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Simplest (cheapest) cable modem

I’m thinking about switching to O2 but I have quite a complex network at home. I use my own router (openwrt) with several VPNs, few WiFi APs, and complex routing. So I don’t want to switch to FrizzBox and setup it from scratch (not sure if it’s even possible). And especially I don’t want to pay extra fee for router I don’t need.

Is there any simple cable modem for O2? Or really the only option is to by/rent expensive FrizzBox just to switch it to “bridge” mode?


Lösung von Samirah 10 June 2023, 18:00

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24 Antworten

Hello @Samirah,

welcome to our o2 Community 😊

I am pleased that you want to switch to us with your connection. If I understand you correctly, you have opted for a cable connection. Is your current connection also via cable or is it a classic DSL connection via the telephone line? Which router are you currently using?

Since you also use VPN, please note that our cable connections are always DS Lite, so a real dual stack is not possible. This can sometimes cause problems with VPNs.

Of course you can use your own router on our connections. DOCSIS 3.1 is required for our cable connections.

Please let us know if you have any further questions.

Best regards


Hi @o2_Giulia,

Thank you for your response. According to o2 web site only cable and dsl connections available for my address. I prefer cable connection over dls since I experienced some troubles with dsl before (another provider and I’m not sure if I’m right but it looks like dsl connection less reliable. The connection was interrupted several times a month especially during lightning etc.). At the same time I didn’t have any issues with cable connection. Now I’m using cable connection from Vodafone. They provide a router without a fee and I managed to switch it to bridge mode.

As far as I know OpenWRT supports DS Lite so it shouldn’t be a problem to setup ipv4 and use vpn over it (in case you provide necessary options for DS Lite of course).

As for DOCSIS 3.1 modem: according to documentation DOCSIS 3.1 and DOCSIS 3.0 are backward compatible and DOCSIS 3.0 should work in DOCSIS 3.1 network. The main difference is that DOCSIS 3.1 supports 10GBs and DOCSIS 3.0 - 1Gbs. In fact I don’t see even 1Gbs tariff at my home address. So why do I need DOCSIS 3.1 and DOCSIS 3.0 is not enough?

Anyway assuming that I need to buy a cable modem did I get it right that all I need is support DOCSIS 3.1? On Amazon I found ARRIS Surfboard SB8200-RB it supports DOCSIS 3.1 but it looks like it was made for USA market… Will it be compatible? Also I’ve read that provider need to “activate” device. What the activation procedure is? Will it be possible to activate that device in case I decide to buy it?

Hello @Samirah,

unfortunately, I can only offer you little support when choosing your own router. We optionally provide our customers with the Fritzbox 6660 (4,99 EUR/month) or Fritzbox 6690 (8,99 EUR/month) as a rental device. Maybe that would be an option for you too.

The Arris router is probably not compatible, at least I haven’t heard of it yet.

The router is always activated via the CM-MAC address of the router. Automatically with our rental routers, with your own routers you have to give the MAC address by phone or online.

Best regards


Unfortunately renting is not an option for me (especially as far as I’ve read it’s quite complicated to switch Frizbox to bridge mode).

Probably you could clarify information about DOCSIS 3.1 and DOCSIS 3.0. Will 3.0 work in your network? I have found another modem `Axing CMO 1-01 DOCSIS 3.0` and many users say it works in Vodafone network, so it looks like it is compatible with at least one cable Internet provider in Germany.

I cannot believe every o2 user use only Frizbox router, probably your support have some success stories from another users?

Moin Moin,

the minimum requirements are DOCSIS 3.1!

You can use the TC4400-EU KABEL-MODEM.

The Problem with O2 Cable is there some restriction in the O2 cable network.

The FRITZ!Box will not work in bridge mode with in the O2 network! 

Hello @Samirah,

unfortunately, we can not recommend or offer support for routers that are not rented from us.

But maybe the answer from schluej will help you a little bit further?

Best regards


Thank you @schluej !

It’s really a hard decision to make.

Either use that Fritzbox in router mode (setup dmz in my local network with all possible disadvantages).

Or to find where to buy TC4400-EU (by now I could find only one place and only pre-order) with hope I can setup it in o2 network.

@o2_Giulia right now I’m not ready to spend time and money to try any of those approaches :( But probably if I chose DLS it will be much easier? What are your requirements for DSL modems? Probably you can suggest some simple and cheap devices?

Using the FRITZ!Box in bridge mode will not work with O2 cable Network!

With DSL there is only 2 restriction, because O2 is not checking the Hardware, it must run with the German DSL network and you must be able to configure the router/modem.

I think DSL would be better 4 U. You will get dual stack and a fill working IPv4 and IPv6 connection.

Hello @Samirah ,

as schluej has said, if you are choosing to go with a DSL connection the only thing that you need to look out for is that the modell that you choose is compatible with the line itself. So if you get a german Fritz.Box there will be no issue. You can get some of the 75XX models for relatively cheap and I’d say that just about everyone can generally recommend these.


Kind regards, Sven

Thank you,

  Your main recommendation is to buy FritzBox. Generally speaking they are very good quality routers for SOHO with several gbits ports, wifi6, etc. But since I need only a modem I don’t think it’s worth buying it.

  I don’t understand how can I chose which modem to buy. For example I’ve found Zyxel DX3301-EU (~50 EUR). It’s a router with WiFi-6, but I could turn on bridge mode and use it as a modem just for 50 EUR. At the same time I’ve found Zyxel VMG4005-B50A-EU01V1F (~100EUR). It’s just a modem, costs 2 times more!

  I don’t want to invest in modem much money, especially to pay for router/wifi features. I just want as dumb and as cheap modem as possible, I'll never use it as a router. Is there a way to get a list of supported modem so I could select the cheapest? I tried to find something on Amazon. There are even some adls routers around 20 EUR. How can I make sure they will work in you network? Is there standards name? Probably you could suggest some online catalogs where I could easily find a proper modem for your networks (I've used https://e-catalog.co.uk/ in UK for example)?

Than buy a FRITZ!Box 7520 from eBay up to 250MBit. This router is working with the o2 Network, auto configuration.

Forward alls Ports to your router and be happy with a working internet connection.

And switch off all thinks you don’t want to use.

Thank you for your suggestion!

I’ve read a bit more about it and it looks like the device is too smart for me. The manufacturer says:

Requirements / Restrictions

  • The FRITZ!Box must set up the internet connection itself.
  • The Internet connection provider must allow the establishment of multiple PPPoE connections.

So, if I got it right, it doesn’t support “true” bridge mode. Is o2 supports multiple PPPoE connections? If not and I must forward all ports as you suggested, probably it won’t work for me. For example I experienced some issues with one foreign SIP provider. The reason was provider’s router. As soon as I switched it to bridge mode I have experienced no problem at all.

Hello @Samirah ,

o2 generally does not allow multiple PPPoE connections. I hope this helps you with your decision making.


Kind regards, Sven

It looks like 7520 is supported by OpenWRT (including dsl modem). So probably it would be not so bad idea to flash it and use in “real” bridge mode.

Just wondered if the dsl and cable technologies in O2 are only for individuals? I can’t believe even a small company would be satisfied with two possible options of SOHO routers...

It looks like I won’t receive any new information about the topic.

There is a summary (many thanks to @schluej ):

About cable modem:

  • In Germany custom version of DOCSIS 3.1 standard is used. So most likely modem which wasn’t made precisely for German market won’t work.
  • O2 support will help only with “default” Fritzbox 6660, Fritzbox 6690 rented routers.
  • In fact it’s very hard to find proper modem. It looks like only TC4400-EU is supported by O2 (in addition to Fritzbox). But it’s quite expensive and really hard to buy. Also I haven’t seen O2 support confirmed the modem is supported.
  • Fritzbox doesn’t support bridge mode! In other words if you want to use your own router the only hope fir you is TC4400-EU.

About DSL modem:

  • No one provided specification of needed DSL modem.
  • Only provided options were Fritzbox 75XX
  • The same as for cable modem: Fritzbox doesn’t support bridge mode!
  • However on e-bay could be found obsolete Fritzbox 7520 which will work with O2. Moreover it looks like the 7520 (not AX version) is supported by OpenWrt, so after flashing it would support bridge mode. However I haven’t tested it myself yet.

In other words despite the fact O2 will work with “non-default non-Fritzbox devices” on paper, it’s really a hard way on practice. And since Fritzbox cannot support bridge mode it’s hard (sometimes impossible) to integrate it to your network.

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IAbout DSL modem:

  • No one provided specification of needed DSL modem.


@Samirah For DSL you could use the Be.Ip routers from bintec (plus 100Mbit /swift 250MBit) too.

But they are not supporting bridge mode too. But you would be able to route your traffic.

Modems are not very common in Germany.

I used 4 years a bintec router behind a FRITZ!Box (O2 has „Zwangsrouter“). I just route all the traffic from the FRITZ!Box to my bintec router. All ports has open and forward to my bintec, works fine for me.

If you have enough network know how, that should no be a big thing.

The FRITZ!Box 7520 is not obsolet, it is just provider router that even will work with other networks. It is just a 7530 with only 2 1MBit ports and USB is limited to 2.0. It is a inexpensive way for a nice 250MBit router that is working find.

The limit is just your mind.

I’m not so familiar with DSL technology, but I don’t see ADSL, ADSL2+, VDSL, super vectoring, Annex-J and other words that I usually see in modem’s documentation. It looks like it’s more about how telephony will work over bit stream connections (BSA), but not the DLS transport itself.

I’ve found a topic here: 

Not sure if the article is not obsolete, but even here haven’t found exactly modem specification I need.

If U use 250 MBit U need Super vectoring Modem, could be used with lower speed too.

If U use up to 100 MBit U need a VDSL Modem.

Information about xDSL U will find her https://www.telekom.de/hilfe/downloads/1tr112.zip

The Topic is old but not complete out dated.


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As already mentioned by @schluej , the Telefónica interface description (see page 2) refers to Deutsche Telekom's Technical Guideline (“Technische Richtlinen”) 1TR112 regarding DSL specifications. This is due to the fact that Telefónica itself does not operate any DSL technology in Germany, but purchases this as a wholesale service from the local line provider.

@blablup Thank you for the information, it explain most things!

Just curios, did I got it right that the same thing applies for Cable technology and other internet providers? If so how could it be possible that some say that DOCSIS 3.0 modems works with some providers, but doesn’t work in O2.

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Telefónica Deutschland also does not operate any technology for cable TV/DOCSIS connections. These wholesale services are purchased from local cable network operators Vodafone and Telecolumbus. The contracts between the cable network operators and Telefónica probably stipulate that only DOCSIS 3.1 is allowed. 

And cable modems with DOCSIS 3.0 will not be added to the white list for allowed CM-MAC.

And i think they would clean up the cable network from DOCSIS 3.0 devices.

Hello @Samirah,

I am very happy about this great thread, a lot of important information has already been collected here. I hope it helps you in making a decision.

Thanks to @blablup and @schluej for your great help 💙

Please let us know if you have any further questions.

Best regards


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