My understanding is that with the modem in bridge mode, it is just a passive device, not doing anything including authentication.
This whole problem arose because I needed a block of IPs that need to work through a PIX. The sent me this Zyxel modem/NAT router which only has one addressable interface. So we put it in bridging mode, but the PIX does PPoE, not PPoA, so I bought the 1721 router that is supposed to do PPoA. Now I know that they DO have the ADSL WIC for the 1721, but Cisco's docs on the 1721 show it able to be configured either AS the DSL modem (with ADSL WIC) ir behind the modem (Ethernet WIC).
I can't help but think I am somehow overcomplicating this.
Thanks,
-Al
Al