loss of DHCP traffic on VMs?

Joseph Bernard jhb at clemson.edu
Thu Apr 3 16:23:21 UTC 2014

So it looks like the UDP packet loss is due to a difference in the way virtual NICs buffer vs physical NICs.  I am testing a physical NIC connected to a VM using DirectPath I/O and can run it to 300Mbps before any noticeable packet loss.   I see the same amount of packet loss at 60Mbps on a virtual NIC.  If I increase the TCP socket buffer size greater than the default, I can run either at 900Mbps with no packet loss.  Now I just have to find the magic setting that fixes the difference.

We don't generate 60Mbps of DHCP traffic, so our problem is most like a different issue, but fixing the UDP packet loss couldn't hurt.

Joseph B.

On Mar 28, 2014, at 3:26 PM, Joseph Bernard <JHB at clemson.edu<mailto:JHB at clemson.edu>> wrote:

I didn't even realize that was an option.  I will try to see if that solves our problem.

Joseph B.

On Mar 28, 2014, at 1:39 PM, Steve Clark <sclark at netwolves.com<mailto:sclark at netwolves.com>>

On 03/28/2014 01:23 PM, Joseph Bernard wrote:

We had an issue with DHCP yesterday which lead me to test UDP traffic from the VM we loaded on our VMware ESXi box.  I found that after around 25Mbps UDP starts to lose packets, and it gets worse and worse the fast you go.  I tried every virtual card type (e1000, VMXNET2, VMXNET3) with the same results.  I even tried on different servers, and they were also showing UDP packet loss.  Physical boxes on the same network showed no packet loss until I pushed their NICs to the limit (1Gbps).

Has anyone faced DCHP problems by using a VM because of this?  If so, were you able to fix it or did you just move back to a physical box?

Joseph B.

Did you dedicate a nic to the virtual machine or are going thru a vswitch. We got dramatically better results by taking the nic out
of the vswitch dedicating it to the virtual machine. This was in a packet capture environment.

Stephen Clark
NetWolves Managed Services, LLC.
Director of Technology
Phone: 813-579-3200
Fax: 813-882-0209
Email: steve.clark at netwolves.com<mailto:steve.clark at netwolves.com>

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