in 4.3.10's UI it shows 1500 :)





В петък, 30 октомври 2020 г., 13:25:05 Гринуич+2, Dominik Holler 
<dhol...@redhat.com> написа: 







On Thu, Oct 29, 2020 at 9:36 PM Alex K <rightkickt...@gmail.com> wrote:
> 
> 
> 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:
> 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_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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>> 
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