Usually it is quite straightforward to configure a modem in bridge mode. However, this one turned out to be a bit non-intuitive (as far as I am concerned).
The manual for this modem shows you how to setup bridging. However, my modem didn't show the same menu's as are in the manual. As it turns out the modem is delivered with the 3.11 TUE1 range of firmware. This is seems to be a specific firmware that is being managed by the network provider. It is also not possible to flash the modem with the firmware available from the Zyxel site (which has newer firmware versions).
So I tried to setup bridging for this firmware. Initially it all seems logical. You go to the Network Settings menu and select Broadband. You are presented with the following screen.
So naively I selected my ADSL interface and put it in bridging mode.
But that is not enough. In stead of having to add your local network to also do bridging, you have to configure the Ether interface in the Broadband menu (!!) and also set it to bridging. In the manual it says this interface it the WAN interface on the modem and is used to connect an external modem. As it turns out in this firmware it is the local 4 port switch part of the modem. When also set to bridging you can now request a dhcp lease through one of the 4 ports and get a lease with a public IP adres from the provider.
You do need to disable the rest of the functionality in the router. But even when you disable all functions, I still kept running into problems with VOIP. I have an asterisk setup and it wasn't able to connect to any of my providers. It looks like the Zyxel is intercepting VOIP traffic even though it is disabled on the router. As soon as I switched the Zyxel for another modem in bridging mode, everything worked fine with Asterisk.
A nice feature of the Zyxel is the statistics it displays on the ADSL line. It shows all the tone bands and the number of bits it is encoding per band (for more info on ADSL look at this page: adsl technology it is quite useful).
So with a bit of scripting you can get the ADSL line data out of the modem and store it in a SQLite database. Using GNUplot you can even get some nifty 3D graphs of the line quality over a period of time.
The script is pretty simple, you can download it through the link below.
Don't forget to fill in the IP address of the modem and the user name and password (yeah, it isn't very secure, I know, improve it if you want).