@PeterGeelhoed 

The name "deco1" is statically linked to  the IP address of a Deco?
Does any local device use the Deco as DNS server?

  @PeterGeelhoed 

This is Internet connectivity health check. There is no actual traffic, just DNS queries. If you manage to block all DNS queries from Main Deco, the LED on it should switch to red.

I saw these sorts of health checks performed by other routers, so it must be common. Also note, Deco is a consumer grade WiFi mesh: not very many home users will notice or will care that Deco does its own DNS queries, which usually go to their ISP DNS servers.

Your suggestion to use just tp-link.com makes sense on the first glance, but as any simple solution it could bring its problems. If someone nukes that one domain, every Deco mesh worldwide may assume Internet is down. 

It is quite hard to down multiple domains Deco is using for Internet connectivity health check, even you haven't managed to do that in your LAN environment.

  @PeterGeelhoed I care about the internet, and I am satisfied that the LED of my Deco turns red when my mesh is disconnected from Internet.  That is one of the numerous advantages, for my domestic usage, of Deco versus a regular Access Point.

yves_b wrote

  @PeterGeelhoed I care about the internet, and I am satisfied that the LED of my Deco turns red when my mesh is disconnected from Internet.  That is one of the numerous advantages, for my domestic usage, of Deco versus a regular Access Point.

To add to this. At my house, ISP cable gateway is located in the far corner of the basement, while Deco nodes are at every floor and in my office. I find it convenient to see what is the Internet connectivity status by just looking at LED of Deco M5 - there is always one nearby. This is quite useful feature that I would not want to lose just because I switched Deco mesh from Router to Access Point mode.

We occasionally lose Internet, when it happens it is often for just a brief period of less than 5 minutes and then it comes back on its own. It helps me with member of my household who is not in IT profession: all I had to do is tell her "when light on the top of Deco is red - it means Internet is down and you can't watch cartoons on your tablet until it is green again."