I believe that this problem is caused by Rachio using the BROADCAST flag in its DHCPDISCOVER (and DHCPREQUEST) packets. This requests the DHCP server to send the reply as a broadcast packet. It would normally be used by older devices that are incapable of receiving unicast packets before the protocol stack has been set up. IMO, it’s very unlikely that Rachio is one of them; if they simply turned the bit off (in new firmware releases) the problem would be permanently fixed. However, if their hardware really does have this limitation, we need to do something at the customer end. See
or see https://tools.ietf.org/html/rfc2131 for the gory details.
Broadcast packets are expensive on Wi-Fi for several reasons. Among others, they must be sent from all APs on the LAN, as well as at a low speed. There is some useful detail here:
Some APs ‘handle’ the issue with a ‘block LAN to WLAN broadcast’ option, which is on by default. If that’s your case (the controller eventually hears the reply from a more distant AP), turning this off should fix your problem.
I’m not familiar with EdgeOS, but mapping the Rachio to a static IP may cause it to issue an indefinite or much longer lease time, which may mitigate or eliminate the trouble.
If neither of the above are applicable, and assuming that you have far fewer devices than the subnet allows for, i.e. you have no need to reuse LAN addresses, consider setting the DHCP lease time to e.g. 90 days (and overriding any shorter time that the device requests). I’d expect this to greatly reduce connectivity loss, unless the requests you are seeing are a result of having lost association / authentication at the Wi-Fi level, rather than the lease having run out.