Doubts of the beginner

Hello everyone,

Forgive me if my questions are stupid, but I'm very inexperienced about networking.
I SEEM to have understood that it is not recommended a device that acts as both a modem and a router, while instead it is often recommended the purchase of a good router positioned in cascade to the modem provided by the ISP. Why this thing?
But if I misunderstood, for me it would be preferable to buy a great device that does both a modem and a router, so as not to double cables and total footprint. In this case, could you then recommend a good modem router that supports OpenWRT / LEDE and FTTH fiber?

Thank you in advance!

If you have FTTH, you must use the Optical Network Terminal supplied by your ISP. There is simply no alternative to that. If you're lucky, your ISP can supply one that operates in "bridge mode" like mine. That is the only realistic way you can use an OpenWrt router with FTTH. Sorry.

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I don't think there is anything that can replace the ONT and run OpenWrt, unless your fiber provider supports a generic SFP module. Then technically the modem is in the module.

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Thanks for the clarifications.
I really have to confess that I wrote FTTH because I would like to have it in the future, and then receive advice by which to buy a device that would allow me to find myself already "prepared" for that circumstance.
Instead, I currently only own the FTTC. Also in this case is it not possible to replace the cheap device provided by the ISP?

It depends on what you want to do. Those devices with a DSL modem that is supported by OpenWrt are simply quite old and not very powerful. They might work okay as bridged modem only, but do not have enough CPU cycles to do much more. Personally I got a cheap BT HomeHub5a with the intention to make it my all-in-one dsl-router, but got disillusioned pretty quickly (it now serves as bridged modem, still running OpenWrt and feeds a slightly more powerful OpenWrt WiFi router). I can sympathize with the charm of an all-in-one solution, but for DSL that simply is not a realistic option, but see below.

There are dsl-modems in SFP form factor that can be used in OpenWrt routers with an SFP-"cage", but make sure to research compatibility between router and SFP-module before purchase ;). AFAICT, getting line statistics from such a SFP-dsl-modem seems hard to impossible.
Now depending on the router, it might also be compatible to SFP-modules for the different FTTH-options.
But realistically, I would try to solve the current challenge with current devices and tackle the FTTH challenge once it actually materializes at your home, no?

P.S.: One of the options of a OpenWrt based router with an SFP cage would be the turris omnia, but if you start searching the Forum you will see that it has at least as many disappointed users as happy ones.
Disclaimer, I am happy with my omnia, but I do not do anything demanding/bleeding edge on it, nor do I use the SFP cage.

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Some ISPs, like Germany's Deutsche Telekom, will also provision GPON service to compatible 3rd party ONTs, so the "supplied by your ISP" part might have some flexibility, but independent of that the device on the end-user side needs to match the device/technology the ISP uses. So before you know what your ISP will use/uses it will be risky to purchase anything.

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Thank you for the very comprehensive technical explanation. I conclude that the solution of the 2 in 1 modem router device is definitely out of my technical capabilities. I try to open another discussion for advice on more targeted purchases. Thank you again!

The disadvantage of both in one is twofold for me. If one fails or is misconfigured, you can’t connect directly to the modem for Internet access to get help, or swap in a cheap router. If you change ISPs, or they change modulation, you may need a new dual-purpose box. For me, separating the two means I can pick “the best” for each purpose, looking at performance, functionality, cost, reliability, ...

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Right point of view. I thought about it just as I was asking the other question here on the forum. Because in fact I can now buy a good router separate from the modem, and keep it in the future if I ever switch to FTTH. While instead if I bought a 2 in 1 I could not do it and instead I would have to buy another one in order to use the FTTH.