u/bubeli

Hi everyone,
I’m trying to understand some IPv6 behavior in a mixed Swisscom/UniFi network.
This is NOT an IPv6 connectivity problem. IPv6 itself works perfectly. What I’m trying to understand is how UniFi handles IPv6 on the management plane and how UniFi Network learns and reports IPv6 addresses when the gateway is not a UniFi device.
My setup is:
Swisscom Internet-Box 5 Pro, 10 Gbit/s FTTH, acting as IPv4/IPv6 router and firewall.
Connected to it is a UniFi Flex XG running firmware 7.5.10.
Behind the Flex XG I have a UniFi Flex 2.5G 8 running firmware 7.4.2.
The Flex 2.5G 8 has, among other devices, a QNAP TS-264 connected through a 2 x 2.5 GbE ALB trunk.
I also have Mac Studios connected both directly to the Flex XG and behind the Flex 2.5G.
The UniFi devices are managed by a CloudKey Plus running UniFi Network 10.5.67.
There is NO UniFi gateway in the network. Routing, DHCP, IPv6 Router Advertisements, firewalling, etc. are all handled by the Swisscom Internet-Box 5 Pro.
The LAN currently uses the public IPv6 prefix:
2a02:1210:6614:5900::/64
IPv6 itself definitely works
Macs receive both global IPv6 addresses and link-local addresses correctly.
My QNAP TS-264 is explicitly configured for IPv6 Auto-Configuration (Stateless/SLAAC) and receives:
2a02:1210:6614:5900:265e:beff:fe83:9c38/64
I can ping6 that address from a Mac with 0% packet loss.
I can also open the QNAP web interface directly using its IPv6 address.
The important point is that the QNAP is physically behind the Flex 2.5G 8, which is itself behind the Flex XG.
I also ran an Internet speed test against Swisscom’s IPv6 server and obtained approximately 8.0 Gbit/s download and 7.6–7.8 Gbit/s upload, with 0% packet loss.
So IPv6 forwarding through both UniFi switches is unquestionably working correctly.
Problem/question 1: UniFi does not show IPv6 addresses for wired clients
In UniFi Network, under Client Devices, I can enable two columns:
IPv6 Address
IPv6 Link Local
However, both columns show “-” for my wired clients.
For example, I have a Mac Studio connected DIRECTLY to the Flex XG at 10 GbE.
UniFi sees the Mac correctly, including its IPv4 address, but shows “-” under both IPv6 columns.
The Swisscom Internet-Box, however, correctly shows both the Mac’s global IPv6 address and its link-local IPv6 address.
The exact same thing happens with another Mac located behind the Flex 2.5G 8.
Therefore this cannot be caused by the Flex 2.5G being downstream from the Flex XG.
My main question here is whether this is expected when UniFi Network is used with a third-party IPv6 gateway.
Does UniFi normally obtain the IPv6-to-client association from the NDP/Neighbor table of a UniFi gateway?
If there is no UniFi gateway, is it expected that the IPv6 Address and IPv6 Link Local columns remain empty?
Should the UniFi switches themselves be capable of learning and reporting those IPv6 addresses, or does this information fundamentally depend on the gateway?
Problem/question 2: Flex XG has a link-local IPv6 address but apparently no GUA
The Flex XG is managed via IPv4 at:
192.168.1.11
UniFi also reports this IPv6 link-local address for the Flex XG:
fe80::265a:4cff:fe1b:59d
However, neither UniFi nor the Swisscom router shows any global 2a02:… IPv6 address for the switch.
This is interesting because Macs, the QNAP and even the CloudKey Plus on the exact same LAN all obtain global IPv6 addresses without any problem.
I found the following official Ubiquiti release-note entry for UniFi Switch 7.2.123:
[USW-Flex-XG] Fixed a device crash issue while requesting a DHCPv6 address.
So we know that, at least under some circumstances, the Flex XG has DHCPv6 functionality capable of requesting an IPv6 address.
What I have NOT been able to find is reliable documentation explaining exactly how IPv6 addressing of the Flex XG management interface works.
Does the USW-Flex-XG use SLAAC for its management GUA?
Does it use DHCPv6?
Does it support both?
If DHCPv6 is used, does it specifically use IA_NA?
Does its behavior change depending on whether the gateway is a UniFi gateway or a third-party router?
Is it normal for a Flex XG to have only a link-local IPv6 address and no global IPv6 management address?
Is there any way to configure SLAAC, DHCPv6 or a static IPv6 address for switch management?
I have found contradictory claims about this online, so I would be especially interested in first-hand experience, packet captures, SSH/debug-console output or actual Ubiquiti documentation.
Importantly, I do NOT want to assume that “the Flex XG requests a DHCPv6 address” means that it ONLY supports DHCPv6 or that it does not support SLAAC. The release note alone does not prove that.
Problem/question 3: Flex 2.5G 8
The Flex 2.5G 8 is managed via IPv4 at:
192.168.1.12
Unlike the Flex XG, UniFi does not even show an IPv6 Link Local field for this switch.
Yet IPv6 forwarding through it works perfectly, as demonstrated by the QNAP and other devices behind it.
Does anyone know whether the USW-Flex-2.5G-8 currently supports IPv6 on its own management plane?
Does it support SLAAC?
Does it support DHCPv6?
Should it at least have a link-local IPv6 address?
Is the lack of IPv6 information simply a firmware/UniFi Network reporting limitation?
Is IPv6 management implemented differently on the Flex 2.5G family compared with the Flex XG?
What I am NOT trying to fix
I do not actually need to manage the switches over IPv6. Managing them over IPv4 is perfectly fine for me.
I also do not want to modify a working IPv6 configuration just to make an address appear in UniFi.
IPv6 forwarding already works perfectly end-to-end.
What I am trying to understand is why the situation looks like this:
Wired clients use IPv6 perfectly and the Swisscom router sees their IPv6 addresses, but UniFi shows “-”.
The Flex XG has a link-local IPv6 address, but apparently no visible global IPv6 address.
The Flex 2.5G 8 forwards IPv6 perfectly, but UniFi shows no IPv6 management information for it.
Meanwhile the CloudKey Plus, Macs and QNAP all receive normal global IPv6 addresses on the same LAN.
If this is simply a limitation of UniFi management/reporting when using a third-party IPv6 gateway, that is perfectly fine. I would just like to understand the actual technical mechanism rather than start changing a network that already works correctly.
Any experience with Flex XG, Flex 2.5G, UniFi Network without a UniFi gateway, SLAAC/DHCPv6 switch management, or NDP-based client discovery would be very welcome.