[claudex]

>DSL tech is far simpler and it's always a combo unit so I could see a case where you would be allowed to bring your own DSL modem.

Not really, when you want to increase the bandwith, e.g. with vectoring[1], you need to have all neighbor modems to participate, which prevent free modem choice for the users.

>But it just doesn't work like that for DOCSIS or GPON where the cable modems or "ONT" router combo units these days do much more than just media conversion - SIP, PPPoE, IGMP, etc.

In Belgium, the ONT is just media conversion these days, SIP is done on the provider box, so you can have your own GPON SFP.

>so it's just not possible for the ISP to guarantee support for any random modem or ONT.

The ISP doesn't have to guarantee support to let you use your own hardware. It just have to give you the specs to use it and let you plug the ISP box if you can't configure vlan of dhcpv6 client.

[1]: https://en.wikipedia.org/wiki/VDSL#VDSL2_vectoring

[claudex]

Each country regulator in the European Union have to set it's own regulation, but the BEREC (Association of UE telecom regulators) guidelines say that in most case, the free choice of router and modem is what's is required by the EU decisions https://www.berec.europa.eu/sites/default/files/files/docume...