Ah, now that's where things get complex.

I am using a 6086N (http://www.gctglobal.com/stb6086.html)

It's been hacked to have a hard disk and the memory has been maxed out.
I have also installed another Realtek RTL-8139 ethernet as well, making 3 interfaces.

It's running Fedora 12 and "rp-pppoe" as the PPPoE engine.

One ethernet links to the V120, one to a DMZ and one to an internal "secured" network, all using public IP address blocks.

IPv6 "just works" with a small amount of address allocation on the two internal interfaces to give them different IPv6 subnets.

Sorry, probably not a huge amount of help really.