The AirPort Extreme is a wi-fi/LAN router, and, although it is not a modem itself, it has the ability to establish a PPPoE connection via a modem. This is advantageous in that it lets the AirPort handle everything from the internet connection, to NAT to DHCP. The problem in New Zealand at least is that pretty much all our DSL connections are PPPoA not PPPoE. This is where the Draytek comes in.
Unlike the sort of modem your ISP probably sent you the Vigor 120 has the ability to carry out PPPoA to PPPoE passthrough when in modem mode. This means that the AirPort Extreme can use the Draytek to establish what it thinks is a PPPoE connection to the ISP.
The whole thing is surprisingly simple to setup. Only thing is, you may wish to have an alternate internet connection handy in case you manage to stuff things up and need to do some investigative work to get things going again.
Steps to setup are as follows:
1: Plug in your Draytek to power and DSL. Connect a network cable from the Draytek to your portable computer of choice.
2: By default the Draytek device lives at 192.168.0.1, so set your computer to have an IP address of anything else on the 192.168.0.x subnet, go to your internet browser and go to http://192.168.0.1
3: Hopefully you get a login prompt for username and password. Now the Draytek manual that came with by device stated that by default you should not enter anything in either field and just press ok. Usefully this isn't the case. I needed to enter "admin" as the username and leave the password field blank.
4: you should end up with a screen looking something like this:
5: Click on the "Internet Access" link on the left hand side, and then click on the "PPPoE / PPPoA" option. you should get a screen similar to below:
6: All you need to do is tick the little checkbox that is labeled "For Wired LAN" in the "PPPoE Pass Through" section. This configures the Draytek modem to operate in PPPoE passthrough mode and disables all other features (like NAT etc, which we are going to get the AirPort Extreme to do anyway).
7: You can now unplug the Draytek from your computer and plug it into the WAN port on the back of the AirPort.
Now we get into setting up the AirPort :-)
I'm assuming here that you're using the AirPort Utility on a Mac.
8: Open up the AirPort utility, select your AirPort and hit edit. You should get the screen below:
9: Hit the "Internet" tab at the top, you should get the screen below:
To set the AirPort up to handle your internet connection via the Draytek Modem you need to do the following:
Connect Using: Set this to "PPPoE"
Account Name: Your ISP account name
Account Password: Your ISP account password
Everything else can be left as-is.
10: Go to the "Network" tab and you'll get the following screen:
11: You should see that the Router Mode is set to "DHCP and NAT", if not you should set this. You can also configure additional options, DHCP reservations, port settings ,etc. but that isn't required at this point.
12: Hit "Update" and your AirPort will reboot to load the new config. When it comes back up you should be able to connect to it and see on the "Internet" tab that it has an ipv4 address listed (This is the external ip address of the AirPort now that it is connected to the internet.
At this point all should be working and your AirPort serving internet access to your client computers :-)
Hope this is of some use to people.