参考文献：
https://tonydeng.github.io/sdn-handbook/ovs/ovn-internal.html
一、OVN L2 功能包括:
- L2 switch
- L2 ACL
- Supports software-based L2 gateways
- Supports TOR (Top of Rack) based L2 gateways that implement the hardware_vtep schema
- Can provide networking for both VMs and containers running inside of those VMs, without a second layer of overlay networking
二、OVN L3 功能包括：
- IPv4/IPv6分布式L3路由
- ARP and IPv6 Neighbor Discovery suppression for known IP-MAC bindings
- L3 ACL
- Native support for NAT and load balancing using OVS connection tracking
- Native fully distributed support for DHCP
- Supports L3 gateways from logical to physical networks
