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saldon
Enthusiast
Enthusiast

Effects on virtual machines of changing MTU on a network interface in ESXi

I'm running ESXi 6.7 and vCenter 6.7. I have a several different clusters in different geographic locations. They are all managed by the same vCenter. I need VM's in each of these clusters to be able to talk to each other over the WAN links. The catch is these WAN links are encrypted. The encryptors add overhead to each frame without regard for exceeding the MTU size. This results in dropped frames and very poor network performance. I find that if I lower the MTU size to 1400 everything works fine. The catch is that rather than changing each virtual machine to use an MTU size of 1400 I'd rather configure the Hosts to use an MTU size of 1400 and basically force this upon the VM's. We are using standard virtual switches. I can't change to distributed switches. I didn't create the network design but there are reasons I'm stuck with it.

So, if I change the MTU size on the virtual switches to 1400 will they properly fragment the frames for me? 

- Tom
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Lalegre
Virtuoso
Virtuoso

Hey @saldon,

To be honest, I have never seen if it is possible to change the MTU value in vSphere to 1400 (as 1500 is the default) and I do not have a LAB right now but it would be good to give it a try.

What I do know is that if you change the MTU of a portgroup to a desired size, then the packets will be fragmented to the number you configured, however, you need to make sure that the applications that are installed on your servers support the fragmentation and do not have impacts directly on the performance.

Just for the test, I would create a new portgroup with the same VLAN ID and modify the MTU to the desired number. After that, test the connectivity and see the results.

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saldon
Enthusiast
Enthusiast

I think I should be more specific about where I'm thinking of making this change. I'm not looking for a global ESXi change. Rather I'm thinking of setting the MTU size on the uplink port of my vSwitches.

- Tom
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Lalegre
Virtuoso
Virtuoso

Hey @saldon,

Even me got confused for a moment and said to update PortGroup MTU which is not possible. Indeed, if you change the vSwitch MTU it will affect all the portgroups that you have attached to it.

And going with my previous message, I never tested it but if it is VSS and you have more than one host it is worth to empty one host and give it a try. However if you are using iSCSI, NFS or any other specific protocol, you should check on the frame size requirements as they could not be working properly.

However do not stay with my answer entirely as maybe one of the folks from the community can give you the proper and specific one.

 

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