For IPv6 only networks there is a problem with DNS configuration. In the existing network 'provider_net_subnet_ipv6_only', the IPv6 address is assigned by SLAAC which is not assigning DNS address. This is a problem for OCP installation as the ignition files are using hostnames for a source. And without DNS the source hostname can't be resolved and therefore the ignition file can't be fetched.
It is necessary to have DNS configured in advance before the ignition process is trying to fetch the ignition file. This can be done manually during boot process via grub menu via static network configuration by passing kernel parameters. This is current workaround.

We need to find some way how to add a DHCP server to IPv6 only network to pass DNS address dynamically while not breaking the IP address assignment logic done by OpenStack.