vmxnet3 and MTU 4500

[rayvd]

Looking to enable jumbo frames on several of our ESXi 5.0.0 VM's which are doing iSCSI to an IBM Gen2 XIV.

The XIV supports jumbo frames -- but only at MTU 4500 rather than the more common 9000.

Our VM's are using vmxnet3 vNICs and these apparently only lets us choose between an MTU size of 1500 and 9000.  Any way to adjust this to 4500?  Would 9000 work and just auto-drop down to 4500 at the TCP/IP level thanks to MSS?  Should we use the E1000 driver instead which appears to have more MTU size options?

Thanks!

[MKguy]

You should be able to set the a specific MTU independently from the options offered in the device drivers properties via netsh as described here:

http://www.richard-slater.co.uk/archives/2009/10/23/change-your-mtu-under-vista-or-windows-7/

I tested this and it seems to work.

You're probably aware of it but remember to also adjust the vSwitch MTU if you're doing this.

[bseklecki_ge]

Manually forcing the MTU with netsh.exe doesn't work.

"The parameter is incorrect."

The code in the driver must be enforcing these draconian restrictions.  A lot of Cisco IE (Industrial/IOT)-series switches still do not support 9000byte jumbo frames.

[rickardnobel]

rayvd wrote:

Any way to adjust this to 4500?  Would 9000 work and just auto-drop down to 4500 at the TCP/IP level thanks to MSS?

If the remote side supports only 4500 then this will be negotiated for each TCP session. Both sides will automatically use the lowest common TCP Maximum Segment Size.

http://rickardnobel.se/different-jumbo-frames-settings-on-the-same-vlan

You should not have to tweak this by lowering the ESXi side.