correctly (I believe on C3750), but that's not enough. You need to use CBWFQ together with LLQ to give priority to the Voice traffic over data in case of congestion. Do you have such configuration? Can you show us some excerpt from it?

Next, during the testing, you said that your phone hang-up...where you in a call?

To respond to your questions:

1. The start is ok, but we need more details. You are pushing traffic from Smarbit, this is your local end, but where is the traffic pushed to (remote end), who is receiving the traffic?

2. In theory, you shouldn't have any impact over voice if links are 100Mbit, only if you have such a power packet generator that could fill 100Mbit. What do you mean by "This is not normal right as this is supposed to be unicast traffic"? VoIP is also unicast traffic...

I can see that you are determined to solve this issue, and this is OK as it will help you back with gained knowledge, but I have to warn you that troubleshooting QoS / Voice related problems may be more tricky than you think, as it will involve a strong know-how in these areas.

We will help, but you have to come back with more precise details.

Good luck!



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