Set up Sky Fibre Broadband with Unifi USG

I recently moved from eir to Sky for TV, phone and broadband. My old eir modem supported bridging which allowed it play nice with my USG and the rest of the Unifi network. Unfortunately, the Sky modem does not support bridging. The usual advice online is to eliminate the Sky hardware and add a dedicated modem such as the Draytek V120. This is not an option for me as I'm also subscribing to Sky Talk, and that passes through the Sky modem.

That leaves only one option: configure the Sky modem with DMZ pointing to the USG WAN IP address. The downside is that this setup introduces double NAT which may impact some gaming setups, port forwarding etc., but it's the only way I could get interoperability...

The key here is to configure the Sky LAN and USG WAN with IPs on the same subnet, and the USG LAN, (and all devices behind the USG), on a different subnet.

I also elected to maintain DHCP services on my Unifi network, so switched off DHCP on the Sky modem. As I have a few Unifi Access Points, I also switched off all WiFi services on the Sky modem.

Here's how the the LAN settings on the Sky modem are configured. Here I use 192.168.0.1 as the IP address of unit. Note that DHCP and IP6 are also off;

And here's the Sky WAN setup. The DMZ address is set, and configured to match the IP set on the USG WAN (see below).

Looking in Unifi, I have LAN and WAN networks;

The LAN is set up on a different subnet, 192.168.1.x;

The the WAN assigns a static configuration, specifying the IP to match that set in the Sky DMZ field;

(the highlighted icon on the USG info panel here should turn green when everything is set up - I had some kind of cabling problem which is why mine was grey when I snapped this image).

Speaking of cables, the configuration is a phone cable from the incoming phone line termination box to the Sky router as per Sky setup instructions. Then I run an RJ45 from the Sky modem Port 1 to the WAN 1 port on USG. Another RJ45 runs from USG LAN to a network switch.

When this is all set up, there is no access to the Sky admin interface from within the network. To get to it, both during and after setup, it's necessary to connect a computer / laptop to one of the Sky modem LAN ports and access it on 192.168.0.1, having of course configured the access systems network settings to be on the same subnet.

So far, so good. No specific issue have arisen with double NAT, but I'm sure some will be uncovered in the future. 

Maybe someday Sky will permit bridging, or I'll drop the talk service and go for a separate modem. Until then, this setup will suffice.