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I faced a nasty problem after enabling IPv6 in a LAN with two subnetworks (main and guest networks, splited in two VLANs).
I did use stateless IPv6 configuration on both subnetworks, advertised by a Mikrotik router.
After doing that, i had a Windows 10 PC where i got two IPv6 addresses, one for each subnetwork !. Initially i thought that there was an ICMPv6 leaking (RA leaking) between both VLANs, or a leak in the Router OS advertisements. But after checking that, this was not the case.
After scratching my head, i did finally find the culprit : Windows 10 was listening for tagged traffic from the guest network that was available in the cable to this PC machine. The Ethernet adapter was setup without specific VLAN configuration, normally only listening for untagged traffic of the main subnetwork.
Disabling the guest tagged VLAN in the switch for the port of this PC did solved the problem. No more dual IPv6 addresses !!!
This mean that Windows was listening for all ICMPv6 traffic, not only ICMPv6 from the untagged VLAN, but ICMPv6 from all tagged VLANs available in the Ethernet cable !
I report this here because it seems to me that this is a security issue. Alternatively, when a PC get in error two or more IPv6 addresses from different subnetworks, this is triggering IPv6 connectivity problems specially inside web browsers that could revert to IPv4 (IPv4 fallback) because of that.
The solution is to check that switch ports going to IPv6 enabled PCs never have more enabled VLANs than the untagged VLAN of the desired subnetwork (access port only).