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.
Ack, this is a safe choice.
In oVirt's default configuration, an overhead of 58 bytes
( = 20 IPv4 header + 8 UDP header + 16 GENEVE Ethernet header (including 8 bytes options) + 14 bytes inner Ethernet header)
is added.
This is why the default MTU for OVN networks in oVirt is 1442 bytes, which assumes 1500 bytes MTU in the physical network.
The draft:
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 ?
You could check with packet analyzers like tcpdump and wireshark.
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