+++ This bug was initially created as a clone of Bug #1841526 +++

Description of problem: right now VXLAN is not supported by OVN for regular inter-chassis communication. There is VXLAN support for "vtep" ovsdb schema compliant external switches (aka ramp switches). This RFE is to extend core OVN to support VXLAN for regular in-cluster traffic.

Initial upstream discussion: https://www.mail-archive.com/ovs-discuss@openvswitch.org/msg06771.html
Demo implementation: https://patchwork.ozlabs.org/project/openvswitch/patch/20200320050711.247351-1-ihrachys@redhat.com/

Related links:
https://blog.russellbryant.net/2017/05/30/ovn-geneve-vs-vxlan-does-it-matter/

Core OVN work is tracked in bug 1841526. This bug is to adjust Neutron to support the new network type for OVN driver. There are two elements to it:

1) allow VXLAN type (as simple as https://review.opendev.org/734889);
2) read max_tunid from NB_Global object and use it to adjust vni_ranges used by VXLAN type driver.

Since max_tunid is written in the latest OVN only, its presence can be used as a tag that VXLAN support is present.