Stephane,
The good news is that the config for your local 800 is pretty straightforward. You don't have to add any AAA or radius commands to the 800, since only the Shiva talks directly to the radius server.
For this example, we will assume that your radius username is george, and the radius password is apple. On the 800, if you have a dialer interface, you would add these commands there. If you have no dialer interface in the config, then you would instead add these commands to the bri interface:
ppp authentication pap ms-chap chap
ppp chap hostname george
ppp chap password apple
ppp pap sent-username george password apple
That should do the trick. If you happen to have the username and password that the Shiva router will use to identify itself, then you should add this to the 800 config as well. In this example we will say the Shiva will identify itself with hostname/username jack and password jill. At global config in the 800, you would add:
username jack password jill
In order to avoid some really interesting authentication issues, you should add your own username and password in the same way, also at global config:
username george password apple
You may not need to enter either of those "username xxx password yyy" statements into the config for this to work; it depends on exactly how it is implemented. If you do not see a successful connection, on your 800 run the debug of "debug ppp authentication." This will tell you if you are failing authentication specifically.
Good luck!
