The router and the extender sometimes assign the same IP address to 2 devices

[LGH50]

Extended network using Nighthawk RAX43 router and EAX 15 extender.

The combined network performs well for days - weeks, but then connections become unstable.

Reason is, the main router and the extender have assigned the same IP adress to two different devices. 

I tested in both mesh mode and access point mode. The issue seems to occur when a device that has been offline for a while is reconnected. It may then get the same IP address as it had before. In the meantime, that address has been assigned to another device. So, now we have two devices with the same IP address - one talking to the main router, and the other to the extender.

Why does this happen, and what can I do to avoid?

A google search shows I am not alone, but I have not seen a way (setting) to make the system keep better track on available IP adresses. .

[plemans · Accepted solution]

Not a whole lot 🙂

Just set the extender and the 2x devices with issues up as reserved through the primary router.

One other thing to do. If you have mac's/iphones on your network disabled private mac/ip address and icloud relay. 

They can cause different ip addresses to be assigned. there should be plenty but maybe over a couple days if you have multiples, its causing issues

[plemans]

Try going into the primary router and setting those devices with a reserved ip address

[LGH50]

Thanks for your suggestion.  Since we ar a big family, with mobile devices migrating all the time, that may not be a practical solution. 
I keep wondering what it is that confuses the main router, such that it does not keep proper track of assigned addresses. 
At one time, it had actually given the same address to the extender itself (!) as the extender had already assigned to another device. 
is the reality that low cost mesh systems simply are not up to the task - one needs corporate grade hardware to build a stable network?

[plemans]

I'm not saying set them all with reserved, just the couple with issues

[LGH50]

Would you say the extender(s) should be the first one(s) with fixed address(es), selected near the high end of the range. It may be hard for the main router to know the address register of an extender that has been (temporarily) off line. The main router then re-connects the extender with the IP address it had before, and does not check what the extender has been doing in the meantime. 
Does this make sense?

[plemans · Accepted solution]

Not a whole lot 🙂

Just set the extender and the 2x devices with issues up as reserved through the primary router.

One other thing to do. If you have mac's/iphones on your network disabled private mac/ip address and icloud relay. 

They can cause different ip addresses to be assigned. there should be plenty but maybe over a couple days if you have multiples, its causing issues