We don't support VXLAN in ML2/OVN. See rhos-tech email thread [1]  and bugzilla [2]

Certain language in the OVN Migration Guide can be easily misinterpreted as implicit support of VXLAN in OVN. [3]

We should clarify this. 

[1]    rhos-tech email thread VxLAN network type support for ML2-OVN 

[2]   RFE Support OVS to OVN migration from vxlan to vxlan

[3]   ML2/OVS to ML2/OVN in-place migration scenarios that have not been validated