总结：
1、br-int 流表总体是按照Normal 的方式，即常规的交换机的转发方式进行转发。而br-tun 交换机则主要按照流表的方式进行转发。
2、一般情况下，VM发出的ARP请求，会在该VM的所有邻居进行广播发送和查找，大量浪费带宽。当neutron开启了l2 pop后（二次注入功能），
计算节点会学习别的主机发送的免费ARP, 从而在本地存在ARP表项。
3、当本地的VM发出ARP请求时，br-tun交换机会优先查找本地的ARP表项，从而对报文进行ARP应答。
这样的话，就不用发出ARP请求的广播报文。如果br-tun交换机查找不到对应的ARP响应流表，则按照普通的隧道广播的方式进行正常广播发送。
4、ARP响应流表的匹配条件和动作分别如下：
- ARP响应流表的匹配条件是：
br.add_flow(table=constants.ARP_RESPONDER, priority=1, proto=‘arp‘, dl_vlan=vlan, nw_dst=‘%s‘ % ip, actions=actions)
- ARP响应流表的动作如下：
ARP_RESPONDER_ACTIONS = (‘move:NXM_OF_ETH_SRC[]->NXM_OF_ETH_DST[],‘ ‘mod_dl_src:%(mac)s,‘ ‘load:0x2->NXM_OF_ARP_OP[],‘ ‘move:NXM_NX_ARP_SHA[]->NXM_NX_ARP_THA[],‘ ‘move:NXM_OF_ARP_SPA[]->NXM_OF_ARP_TPA[],‘ ‘load:%(mac)#x->NXM_NX_ARP_SHA[],‘ ‘load:%(ip)#x->NXM_OF_ARP_SPA[],‘ ‘in_port‘)5、对匹配条件和动作的解释如下：
self.tun_br.add_flow(table=constants.ARP_RESPONDER, – Add this new flow to the ARP_RESPONDER table
priority=1, – With a priority of 1 (Another, default flow with the lower priority of 0 is added elsewhere in the code)
proto=‘arp’, – Match only on ARP messages
dl_vlan=lvid, – Match only if the destination VLAN (The message has been locally VLAN tagged by now) matches the VLAN ID / network of the remote VM
nw_dst=‘%s‘ % ip, – Match on the IP address of the remote VM in question
actions=actions)
actions = (‘move:NXM_OF_ETH_SRC[]->NXM_OF_ETH_DST[],’ – Place the source MAC address of the request (The requesting VM) as the new reply’s destination MAC address
‘mod_dl_src:%(mac)s,’ – Put the requested MAC address of the remote VM as this message’s source MAC address
‘load:0x2->NXM_OF_ARP_OP[],’ – Put an 0x2 code as the type of the ARP message. 0x2 is an ARP response.
‘move:NXM_NX_ARP_SHA[]->NXM_NX_ARP_THA[],’ – Place the ARP request’s source hardware address (MAC) as this new message’s ARP target / destination hardware address
‘move:NXM_OF_ARP_SPA[]->NXM_OF_ARP_TPA[],’ – Place the ARP request’s source protocol / IP address as the new message’s ARP destination IP address
‘load:%(mac)#x->NXM_NX_ARP_SHA[],’ – Place the requested VM’s MAC address as the source MAC address of the ARP reply
‘load:%(ip)#x->NXM_OF_ARP_SPA[],’ – Place the requested VM’s IP address as the source IP address of the ARP reply
‘in_port’ % {‘mac’: mac, ‘ip’: ip}) – Forward the message back to the port it came in on
6、参考blog: OVS ARP Responder – Theory and Practice
https://assafmuller.com/2014/05/21/ovs-arp-responder-theory-and-practice/