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I have an ASUS RT-AC68U with the Tomato USB firmware and I want to extend the existing wifi coverage (a LTE modem with router) for better reception in the rest of the house. Having the same subnet or essid is of lesser importance - no devices need to talk to one another. So I got this working somewhat, but with problems. The existing network has ESSID "Net A" and subnet 192.168.0.0 (192.168.0.1 is the LTE modem). This is what I did:

  • disabled DHCP on the default br0 bridge setup
  • set the gateway to 192.168.0.1 on the br0 bridge setup
  • on the 2.4 Ghz network I set the "Mode" to "Wireless Ethernet Bridge" and entered the network details of the host network ("Net A").
  • on the 5 Ghz network I set the "Mode" to "Access Point" and the ESSID to "Net B"
  • I set the IP address of the device to be 192.168.0.2 (I think)

What works:

  • connecting to the 5Ghz "Net B" works great and it connects wirelessly to the host LTE router

What does not work:

  • connecting to "Net A" while ASUS is powered on
  • connecting to the Asus admin interface using the assigned IP of 192.168.0.2 (tried both "Net A" and "Net B")

If I power off the ASUS I can again connect to "Net A" without issues, meaning it seems to interfere somehow with the setup of the LTE modem. I thought that it might have been somehow due to me connecting to the Asus router using "Net A" (somehow - I do not know if it makes sense), but arp -a (which I believe should show which external MAC resolves to which IP) never changes for 192.168.0.1. Should not this change if I connected to the Asus?

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  • 1) If you connect some devices to "Net A" and then power on the ASUS, do all those devices lose their connection? 2) Do you have any wired devices connected to the LTE modem's ethernet ports (if it has any), and are those reachable from Net B? 3) Do you have any wired devices connected to the Asus' LAN, and can those reach 192.168.0.2?
    – grawity_u1686
    Commented Dec 19, 2020 at 15:03
  • 4) Can you just avoid Wi-Fi bridging and run an ethernet cable from LTE modem (if it has a LAN port) to the ASUS?
    – grawity_u1686
    Commented Dec 19, 2020 at 15:08
  • I don't have any wired devices, nor the possibility of running cables to avoid bridging (which is why I set the bridge up). I only ran a cable router to router when setting up the Asus to see that the basic networking worked
    – oligofren
    Commented Dec 19, 2020 at 15:20

2 Answers 2

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Having the same subnet or essid is of lesser importance - no devices need to talk to one another

Having the same ESSID isn't about letting devices talk to one another – it's about letting them seamlessly roam from one AP to another as you move around (before the old signal drops to unusable levels).

but arp -a (which I believe should show which external MAC resolves to which IP)

No, it shows the opposite. ARP is used to resolve IP addresses to MAC addresses, not the other way around. (Also, there is no such thing as "external" MAC here.)

never changes for 192.168.0.1. Should not this change if I connected to the Asus?

No. Bridges are supposed to be fully transparent at MAC layer. If the entire network is bridged, then it is completely normal that 192.168.0.1 will resolve to the same MAC address everywhere.

Wi-Fi is one big exception to the above rule of bridges being transparent, but in your case it goes in the opposite direction – it's the IP addresses of "Net B" that will appear to have the Asus' MAC when resolved from Net A.

Your Asus, being an ordinary Wi-Fi client of the LTE modem, is not allowed to send packets "from" arbitrary MAC addresses – instead it has to rewrite all outbound packets so that they appear to be sent from the Asus' MAC instead, and it has to spoof ARP in the same way. So devices on "Net A" will always see the Asus Wi-Fi MAC for any IP address that's on "Net B" (due to Wi-Fi bridging limitations). In addition, this rewriting seems to completely break IPv6.

However, this limitation only applies to Wi-Fi clients, not access points, so any device on both nets should see the correct MAC for addresses that are on "Net A". Your LTE modem's MAC address should not appear to change.

If I power off the ASUS I can again connect to "Net A" without issues, meaning it seems to interfere somehow with the setup of the LTE modem

I do not have an answer for this, though I do have a guess. (Can't do much better if you have no means of testing anything else but Wi-Fi...)

As I mentioned above, Wi-Fi clients are an exception to bridging... but there's an exception to that, too – the bridge client and the AP can agree to use the so-called "WDS bridge" or "4addr" mode, which allows for 100% transparent bridging, preserving MAC addresses in both directions.

So my wild guess is that the ASUS tries connecting in the "WDS bridge" mode and this makes the LTE modem start using this mode for all clients, whether they're bridges or not, thus making it impossible for normal clients to connect. (I haven't actually seen this happen, but it's technically possible.)

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  • With regards to "external MAC", I meant exactly what you wrote, it was just ambiguous what "external MAC" implied. After 3 hours of trying to debug the thing, I noticed that while my laptop suffered from the mentioned weirdness, no matter if "forgetting" the wifi, reconnecting, flight mode, etc. my android connected without issues to Net A. So did other devices. After unplugging the LTE modem, it seems to have resolved whatever issues it had with my laptop, making the network setup work exactly as intended. Still giving you an upvote for the in-depth explanations, of course.
    – oligofren
    Commented Dec 19, 2020 at 20:50
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I used 3 hours of trying to debug this thing, trying all kinds of things like making Windows "forget" the wifi, reconnect, put into flight mode, etc. All with no go. I then noticed that while my laptop suffered from the mentioned weirdness, my Android phone connected without issues to Net A. And after testing, so did other devices. Until this point, I had not considered rebooting the LTE modem, as I had changed no settings after verifying it worked initially and seeing that the bridging seemed to work. Seeing other devices working and having tried every possible workaround on my laptop made me suspect that this was some kind of bug on the LTE modem. After all, router software seems to be some of the buggiest I encounter.

After unplugging the LTE modem, it seems to have resolved whatever issues it had with my laptop, making the network setup work exactly as intended.

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