Comment 6 for bug 1722534

> And I really do need a config file per group of links and bridges and IPs.
> Currently I dont see a way to implement this with systemd.networkd.

networkd's config design does tend to assume more of a 1:1 configfile-to-interface relationship, although it is possible to use a single network config file for multiple interfaces, by setting the [Match] section more widely (e.g. matching Name= on a pattern, like Name=ens3*). However I suspect you don't want to configure lots of interfaces in identical ways, so that may not help.

If you can attach/paste your ifupdown config file(s), I may be able to suggest corresponding networkd config, or at least it might help me understand typical telco networking config needs. Also you may want to check upstream systemd issue tracker to see if anyone has asked for changes to better accomodate telco needs, or open an issue yourself:
https://github.com/systemd/systemd/issues

There's also the systemd mailing list if you want to email them, or search for telco-related discussions:
https://lists.freedesktop.org/archives/systemd-devel/

Finally, you also can look at using netplan, which is a frontend for systemd-networkd (or NetworkManager) and its config files might be better for your use case - you can have one netplan config file to configure multiple interfaces, unlike systemd-networkd (netplan will auto-generate any/all needed networkd config files from the netplan config data).
http://manpages.ubuntu.com/manpages/hirsute/en/man5/netplan.5.html