in 4.3.10's UI it shows 1500 :)
This might be just a misleading representation of the default value or an unknown value.
If "Automatic Synchronization" is enabled for the ovirt-provider-ovn in oVirt Engine,
the value should be updated in Engine to the correct after the next synchronization in the background.
В петък, 30 октомври 2020 г., 13:25:05 Гринуич+2, Dominik Holler <email@example.com> написа:
On Thu, Oct 29, 2020 at 9:36 PM Alex K <firstname.lastname@example.org> wrote:
> On Tue, Oct 27, 2020, 02:49 Strahil Nikolov via Users <email@example.com> 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)
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.
>> 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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