Each of the built-in NIC config templates in the network/config/* directory of tripleo-heat-templates has a special version called controller-v6.j2.yaml which is no longer needed. The original reason for this file was to have a version of the Controller NIC configs that had an IPv6 default gateway on External and an IPv4 default gateway on the Control Plane. The role.role.j2.yaml file in each of those directories will now produce the correct routes if the roles_data.yaml is updated.
For example::
default_route_networks: ['External', 'ControlPlane']
If the above is placed in the Controller definition in roles_data.yaml, the generated NIC config templates will contain a default route on both the External and ControlPlane networks. When the External network is IPv6 and the ControlPlane is IPv4, these two default routes do not conflict, and the end result is the same as including controller-v6.yaml.
We originally left both versions in each NIC config template directory to support upgrades, but we can document that users should use controller.yaml instead of controller-v6.yaml when upgrading from a previous version.
Is it still an issue?