UniFi L3 Routing: Nice Feature, Where Did My IPv6 Go?

I moved one test VLAN from my UDM Pro to a USW Pro Max 16 PoE. IPv4 worked. IPv6 had disappeared from the VLAN’s configuration.

The network was working perfectly well before I touched it. In my defense, it was called Testing.

Because I can

In October 2026, I had the UDM Pro routing my home network again, with the Pro Max handling switching. I’d already written about using the UDM Pro at the edge of my homelab, and I wanted to give UniFi another go. I had an L3-capable switch, a spare VLAN, and some time to see what happened when I moved the gateway.

In UniFi Network, I opened the Testing network, VLAN 2 (192.168.4.0/24), and changed its router from the UDM Pro to the switch. Or tried to. The iOS app said, “Something went wrong. This item cannot be edited.” I made the same change in the web interface and it went through.

I connected to the test VLAN. IPv4 worked. Nice.

Goodbye, IPv6

The web interface warned me that changing the router would remove the network’s IPv6 configuration. It did exactly that. Once the switch owned the VLAN’s IPv4 gateway, UniFi no longer configured an IPv6 gateway or sent IPv6 Router Advertisements for that VLAN.

For a moment, the network overview still showed the old IPv6 prefix. I thought maybe the UDM was still doing something for IPv6. The overview was showing stale information.

The switch could still forward IPv6 frames at Layer 2. What I lost in this setup was the UniFi-managed IPv6 gateway and the Router Advertisements that clients had been getting from the UDM. A working IPv4 connection and an IPv6 prefix still visible in the controller were a pretty convincing impression of a healthy VLAN.

Why this bothered me

I like UniFi. The topology view is useful, and having my APs, cameras, switches, and gateway in one place makes it easy to see what’s going on. It also makes an experiment like this look like a simple dropdown change. I wanted to move the VLAN gateway and keep both address families working.

I run AS215855 for fun, so of course I care about IPv6. But you shouldn’t need your own ASN to expect a dual-stack VLAN to stay dual stack when you move its gateway.

Of course I opened eBay

Within an hour I was looking at a MikroTik CRS310. If you’ve read about the RouterOS upgrade that knocked my Comcast connection offline, you already know how much time a surprise involving Router Advertisements can cost me on that side, too. That one looked like a DHCP problem for longer than I care to admit.

After this test, I decided to leave the dual-stack VLANs on the UDM Pro. The Pro Max could keep switching traffic and powering the APs. That was already working when I woke up.

Anyway. I didn’t buy the CRS310. Yet.