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On my CentOS 5.6 ESXi 4.1 VMs using VMXNET, it looks like they are attempting to send jumbo frames:

enter image description here

However, all the interfaces are set to MTU 1500. What is causing this behavior?

Update:

  1. I would think that regardless of the switch, wouldn't those host honor the MTU on the interface?
  2. esxcfg-vswitch -l already shows 1500

Update 2:

  • Changing to e1000 works around this issue it seems.

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1 Answer

ESX/ESXi's vSwitches are just that, switches, nothing more or less, and can happily handle both jumbo and regular frames. By default ESX/ESXi itself won't use jumbo frames (for v4.0-v.4u1 anyway) without the instructions below. So I suspect that your Centos guest itself is trying to use jumbo frames, obviously you can switch them off if you're infrastructure can't/doesn't handle them well but overall I wouldn't worry about it.

To switch on ESX/ESXi jumbo frame support do the following either from an SSH session or via the VMa;

esxcfg-vswitch -l (which lists the current vSwitch MTU)

esxcfg-vswitch -m 9000 vSwitch0 (which sets the MTU at 9000)

then use esxcfg-vswitch -l again to check it's set, repeat for other vSwitches if you like.

If you have a cluster I'd be tempted to do these changes with the host in maintenance mode then immediately reboot and check MTU again before putting the host back into the cluster.

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Shouldn't the guest not attempt to exceed its own MTU though? –  Kyle Brandt Jul 25 '11 at 14:21
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