Consider following topology to testing trunk/subport use-case.

1) 3 networks and 3 ipv6 subnets

openstack network create net0

openstack network create net1

openstack network create net2

openstack subnet create --network net0 --subnet-range 2001:db8:0:3::/64 subnet0_IPV6 --ip-version=6 --ipv6-address-mode=slaac --ipv6-ra-mode=slaac --allocation-pool start=2001:db8:0:3::2,end=2001:db8:0:3:ffff:ffff:ffff:fffe

openstack subnet create --network net1 --subnet-range 2001:db8:0:4::/64 subnet1_IPV6 --ip-version=6 --ipv6-address-mode=slaac --ipv6-ra-mode=slaac --allocation-pool start=2001:db8:0:4::2,end=2001:db8:0:4:ffff:ffff:ffff:fffe

openstack subnet create --network net2 --subnet-range 2001:db8:0:5::/64 subnet2_IPV6 --ip-version=6 --ipv6-address-mode=slaac --ipv6-ra-mode=slaac --allocation-pool start=2001:db8:0:5::2,end=2001:db8:0:5:ffff:ffff:ffff:fffe

2) 3 port on different subnet sharing same Mac-address 

openstack port create --network net0 port0 --disable-port-security

parent_mac="$( openstack port show port0 | awk '/ mac_address / { print $4 }' )"

openstack port create --network net1 --mac-address "$parent_mac" port1 --disable-port-security

openstack port create --network net2 --mac-address "$parent_mac" port2 --disable-port-security

 3) Configuring port0 as trunk and port1 and port2 as sub-ports.

openstack network trunk create --parent-port port0 trunk0

openstack network trunk set --subport port=port1,segmentation-type=vlan,segmentation-id=101 trunk0

openstack network trunk set --subport port=port2,segmentation-type=vlan,segmentation-id=102 trunk0

4) Router created and all IPv6 subnet added to the router.

openstack router create router1

openstack router add subnet router1 subnet0_IPV6

openstack router add subnet router1 subnet1_IPV6

openstack router add subnet router1 subnet2_IPV6

5) VM booted on trunk port. 

openstack server create --flavor ds1G --availability-zone nova:osc-pike-ubuntu16 --image Ubuntu --nic port-id=port0 VM1

Observed the VM's phy-interface getting assigned with all the trunk/sub-ports Ipv6 address(snapshot attached). It should only have trunk-port address.

packet-capture done during booting of VM.