如果不遇到问题，真是不知道这个是干什么的？
https://wiki.openstack.org/wiki/L2population
https://wiki.openstack.org/wiki/L2population_blueprint
L2群体，是一种ML2插件，可以帮助缓解overlay网络的实现的性能问题。 方法就是生成虚拟交换机的转移表，这样的话广播，组播包就可以只关注转移表里有的记录，而不是真正的广播(LinuxBridge or OVS)，这样就可以有效的减少物理网卡上的广播包数量，当然这些广播包产生的原因是使用了overlay的网络VXLan，或者GRE。
https://docs.google.com/document/d/1sUrvOQ9GIl9IWMGg3qbx2mX0DdXvMiyvCw2Lm6snaWQ/edit
Two topics are implemented with this mechanism driver :
populating ARP proxy so that ARP broadcasts are not broadcasted in the network fabric anymore; (only available for Linuxbridge in havana-1, but OVS implementation should land soon with an ebtabless driver)
populating forwarding tables because when ARP proxy is used, virtual switch forwarding tables don’t learn where to send packets anymore.
https://access.redhat.com/documentation/en-us/red_hat_openstack_platform/16.0/html/networking_guide/sec-networking-concepts
https://access.redhat.com/documentation/en-us/red_hat_openstack_platform/12/html/networking_guide/openstack_networking_concepts#l2_population
2.9. Configuring the L2 population driver
The L2 Population driver enables broadcast, multicast, and unicast traffic to scale out on large overlay networks. By default, Open vSwitch GRE and VXLAN replicate broadcasts to every agent, including those that do not host the destination network. This design requires the acceptance of significant network and processing overhead. The alternative design introduced by the L2 Population driver implements a partial mesh for ARP resolution and MAC learning traffic; it also creates tunnels for a particular network only between the nodes that host the network. This traffic is sent only to the necessary agent by encapsulating it as a targeted unicast.
https://docs.openstack.org/ocata/networking-guide/config-ml2.html
L2 population is a special mechanism driver that optimizes BUM (Broadcast, unknown destination address, multicast) traffic in the overlay networks VXLAN and GRE. It needs to be used in conjunction with either the Linux bridge or the Open vSwitch mechanism driver and cannot be used as standalone mechanism driver. For more information, see the Mechanism drivers section below.
