@Alexandre. 

Hi Alexandre, and thanks for your feedback. I followed your instructions, and it was a bit weired. Even Smart DHCP looked to be disabled, I touched the button and a next window open, where the switch button was in fact enabled. I turned it off.

I'm a technical expert in development and programming, and I'm very familiar with networking since a lot of years ago, so I thank you a lot if you want to give me further information about what has happened here.

Thanks for your support.

Best regards.

  @latas2002 Did you verify via the app that  Deco Unit was setup with a different configuration that you did?  Or did you just observe it was acting as a DHCP server when disconnected from Internet?

  @latas2002 

I am going to tell you a story about system design decisions, which are foolproof and could not possibly go wrong … until taken from development/staging to production environment/real life - and do go wrong. If you have IT knowledge and experience in software development, that situation should be familiar to you.

When Main Deco runs in Access Point mode, it still has all router software code. When primary DHCP server in LAN goes down, and in home network that usually means ISP Router went down, Main Deco will take over. Main Deco will become the default gateway, DHCP server and DNS server. Why? Because it can, apparently.

Main Deco does it in a smart way, actually: as long as it runs undisturbed it will provide IP addresses in the same range as were previously configured in ISP Router. Devices already known to Deco mesh, when they renew their IP address, they’ll get the same IP address they have now from Main Deco DHCP service.

This is smart functionality that aims at having home network operational even when primary DHCP server is down. Also, completely unnecessary for a regular home network. In my household, for example, when ISP Router goes down nobody cares if they can still print on network printer or if automatic PC backups to backup server run uninterrupted. What user at my house would be concerned is she can’t view online cartoons on her tablet.

If you bounce Main Deco while ISP Router is down, it creates additional mess: Main Deco resets itself on reboot and starts with default Deco router IP range 192.168.68.*. Devices will be getting IP addresses from that IP range. That’s what happened after power outage you experienced.
Still, that should not be a problem: DHCP lease time from Main Deco is in mere minutes. As soon as ISP Router (primary DHCP server) is back, Main Deco will yield to it and the network should recover promptly.

This is what they call Smart DHCP service: if there is DHCP service active on LAN, I will not start mine. If there is none, I’ll start mine.

In summary: smart design, helps network resiliency, completely harmless, network recovers to its normal state almost immediately after primary DHCP server come online. What could possibly go wrong?

Here comes the proverbial “but:” what would happen if primary DHCP server is also “smart?” Its developers may have thought: “why would we let idiot user start DHCP service from our DHCP server, when other DHCP service is already active? That would only create huge mess.”
For ISP Routers being "smart" is quite unusual, these routers almost always on restart just enable their DHCP service no matter what – thus really promptly recovering home network, taking control back from Deco. Yours is only second time I hear on that forum that is “smart.”
As for standalone DHCP servers, those often come with “smart” functionality. Some allow to turn it off through configuration setting, some don’t. My Synology NAS came with basic DHCP server, I tested it: it is “smart” and there is no option to turn it off. Using it in combination with Deco mesh could cause the same mess on home network you’ve experienced.

Until recently Deco firmware did not have the option to turn off Smart DHCP feature. That caused very heated feedback from people who run their standalone DHCP server with same Smart DHCP functionality and bounce it. During the bounce, Main Deco takes over as DHCP server. After standalone DHCP server comes back, it sees Main Deco running as DHCP server and does not start its own DHCP service. Many hours of troubleshooting could follow…

Nowadays most of new Deco models have firmware that has an option to turn this feature off. For older Deco models where this feature can not be turned off there are workarounds. You can put ISP Router on UPS (Uninterruptible Power Supply) so that it’ll be on while Main Deco reboots during power outage. You can, if there is such configuration option, turn off Smart DHCP feature in standalone DHCP server you are running on your LAN, or in ISP Router.
If nothing else, the specific way of restarting Deco and ISP Router after power outage will recover your home network. I’ll provide restart steps in the next post I make to this forum topic. You may have guessed them already, but there are few gotchas I haven’t mentioned yet.

  @Alexandre. 

I assume that, in the scenario you describe, the ISP router detected another active DHCP router because the Deco forgot where was its WAN port, and acted as a DHCP server on that WAN port as well.  This would not occur in case only the ISP router is restarted.

I assume also that the Deco remained "red" because it is configured to receive a dynamic IP from the ISP router, hence will remain red until the ISP router acts as a DHCP server.  When configuring the Deco in AP mode, is there a way to configure it with a static IP configuration?  That would allow the Deco to communicate with Internet, become green, and stop its DHCP server.

yves_b wrote

  @Alexandre. 

I assume that, in the scenario you describe, the ISP router detected another active DHCP router because the Deco forgot where was its WAN port, and acted as a DHCP server on that WAN port as well.  This would not occur in case only the ISP router is restarted.

 

  @yves_b 

The trigger for the event is DHCP service disappearing from the LAN. That is what forces Main Deco to take over as DHCP server. WAN/LAN port autosensing could only contribute to failure of the recovery process and only under very specific cases.

For home networks where DHCP service runs on ISP router, it is enough to restart ISP router to trigger this issue. Every time you restart ISP router, with Deco mesh running in AP mode and Smart DHCP setting enabled, the issue will be triggered. For most people it'll be unnoticed, because after the ISP router restart the network will promptly return to its normal state.

If you would like to, I can provide you an example of what device on the network will see as a response to its DHCP query when ISP router is down but Main Deco has not been bounced, and when ISP router is down and Main Deco has been bounced. 

  @Alexandre. 
Ok understood. It makes sense. In fact I've got a Synology NAS 1813+ in my network. I will follow your instructions below for a right boot up if it happens again and turning off DHCP Smart from DECO wouldn't do the trick.

Best regards.

  @yves_b It's a timing problem. If you experience a power outage at home, all your devices will try to boot up at same time the power is restored. In my case the ISP router takes long to boot up. It's about 2 minutes. Weird but it is what it is. It's not broken. So in my scenario Deco starts faster. And that was the issue.
 

Cheers.

  @Alexandre. 

If the main Deco is not bounced, will it answer to DHCP requests from its WAN port?