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Actually, links bellow ( regardless they connected GRE tunnels ) answer your question
1. https://answers.launchpad.net/neutron/+question/221476
2. https://visualne.wordpress.com/2013/02/09/quantum-gre-tunnels-ovs/

In case of GRE or VXLAN tenants L2 networks the VLAN tags you see in the output of "ovs-vsctl show" and in output of "ovs-ofctl dump-flows br-tun" (mod_vlan_vid) are only locally significant. This VLAN tags are not really L2 tags added to the frames leaving on the physical interface.They are only used by openvswitch to separate traffic on the br-int, so the different  tap interfaces corresponding to different neutron subnets do not see each other's traffic

Actually, links bellow ( regardless they connected GRE tunnels ) answer your question
1. https://answers.launchpad.net/neutron/+question/221476
2. https://visualne.wordpress.com/2013/02/09/quantum-gre-tunnels-ovs/

In case of GRE or VXLAN tenants L2 networks the VLAN tags you see in the output of "ovs-vsctl show" and in output of "ovs-ofctl dump-flows br-tun" (mod_vlan_vid) are only locally significant. This VLAN tags are not really L2 tags added to the frames leaving on the physical interface.They are only used by openvswitch to separate traffic on the br-int, so the different  tap interfaces corresponding to different neutron subnets do not see each other's traffic

View also https://ask.openstack.org/en/question/54598/what-is-going-happen-on-juno-ml2ovsvxlan-when-tun_id-of-private-subnet-will-become-0x10002/

Actually, links bellow ( regardless they connected GRE tunnels ) answer your question
1. https://answers.launchpad.net/neutron/+question/221476
2. https://visualne.wordpress.com/2013/02/09/quantum-gre-tunnels-ovs/

In case of GRE or VXLAN tenants L2 networks the VLAN tags you see in the output of "ovs-vsctl show" and in output of "ovs-ofctl dump-flows br-tun" (mod_vlan_vid) are only locally significant. This VLAN tags are not really L2 tags added to the frames leaving on the physical interface.They are only used by openvswitch to separate traffic on the br-int, so the different  tap interfaces corresponding to different neutron subnets do not see each other's traffic

View also https://ask.openstack.org/en/question/54598/what-is-going-happen-on-juno-ml2ovsvxlan-when-tun_id-of-private-subnet-will-become-0x10002/
Last sample was built by myself : http://bderzhavets.blogspot.com/2014/11/vxlan-tenants-subnets-and-vlan-tags-on.html