ASA/PIX only support PPPoE as a PPP negotiation method. PPPoA and PPPoE have different methods of negotiating and to my knowledge there is no way to make them tunnel/work in unison from a bridged Cisco router to the Cisco ASA running PPPoE.
Background here is that today I was asked to configure a Cisco 887VA router and Cisco ASA 5510 together so that the ASA would have the public IP of the internet service on it's outside interface. This would mean running the 887VA in bridged mode (which is possible) and running PPP on the ASA 5510. This scenario is fine provided the ISP supports PPPoE for the internet service. If the ISP does PPPoA only... then forget it because the ASA only suppports PPPoE. The reasons why are that PPPoA requires specific ATM drivers/interactions which the ASA will never support (it has to do with ATM being CELL based technology vs packet-based). PPPoE is designed to be encapsulated by Ethernet (i.e. it is more abstracted from the medium used then PPPoA).
This post helped me understand the PPPoE/PPPoA a lot better: http://www.petri.co.il/forums/showthread.php?t=1728
Looks like I'm stuck configuring double-NAT for this one... oh well. If anyone knows better let me know. I tried this in my lab and just couldn't find a way to make this work.
