Why is my 2021 Apple TV 4K sending ICMPv6 router advertisements?
I'm having the same symptoms found in the following HomePod mini thread after adding a 2021 Apple TV 4K to my network:
https://discussions.apple.com/thread/252367920
I have two original (2017) Apple TV 4Ks on my network and just added a 2021 Apple TV 4K. About the same time I connected the 2021, I started getting IPv6 ULAs (with an fdb4: prefix) followed by "No Internet" messages on both wired and wireless clients, both Mac and Windows. This IPv6 address assignment would coincide with those devices dropping their DHCP-assigned IPv4 addresses and usually eventually assigning themselves 169.254.x.x IPv4 addresses. I ran a WireShark packet capture on my iMac ethernet interface as I plugged it in (with iMac Wi-Fi off), and I found the culprit was the 2021 Apple TV 4K.
It was initially difficult to diagnose as my 2021 Apple TV is connected wirelessly only, so I wouldn't see the issue when I took down the APs. No other device is doing this, including the two 2017 Apple TVs. My router doesn't issue RAs; I do not have IPv6 set up on the router for the WAN (even though my ISP supports it) or LAN.
I'm going to try to put a support ticket in, but I have no idea how long it's going to take to find a support representative who understands the problem, let alone is knowledgeable enough on IPv6 to submit a meaningful ticket to engineering. It's breaking connectivity with many of my devices.
I decided not to do any rules to block those messages in case doing so would break HomeKit or something else. I've got the 2021 unplugged for now until I find out more about what's going on.
I see no good reason why Apple TVs should send router advertisement messages! :-D (H/T: R. Hamilton)