当neutron启用L3 agent时，如果在配置文件中配置了external_network_bridge，从这个bridge上出去的包只能是untag的。但在DC中，极有可能被分配的是某一vlan。这种情况下，在配置文件中就要如下设置：
/etc/neutron/l3_agent.ini # Name of bridge used for external network traffic. This should be set to # empty value for the linux bridge external_network_bridge = ""
、# ovs-vsctl show
418f4819-8ad6-4fe5-a959-3605eee4852b
Bridge "br-eth1"
Port "phy-br-eth1"
Interface "phy-br-eth1"
Port "br-eth1"
Interface "br-eth1"
type: internal
Port "eth1"
Interface "eth1"
Bridge br-int
Port "tapa570fb69-fb"
tag: 5
Interface "tapa570fb69-fb"
type: internal
Port "qr-f01fed18-ad"
tag: 5
Interface "qr-f01fed18-ad"
type: internal
Port br-int
Interface br-int
type: internal
Port "tapa7b25e2f-ad"
tag: 1
Interface "tapa7b25e2f-ad"
type: internal
Port "qg-d2f9cb0b-11"
tag: 3
Interface "qg-d2f9cb0b-11"
type: internal
Port "tapa85a86f8-c1"
tag: 3
Interface "tapa85a86f8-c1"
type: internal
Port "int-br-eth1"
Interface "int-br-eth1"
ovs_version: "1.11.0"Neutron provider external network
原文：http://blog.csdn.net/matt_mao/article/details/19088127