freeradius failover-through proxy or other way?

[Martin Lambev]

I'm really beginner in freeradius realm, and in advance sorry if the 
question is immature...

After rading all wiki - freeradius, still is not clear to me, is it 
possible to do failover-through proxy, and how to organize the things 
that I want to accomplish. Explanation fallow:

Now I have the fallowing setup: node 1 - NAS (pptp, openvpn) -> server 2 
(freeradius + mysql as backed)
I red in documentation about 2 or 3 mysql db and how to do fail-over, 
load-balancing and redundancy, but If I do it like that when freeradius 
server fail, the whole setup is down.

I want to add another node as second NAS so the things will become like 
this:


node 1 - NAS (pptp, openvpn) -> server 2 AAA (freeradius+mysql)
node 3 - NAS (  l2tp) -------------^

I want to have redundancy in case server 2 AAA (freeradius + mysql as 
backend) fail, second server 4 AAA to take over with exactly the same 
setup (freeradius + mysql backend).. Should I use freeradius proxy on 
every node??? other solution? So the thigs needs to become like this:

node 1 - NAS (+freeradius proxy?)--|  Internet  |---server 2 master 
(freeradius+mysql, location ex.US ) node 3 - NAS (+freeradius 
proxy?)--|  Internet  |---server 4 slave (freeradius+mysql, location ex.EU )

I want to have mysql db to be updated (to have mirror copy) on booth 
server 2,4 in real time. The purpose of this set up is redundancy  if 
one of the AAA server is down the other one to take over without impact 
over node 1,3 ( temporary user disconnect is acceptable )

Or may be there is other way to do so?

Any advices are welcomed, correction or hints.... anything that can help 
me "see" better :)

Best Regards,

Martin