On Tue, Oct 27, 2020, 02:49 Strahil Nikolov via Users <users@ovirt.org> wrote:

Hello All,

I would like to learn more about OVN and especially the maximum MTU that I can use in my environment.

Current Setup 4.3.10
Network was created via UI -> MTU Custom -> 8976 -> Create on External Provider -> Connect to Physical Network

So my physical connection is MTU 9000 and I have read that Geneve uses 24 bits (maybe that's wrong ?) , thus I have reduced the MTU to 8976.

From the internet draft it seems that the tunnel header + reserved bits comprise 64 bits. A common practice for Geneve implementations is to use 8900 mtu for VMs when having 9000 mtu physical network.

The draft:

https://tools.ietf.org/html/draft-ietf-nvo3-geneve-08

I did some testing on the VMs and ping with payload of '8914' was the maximum I could pass without fragmenting and thus the MTU on the VMs was set to 8942.

Did I correctly configure the test network's MTU and am I understanding it correctly that we need extra 34 bits inside the network for encapsulation ?

I have checked https://www.ovirt.org/develop/release-management/features/network/managed_mtu_for_vm_networks.html but I don't see any refference how to calculate the max MTU.

Best Regards,
Strahil Nikolov
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