When an IPv6 subnet is created without specifying ipv6_ra_mode, it is expected that the VMs with this subnet obtains global IPv6 address from non-OpenStack router using SLAAC.

openstack subnet create --network network1 --subnet-range 2001:db8:0:3::/64 subnet1 --ip-version=6 --ipv6-address-mode=slaac

Currently, ACL is dropping these packets in VM ingress anti-spoofing table (240) which results in VM not obtaining global IPv6 address from external router.