I seem to remember seeing a good diagram somewhere that shows the protocol stacks for each encapsulation method and helps to explain the modem/bridge mode - I'll have to have a look around and see if I can find it again.
As for PPPoE relay, unfortunately it's simply the modem mode PPPoE ----> PPPoE and nothing more which is a shame because PPPoA ----> PPPoE would be very useful indeed.
I have a Draytek Vigor 110 and this can act as a pure PPPoE ----> PPPoA bridge and it does it very well indeed. I believe the ADSL Nation X-Modem M3 is another device with this facility.
I bought the Draytek because I use a Linksys WRT54GL running Tomato and needed the PPPoE to PPPoA bridge on ADSL Max, however after a while I decided to play around with a Netgear and found that it connected at a better rate. The only problem is the lack of PPPoE to PPPoA bridging, so what I did was simply set the encapsulation of the Netgear to PPPoE (which is really PPPoEoA) and it worked well for me, albeit with a slightly reduced throughput vs PPPoA due to the larger overheads with PPPoE. I'm with Be now and use ETHoA (IPoATM) and bridging the BeBox to the Linksys with a static IP.
The point-to-point tunnelling feature (PPTP ----> PPPoA) of the Speedtouch was something I also considered when intially figuring out how to use a 'cable' router with ADSL but I didn't go down that road because I was finding that trying to configure and play around with a speedtouch required numberous reboots which isn't really what you want on a longish line with a sensitive profile, but it apparently does work and Revs Per Min on the whirlpool forums seems to be a very good source of knowledge for the more advanced features of speedtouch routers.
I'm not sure how (and if) the PPTP works with Netgears - I think that it's somehow tied up with VPN settings.
Edit: Found image, originally posted on DSLReports forum
