Server Fault is a question and answer site for system and network administrators. It's 100% free, no registration required.

Sign up
Here's how it works:
  1. Anybody can ask a question
  2. Anybody can answer
  3. The best answers are voted up and rise to the top

I have a small flat LAN based on 1Gbit switches from 3Com. My central switch in server rack is connected to a couple of virtualization servers VMware ESXi and PBX Alcatel. Not long ago I started to notice that when I use live migration of virtual machine and therefore generate traffic about 300-500 Mbit per second then IP phones users experience problems. Sound start to tremble. I think that problem appeared at some moment without any major changes in environment. Maybe I should just replace a switch, or there are methods to investigate problem root (I'm not a network engineer)?

share|improve this question
Do the switches support QoS? Is that enabled? – Hennes Dec 13 '12 at 13:39
Hi! Where should I apply the QoS policies if VM traffic never leave switch and therefore never use the same ethernet line with SIP and RTP traffic? – Dmitriy Dec 13 '12 at 14:39

Separate the switches... Move the voice to its own small switch. I'm assuming your voice, data and VMWare storage/vMotion networks are on different vlans... If not, that's a different project :)

Can you specify the model of 3Com switch you're using. That could help determine if quality-of-service (QoS) rules can be applied.

share|improve this answer
Thanks! It's not quite possible to separate Voice to it's own vlan because my sip phones act as a switch for PC. So Phone is connected to switch and PC to phone. I usa 3Com 2952 switch. It has QoS settings, yes, but ports on the switch are separate and independent, and VMware traffic doesn't leave the switch so it cannot use the same port (uplink for example) as SIP traffic - that's strange. – Dmitriy Dec 13 '12 at 14:11
This is not an uncommon scenario... But it's a deeper networking discussion. This can be accomplished with switchport trunking and assigning a vlan tag to your phones (and a native vlan for your PC's)... But at this point, you understand the cause of the issue. – ewwhite Dec 13 '12 at 14:25

Either get QoS set up on your current switch or purchase a new switch and move all of your voice lines to it. It likely doesn't need to be gigabit-capable, so it shouldn't be expensive.

share|improve this answer
Thanks! As I wrote at previous comment, I cannot separate SIP links because they are the same for Phones and PC's because my IP phones are actually switches for PC's. But VMware traffic never use the same switch port as SIP traffic. The only part of the that is shared between connected devices is backplane, but maximum traffic is 500 Mbit/sec. I'm not a network engineer but that seems quite strange for me. – Dmitriy Dec 13 '12 at 14:22

Your Answer


By posting your answer, you agree to the privacy policy and terms of service.

Not the answer you're looking for? Browse other questions tagged or ask your own question.