Started looking for info on this topic and this thread was the most helpful so I thought I'd update it with some info I learned today with some help from and AT&T tech.
I was trying to add a Cisco E3000 (aka Linksys) router to my home network where a 2WIRE 1701HG gateway was already was already handling my internet connection.
I switched the one computer I had cabled from the 2WIRE to the E3000 and ran a 2nd cable from the E3000 to the 2Wire gateway. I let the Connections software do its thing. Then the problems started. I couldn't access the internet. The 2WIRE would redirectly me to an internal page saying it had detected a "router behind a router" condition and asked if I wanted to resolve it. Basically it said it was going to move the E3000 to the DMZ Plus zone and "share" the one internet address it was getting from AT&T. I didn't like that idea.
So, I figured that part of the problem was that the E3000 had defaulted to having NAT on. So I turned that off. At this point I ran out of time so I pulled out the E3000 until I could do more research. I returned my home network to it's original condition. It wouldn't let me access the internet. I rebooted the 2Wire by pulling the power plug for 10 secs. I rebooted the computer now cabled directly to the 2Wire. The 2WIRE kept complaining (paraphasing here) "Although your connection seems to be fine now, you have a cache problem if you're seeing this page. Please reboot your browser to resolve the problem"
I repaired the connection so it would dump the associated caches. Rebooted everything again -full power down and restart. No go.
So I called AT&T tech support and they were very helpful.
Turned out I need to do a factory reset on my 2WIRE. Using a pen, hold the reset button in on the back of the unit for 30 secs. Now log back in and redo your settings. For me this was: Under Broadband catagory, set connection type to PPPoE, enter email address as login, enter pwd and confirm it, then save.
I was good to go.
The tech gave me some helpful advice I'm going to try out tonight. First, there's a way to see all the settings in the 2WIRE without having to tab around through it's interface. Enter the main 2WIRE setup address into your browser and add /mdc like this:
http://192.168.1.254/mdc and you will see the Management and Diagnostic Console. Now there's a list of links on the left side. If you go down to the Advanced - Configure Services you will see an option to turn off the router portion of the gateway and make it into a "dumb" modem. The instructions at the top of the page tell how to reconnect to the 2WIRE once it's in this mode, but the tech told me that I'd need to do a factory reset to get the gateway back into "smart" mode.
After that, I'd need to go into my E3000 and configure it to be my primary router. I'll have to see if I can find a menu to allow me to: set PPPoE, and set my login/pwd. I'll have to remember to turn NAT back on.
Oh, one warning. The E3000 has a guest account option that will allow guests to access the internet wirelessly but not connect or use anything else on your personal network. I'd put a new pwd on this and thought I was good. But when I viewed my wireless network list from my notebook, the guest account was not only showing as unsecured, it was not pwd'ed, and I could access other computers shared info on my network. So until I research this further, the guest account feature is OFF.