[ovirt-users] Re: Question about OVN MTU

in 4.3.10's UI it shows 1500 :)

В петък, 30 октомври 2020 г., 13:25:05 Гринуич+2, Dominik Holler < dholler(a)redhat.com&gt; написа: On Thu, Oct 29, 2020 at 9:36 PM Alex K <rightkicktech(a)gmail.com&gt; wrote: > > > On Tue, Oct 27, 2020, 02:49 Strahil Nikolov via Users <users(a)ovirt.org&gt; 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: > 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 ? >> You could check with packet analyzers like tcpdump and wireshark. >> I have checked https://www.ovirt.org/develop/release-management/features/network/managed... but I don't see any refference how to calculate the max MTU. >> >> >> Best Regards, >> Strahil Nikolov >> _______________________________________________ >> Users mailing list -- users(a)ovirt.org >> To unsubscribe send an email to users-leave(a)ovirt.org >> Privacy Statement: https://www.ovirt.org/privacy-policy.html >> oVirt Code of Conduct: https://www.ovirt.org/community/about/community-guidelines/ >> List Archives: https://lists.ovirt.org/archives/list/users@ovirt.org/message/CKNS5T5KE2W... >> >> > _______________________________________________ > Users mailing list -- users(a)ovirt.org > To unsubscribe send an email to users-leave(a)ovirt.org > Privacy Statement: https://www.ovirt.org/privacy-policy.html > oVirt Code of Conduct: https://www.ovirt.org/community/about/community-guidelines/ > List Archives: > https://lists.ovirt.org/archives/list/users@ovirt.org/message/VIDP3MLYHWG... > > _______________________________________________ Users mailing list -- users(a)ovirt.org To unsubscribe send an email to users-leave(a)ovirt.org Privacy Statement: https://www.ovirt.org/privacy-policy.html oVirt Code of Conduct: https://www.ovirt.org/community/about/community-guidelines/ List Archives: https://lists.ovirt.org/archives/list/users@ovirt.org/message/7UN5DXXKXN3...