Alternative Routing

Hi,

I have session-groups containing 4 session agents each.

Each session-group is in a different realm which sits on a different network-interface (actually clan).

I want the system to route as follows:

Try to route the call through any one the session-agents of session-group no.1.

If they all fail try to route through any one the session-agents of session-group no.2,

If they all fail try to route through any one the session-agents of session-group no.3.

What would be the correct/best way to configure the acme to achieve the above ?

Cheers,

Dan