LAN DHCP fails when WAN disconnected?

[Gizli]

Hi.

I'm a relative noob to these forums and ASUSwrt-Merlin in general. I've got the latest Merlin firmware installed on my RT-AC66U B1. No problems for many months using the default settings. Then, last week, complete chaos. My ethernet cable from the ISP router to the RT-AC66U B1 had failed. With a new cable, all was well again.

What I don't understand is that my LAN side was a mess along with the WAN connection. None of my LAN devices could talk to each other and the RT-AC66U B1 DHCP Sever wouldn't hand out IP #'s in the IP Pool range - some of them I had even manually assigned IP #'s in Merlin's table. I would have thought that Merlin's DHCP server would have kept on working on the LAN side independently. As soon as I replaced the cable, all was well again.

So, I guess I'd like know if this is typical/correct Merlin behavior and understand a little bit more about what's going on.

Thank you in advance for your comments.

 

[dosborne]

Welcome

"I've got the latest Merlin firmware installed"
Pet peeves of mine, that statement is not useful. Please post actual version numbers. "Latest" means different things to different people

You are correct that the internal lan and functions like DHCP should be independent of the wan status. Is it possible that your ISP router was (or still is) configured as a DHCP server as well? In any case, rebooting (or renewing a lease) on a client should have restored internal functionality.

My second guess would be misconfiguration of the network settings such as default gateway.

If there is a "next time", examine the configuration on a client device and see if you can determine (from logs etc) the DHCP server that the client got a response from, the default gateway, etc

 

[Gizli]

I completely agree. I could have been more clear.

The RT-AC66U B1 is running ASUSwrt-Merlin 386.2_6.

The IP # I was getting on ethernet to the router was a self-assigned # - the computer was giving itself a number and could not connect to the ASUSwrt-Merlin interface. However, on wifi, the router was giving it the IP # and I could monitor and adjust ASUSwrt-Merlin.

The ISP router is doing DHCP, as it hands out #'s when connected via ethernet or wifi. The ISP router specifically gives my RT-AC66U B1 a static IP #. In addition, The ISP router has its DMZ active in order to pass all traffic through to the RT-AC66U B1. Throughout the madness, I always was able to connect to either router via wifi and login to its interface.

Thank you for confirming my understanding that, in theory, the WAN and LAN should work independently of each other. That's what I've always thought; however, that was not what was going on, as far as I could tell.

My hope is that there will *never* be a next time!

 

[dosborne]

Dmz should not be required (a lot safer to forward individual ports), unless you are doing something very specific. I suspect the configuration is the source of your issues. But, without knowing more, I wouldn't want to jump the gun.

Typical scenario is ISP router configured in gateway mode, connected to wan port of secondary router. Lan subnets of both routers being unique from each other.

Maybe you have specific reasons for your configuration though.

 

[mistalanious]

I'm having this same issue since the update. None of my devices that are wired in are able to get an IP but wireless is working fine. I've been going in circles rebooting my switches. I did a factory reset after I updated the firmware as well.

 

[Gizli]

I'm having this same issue since the update. None of my devices that are wired in are able to get an IP but wireless is working fine. I've been going in circles rebooting my switches. I did a factory reset after I updated the firmware as well.

Just so I'm clear, my issues did _not_ coincide with updating to ASUSwrt-Merlin 386.2_6. Things were running fine and then ka-flewie! I hadn't made any setting changes or do a firmware update in weeks.

 

[Gizli]

Dmz should not be required (a lot safer to forward individual ports), unless you are doing something very specific. I suspect the configuration is the source of your issues. But, without knowing more, I wouldn't want to jump the gun.

Typical scenario is ISP router configured in gateway mode, connected to wan port of secondary router. Lan subnets of both routers being unique from each other.

Maybe you have specific reasons for your configuration though.

Yeah, my lame reason is LAZINESS for the DMZ. I run a Synology NAS on the secondary router and didn't want to input the different ports individually. But I'll take your advice and put them in soon.

As part of my diagnosing the problem, I tried 3 different routers, one of which was a brand new ASUS RT-AC66U B1 with stock firmware, none of the settings changed except for the admin user and password. That router still had problems. As soon as I changed out the cable, all routers worked fine.

 

[Ranger802004]

Welcome

"I've got the latest Merlin firmware installed"
Pet peeves of mine, that statement is not useful. Please post actual version numbers. "Latest" means different things to different people

You are correct that the internal lan and functions like DHCP should be independent of the wan status. Is it possible that your ISP router was (or still is) configured as a DHCP server as well? In any case, rebooting (or renewing a lease) on a client should have restored internal functionality.

My second guess would be misconfiguration of the network settings such as default gateway.

If there is a "next time", examine the configuration on a client device and see if you can determine (from logs etc) the DHCP server that the client got a response from, the default gateway, etc

Are you using IPV6 and do you have a ISP router bridged with your ASUS router? Maybe your router is acting as a passthrough for the DHCP server on the ISP Router?

 

[ColinTaylor]

Are you using IPV6 and do you have a ISP router bridged with your ASUS router? Maybe your router is acting as a passthrough for the DHCP server on the ISP Router?

You're replying to a post that's almost a year old.

 

[Ranger802004]

You're replying to a post that's almost a year old.

And now so are you ‍