When we initially set up flannel for Tools Forge, we picked vxlan since it was the only realistic option - udp was just too slow, and there weren't any other options.
There's now host-gw, which I think is a great fit - and much better than vxlan for our use case. It requires that all the nodes be on the same layer2 substrate, which I think we are (and will continue to be even with Neutron, I think)?. It just adds route table entries in the hosts, and hence is much easier to debug and understand than whatever magic VXLAN is. It's also a bit faster.
ip route show from a host in a 6 node k8s cluster:
[root@doc-10 INSTALL]# ip route show default via 10.1.1.46 dev em1 proto static metric 100 default via 188.8.131.52 dev em2 proto static metric 101 10.1.1.0/24 dev em1 proto kernel scope link src 10.1.1.230 metric 100 10.244.0.0/24 dev cni0 proto kernel scope link src 10.244.0.1 10.244.0.0/16 dev flannel.1 10.244.1.0/24 via 10.1.1.231 dev em1 10.244.2.0/24 via 10.1.1.232 dev em1 10.244.3.0/24 via 10.1.1.233 dev em1 10.244.4.0/24 via 10.1.1.234 dev em1 10.244.5.0/24 via 10.1.1.235 dev em1 10.244.6.0/24 via 184.108.40.206 dev em2 220.127.116.11/24 dev em2 proto kernel scope link src 18.104.22.168 metric 100 172.30.6.0/24 dev docker0 proto kernel scope link src 172.30.6.1 blackhole 192.168.229.0/26 proto bird
You can see the /24s in the middle - those are just the flannel IPs. This has made my life *much* easier!
If Neutron changes the network topology in a way this isn't possible, then it doesn't make sense to move to this. Otherwise, it's a great benefit IMO.
Note that this isn't applicable to prod k8s at all.