Entire home network goes berserk when router's WAN uplink ceases

Hardware Hub Networking
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Been having internet problems for a bit where if my WAN uplink on the router disconnects, all devices connected via our network switches are completely disconnected from anything and everything, including from each other on the LAN.

My router (for which I have tried multiple, from a RT-AC68U C1 to an Archer A7: EU v5 running stock, OpenWRT and DD-WRT) is connected to via LAN1 and LAN2 to two TP-Link TL-SG105S’es.

The first switch connects to my printer, my NAS running multiple Docker containers, my TV and a laptop and the second switch connects to my desktop and my server which runs multiple VMs.

So, having access to my LAN devices regardless of the WAN’s status is pretty important. Unfortunately, these are unmanaged switches so I’m not exactly sure how to force it to stick to a particular subnet (10.0.0.x/24) or even change the firmware.

Connecting LAN1 directly to my desktop gives me an IP like 10.0.0.102 and lets me access everything (which in this case is just my router since everything else is still behind the switch) but connecting via the switch works for a fraction of a second before I’m given some absurd IP like 169.254.225.38, which some Google-fu means to indicate that either

  • the uplink is disconnected; or
  • it cannot find the DHCP server

Both aren’t true, the lights are going blinky-blinky, directly connecting over WiFi and checking the router settings does indicate that something is populating LAN1 and LAN2 and the router is serving as an authoritative DHCP server.

I’d hate to throw these switches away but I’m not sure what to do. I’m currently making do by directly attaching my NAS to LAN1 and server to LAN2 and connecting my printer to the NAS and but I’m not a fan of this approach.

I have no idea why the switches in any capacity care about the WAN uplink of the upstream when they’re supposed to route packets between devices on the LAN and I’m on my third router now (an ISP branded router with details I can’t really share with you cause there aren’t any) and I’m having the same problems so I’m now certain that it’s the switches acting funky.

I’m a networking noob who just likes the security of a wired connection over wireless and so I just did what I think works and now I have no idea how to proceed.

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Sounds like the router is locking up when the WAN disconnects, which may be more of the router causing the disconnect rather than an ISP issue.

The unmanaged switches cant do anything for devices because they only send traffic out their ports, they have no real packet sorting or anything on their own and are relying on the router to actually handle all the network routing and addressing.
You could get around this loss of network by assigning static IPs to all your devices. That way when the DHCP server on the router is down the network devices still have their IPs and the switches know what to do.

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Sounds like the router is locking up when the WAN disconnects

I hope this isn’t “expected behavior” because I’ve switched between multiple routers and having them all lock up as a bug rather than a feature either means I’ve won the crappiest lotto ever or this is expected behavior.

You could get around this loss of network by assigning static IPs to all your devices

That’s what I used to do initially.

I went through all my settings and realized that the problems started when I set all my formerly statically assigned devices to DHCP and then forcing their allocations to be static via MAC address detection.

I switched them back to statically assigned addresses and things started chugging again.

This still doesn’t explain the erratic behavior, so I’m not marking this as solved but at least I can keep going for now.

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WAN loss is somewhat common. That causing trouble would be like a Bakery catching fire when cake runs out…

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That’s an apt metaphor for the sudden change in DHCP behaviour, what really did it for me that it was funky functionality, not a funky unit was that I was fed up of dealing with all of this and told my ISP tech “just get me a router that you guys know how to service” and they got one, we installed it, I used a direct connection so I could get some work done and then one day decided to try it with the switch.

It didn’t work so I thought perhaps it’s the switch that’s at fault, so I grabbed a known-good switch and tried again and it still didn’t work, I knew the router unit was good, I knew the switch behaviour was good, the only difference between the “known good” network and the broken network was that “known good” had completely statically allocated clients and broken’s clients were dynamically allocated static MAC-bound IPs through DHCP.

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You are doing something wrong, there’s millions of routers if not billions out there not stopping lan traffic when wan goes down, so you are definitely doing something wrong

First, use only one switch, an 8 port switch is 30 usd… get one to rule out you messing with uplinks from the switch.
If you really need to go with two switches, daisy chain them

Router lan->sw1 port1
Sw1 port 2 -,> sw2 port 1
Make sure you are using the router lan- port, not the wan
Make sure DHCP is on on the router, and DHCP is set up properly on the clients
Use different lan cables, maybe you have an uplink one that does not work…
Wan going down on the router must not affect internal lan operations

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If you really need to go with two switches, daisy chain them

Already done.

Make sure you are using the router lan - port, not the wan

Already done.

Make sure DHCP is on on the router, and DHCP is set up properly on the clients

Already done.

Use different lan cables, maybe you have an uplink one that does not work

Already done.

Wan going down on the router must not affect internal lan operations

I agree, still doesn’t work.

Tried all the above steps and went above and beyond that. Tried different routers, firmware, cables, different uplink methods (fiber vs copper), different subnets.

This isn’t my first networking rodeo and I wouldn’t be posting here if I hadn’t tried all the usual troubleshooting steps.

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In your original post you said you have both connected to the router …

It is not rocket science and it must work
If you get an IP when connected directly to the router lan- port you must get the same when going router-switch-lan port
If not, either

  • The switch is faulty
  • The router is applying some weird security policy on lan
  • The router is faulty
  • Your PC/nic can’t refresh DHCP properly
  • Your PC firewall is acting up

I would suggest changing only one thing at a ttime, with the exception of the client that, especially if it is window, may benefit of a reboot between trials…

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The router is applying some weird security policy on lan

The only common variable that comes to mind is that all routers I tried aside from the first one are TP-Link routers but I didn’t think much about that since I tried OpenWRT and DD-WRT…

In your original post you said you have both connected to the router …

My bad, that was a configuration I did while troubleshooting but they’ve been daisy chained for quite a bit of time now. Sorry about that.

Your PC firewall is acting up

Not impossible, could be Windows this whole time.

I’m currently sticking to static allocations and bypassing DHCP altogether because I kinda ran out of steam to look into this and just wanted things to work but I’ll probably spend more time troubleshooting once I build up some steam.