But I use it in full bridged mode and it works a treat.

I have my pfsense firewall configured to use pppoe with my adsl username etc. I was getting adsl from enta in the UK if that helps anyone wanting to use pppoe with the zyxel 660r in bridged mode.

I am waiting for BT to move the wires in my exchange tomorrow and then I will be configuring the 660r in full bridge mode with O2 adsl2+ Annex M so I will report back if there are any memory issues using full bridge rather than half bridge.

I would recommend to anybody who is considering buying a ZyXEL ethernet modem to achieve this kind of setup to contact the ZyXEL UK technical support first – they were very helpful when I contacted them, and actually mirrored my initial connection with an ADSL2+ line set up in their offices, in order to test and ensure that the specific model I had was actually fully compatible (and they then offered advice and the correct commands to enter when it was confirmed the modem was compatible).

I trawled all over the net as a matter of curiosity to see whether other ZyXEL devices can achieve half bridge mode, and it seems that not many are. However, I can almost unreservedly recommend the P660R-D1, it’s a cracking little device which puts out very little heat and has a tiny footprint as well. You can scuttle it away in the smallest of corners and, quite literally, set and forget. It wasn’t my first choice of modem (I would’ve preferred to have bought a Draytek) but budgetary restraints required me to go with the cheaper option – and as far as I can tell, since I installed it, it’s worked without a bit of fuss.

The single caveat I read about during my research into the device is that if you’re running in ADSL2+ mode in Annex M, the device tends to ‘choke up’ quite quickly. It runs almost all the time with a good portion of its internal memory already used (74% or so from memory, pun unintended) – so if you’re a heavy P2P user or someone who is going to be seriously thrashing their ADSL2+ connection, you may wish to consider alternative devices alongside the ZyXEL.

If space and heat output isn’t a concern, you can get a Sky-branded Netgear DG834T for a few quid from eBay, and it’s relatively trivial to flash it with something like the UberGT or DGTeam firwmare to unlock its full potential. That device performs very well on LLU ADSL2+ ISPs such as Be* (due to the Broadcom chipset used, which is very compatible with the chipsets in the DSLAMs). However, for the circumstances my setup was situated in, the ZyXEL filled the requirement very nicely, and with no fuss at all, once I’d figured out the half bridge mode!