3G USB for WLAN, Ethernet as "dumb" switch

I have a TP-Link MR3420 router. (It's definitely V2 or higher, I don't think it's really relevant except for the flash file)

Currently, I use it on the stock firmware to provide 3G connectivity to my WiFi smart sockets. But now I need to set up some IP cameras via Powerline, because my main router is too far away and I don't want to use a long Ethernet cable and a separate switch.

So my goal is as follows:

  1. Use the 3G stick to provide connection to my WiFi sockets
  2. Use the LAN ports as a "dumb" router to connect the MR3420 to my main router via Powerline and to connect the cameras to the MR3420

Is it achievable with OpenWRT? If so, how do I do it correctly?

AFAIU I need to:

  1. Bridge the 3G and WiFi interfaces
  2. Disable DHCP and set the IP and gateway manually

Am I right? Is there anything else that should be done?

What is your main router (and does this have an internet connection?)

It's a Mikrotik rb4011igs+rm, it's connected to the internet via PPPoE with a bridge to a PON terminal.

When you say 3G, are you referring to cellular connectivity?

What is a "dumb router"? You can use the device as a basic Layer 2 switch (which has no routing functions) or a router (which is a Layer 3 operating mode). It's not clear what you want here, though.

Can you draw a diagram of your desired network topology? a photo of a sketch on paper is sufficient.

By 3G stick I mean a USB dongle that provides connectivity via cellular 3G network. An old one, not 4G capable.
But still I've labeled it as 4G USB on my diagram for better understanding.

By "dumb router" I mean a Layer 2 unmanaged switch. I meant "dumb switch".

Does the wifi socket (and the wifi on the MR3420) interact with the upstream wired network, or are you only expecting it to use the 3G/LTE network connectivity?

The sockets and the MR3420 only communicate via WiFi, and the MR3420 uses cellular USB modem right now to provide the internet connectivity to the sockets.

To clarify:

  • Wifi smart sockets do not need any ability to connect to the rest of the network and vice versa.
  • The smart sockets obtain internet connectivity via the USB cellular modem
  • The USB cellular modem does not provide internet connectivity to any other parts of the network.
  • You're looking to use the MR3420 basically as 2 independent devices that happen to be in one physical box..
    • an unmanaged ethernet switch
    • a wifi AP + router for the cellular connection (no ethernet connectivity for this network)

Is all of that correct?

  1. Correct.
  2. Via Wi-Fi to MR3420 which has USB cellular modem in it (just to clarify once again).
  3. Correct.
  4. Absolutely.

sure... this is really simple. You'll basically just remove the ethernet ports from the lan interface.

Is the wifi smart plug + 3g cellular USB stick stuff working currently?
How many ethernet ports do you need for the IP camera(s)? (obviously one port uplinks to the powerline -- how many downstream IP cam ports?)

Yes, the 3G+sockets stuff is working correctly, but the MR3420, as I've written in the original post, is on the stock firmware right now. I didn't flash to OpenWRT yet because I needed to make sure that my goals are achievable.

I will be connecting 4 IP cameras. The MR3420 has 4 Ethernet ports and a WAN port, but I'll just remap it to an Ethernet port and plug the uplink there.

Ok... so yes, this is totally possible with OpenWrt.

I think I overlooked that bit...

The problem is that your hardware is so old and limited that you will have to run a very old version of OpenWrt that is EOL and unsupported. It will have many known security vulnerabilities and is not considered safe to use anymore.

The latest release version of OpenWrt (technically LEDE) for your device appears to be 17.01.7. That is over 6 years old and has not been supported for the last 4. There has been a huge amount of change with respect to the configuration syntax, and it is likely that people here just won't even remember how to configure this properly (it's like asking people to speak fluent Middle English today -- there are similarities, but a lot of major differences, too).

With that in mind, I would actually recommend keeping your existing configuration as-is, and just buy a cheap unmanaged switch for your cameras -- a 5 port device can be purchased new or used for pretty cheap (~$10 USD).

I'm behind the NAT on the 3G without any external access, so I don't think any attacks on my MR3420 are possible.

Still, I understand that you're right and it's easier to do it that way.

Don't be so sure... don't forget about local hacks like KRACK and the like. And you probably can't trust the 3g stick entirely, either.

Yeah... in principle the idea is fine, but with that hardware, I think you'll be best served by the simple solution of adding a small switch.

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Thanks! :slight_smile:

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