Use case :

1. create network
2. create subnet
3. create vm
4. create router
5. add subnet to router
6. create external network and subnet
7. create external bgpvpn 
8. associate external bgpvpn to external network
9. openstack router set router1 --external-gateway myextnet10 --disable-snat
10. openstack router set router1 --external-gateway myextnet10 --enable-snat

Notice that the 21->26 flow is missing