Comment 5 for bug 1877301

So in case #1 (User creates a l3 router then sets external gateway and adds an IPv6 subnet to it and create a VM on the subnet), with prefix delegation, since the upstream router has provisioned the prefix, it sets the next-hop for the entire prefix to the neutron router. So all traffic will flow into the subnet automatically, proxy NDP is not required.

For case #2 (advertise a part of VMs to external network), there is no way to do that currently. Can you provide the use case for this? Since if you're trying to not allow incoming traffic that can be accomplished using security groups.