Hello
Since you see, both routers (nano and linksys) do not want to negotiate. Some suggestion?
regards:
pd: win/linux pppoe clients auth okay-. 
Check encryption configuration on both ends.
Make sure they are matching one another and there is not different policy on them.
im try to disable encrypt on concentrator… im try to default, pap,chap,mschap1 and2 , … try only with mschap1 , mschap2… but, it not work.
Regards