Setting MTU globally


Sep 2, 2019

I have a requirement that all traffic leaving proxmox is using a certain MTU lower than 1500, including traffic from VMs

What is the way to configure this? Should we just set the MTU on the public interface and the bridge?

To ensure all traffic is using a certain MTU, must the MTU be configured for interfaces inside VMs too?

You should set it on all interfaces, otherwise fragmentation could happen. For the guests, setting mtu in the respective hardware tab should be sufficient. You can test with tracepath though, if the mtu "mismatch" already happens on the source machine/host or if a network device has to fragment the packages.
Thanks for your reply ph0x!

I'm assuming that setting guest's MTU will be mandatory then?

Unfortunately I'm not seeing the mtu in the hardware tab.
I'm not sure if the guests will respect the host's MTU value if you don't set it explicitly, but you can test this as well. If they don't respect it, the host has to do the fragmentation.
Double click the NIC entry (maybe check the "advanced" box) and then you can enter an MTU value.


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