Upstream:

[Match]
Name=ppp0

[Link]
RequiredForOnline=routable

[Network]
DHCP=ipv6
IPv6AcceptRA=true
KeepConfiguration=true

[DHCPv6]
PrefixDelegationHint=::/56

[IPv6SendRA]
Managed=true

Downstream

[Match]
Name=vlan100

[Link]
RequiredForOnline=routable

[Network]
ConfigureWithoutCarrier=true
DHCPv6PrefixDelegation=true
IPv6AcceptRA=false
IPv6SendRA=true
Address=192.168.178.1/24
Address=fe80::1/64

[DHCPv6PrefixDelegation]
SubnetId=64

[IPv6SendRA]
DNS=fe80::1
EmitDNS=true
RouterLifetimeSec=300