You should take a look at our Proxy & Roaming RADIUS Server (PRRS). In its
proxy mode, the PRRS is simply a packet dispatcher between the NAS'es and
the RADIUS server, caching every reply received from the RADIUS server in
the process. If it goes down, the PRRS replies from its cache and if the
user is not in the cache, the PRRS can also build a "default" authentication
reply, based on the business rules configured in the PRRS.
While the RADIUS server is down, the PRRS acknowledges accounting packets
and stores them in a chronological queue until the RADIUS server returns, at
which point they are forwarded. This way, ISPs using online time to do
billing don't lose revenues this way.
The PRRS also has a watchdog engine that monitors its state and restarts the
service if it fails, so it is very unlikely the PRRS will fail to reply for
a period longer than 20 seconds.
This redundancy has a few advantages over using the "secondary server"
method.
You can see more details on the product at http://www.vircom.com/prrs/