openstack router create router
openstack network create N1

openstack subnet create --network N1 SUB1 --subnet-range 10.10.10.0/24  
openstack router add subnet router SUB1

openstack subnet create --network N1 SUB2 --subnet-range 2001:db8:0:2::0/64 --ip-version 6 --ipv6-ra-mode slaac --ipv6-address-mode slaac
openstack router add subnet router SUB2


openstack network create N2
openstack subnet create --network N2 SUB3 --subnet-range 20.10.10.0/24  
openstack router add subnet router SUB3

openstack security group create X
openstack security group rule create X --egress --dst-port 1:65535 --protocol tcp
openstack security group rule create X --egress --dst-port 1:65535 --protocol udp
openstack security group rule create X --egress --protocol icmp
openstack security group rule create X --ethertype IPv6 --egress --dst-port 1:65535 --protocol tcp
openstack security group rule create X --ethertype IPv6 --egress --dst-port 1:65535 --protocol udp
openstack security group rule create X --ethertype IPv6 --egress --protocol icmp
openstack security group rule create X --ingress --dst-port 1:65535 --protocol tcp
openstack security group rule create X --ingress --dst-port 1:65535 --protocol udp
openstack security group rule create X --ingress --protocol icmp
openstack security group rule create X --ethertype IPv6 --ingress --dst-port 1:65535 --protocol tcp
openstack security group rule create X --ethertype IPv6 --ingress --dst-port 1:65535 --protocol udp
openstack security group rule create X --ethertype IPv6 --ingress --protocol icmp

neutron port-create N1 --name=port1 --security-group X
nova boot --image cirros-0.3.4-x86_64-uec --flavor m1.tiny --nic port-id=$(neutron port-list | grep "port1" | awk '{print $2}') VM1

neutron port-create N2 --name=port2 --security-group X
nova boot --image cirros-0.3.4-x86_64-uec --flavor m1.tiny --nic port-id=$(neutron port-list | grep "port2" | awk '{print $2}') VM2

// here, everything works fine. you can connect to VM1 and ping VM2 with IPv4.

openstack router delete subnet router SUB2

// here, IPv4 routed traffic does not work. you can not ping VM2 from VM1 with IPv4.