Since this bug [1] is fixed now in mitaka, the straightforward solution seems to add an new event in ML2 so that the bgpvpn plugin will be notified of a port_update that reflects the attachment of a port to a router.
If both the router and the network of the port are already attached to a bgpvpn, we should send an exception that will prevent the attachment of that network to the router.
Since this bug [1] is fixed now in mitaka, the straightforward solution seems to add an new event in ML2 so that the bgpvpn plugin will be notified of a port_update that reflects the attachment of a port to a router.
If both the router and the network of the port are already attached to a bgpvpn, we should send an exception that will prevent the attachment of that network to the router.
[1]https:/ /bugs.launchpad .net/bgpvpn/ +bug/1475093