Comment 12 for bug 1471316

Extracted from /etc/netron.conf

# Default Subnet Pool to be used for IPv6 subnet-allocation.
# Specifies by UUID the pool to be used in case of subnet-create being
# called without a subnet-pool ID. Set to "prefix_delegation"
# to enable IPv6 Prefix Delegation in a PD-capable environment.
# See the description for default_ipv4_subnet_pool for more information.
# default_ipv6_subnet_pool =

it doesn't indicate that cidr specified in subnet-create will override the default subnet pool.

I'd like to suggest that to use prefix delegation in a cloud, the admin should create a shared pool for prefix delegation in the admin project, and openstack should reserve the name 'prefix-delegation' (or some sort) for PD. A few benefits with it:
  . when hint with PD is supported, the hind can be specified by the 'cidr'
  . a service plugin can choose to provision its PD server with the pool

So the default pool still maintains the same semantics as specified in above:
   -- if it's not None, and the user doesn't specify --subnetpool, it will be used for creating the subnet. If it's specified as prefix-delegation, then PD will be used. If the user specify --subnetpool, it will override the default

Another issue with the existing default subnetpool definition is the use of UUID in the config. This becomes a chicken and egg problem as the neutron server relies on the neutron.conf to be started so that the default pool can be created.

The default pool should be specified as name. And to make it further, the pool parameters may be specified in the config and neutron server can create the pool in the admin project as part of starting up the service.