there is no "buffers to the real".
So not sure what your concern is.
The CSS normally uses probes to detect if a server is available, and if not another one from the list is being used.
What you can do is create a service pointing to the default gateway - the service will be configured as transparent-cache, and will be used as a sorryserver under the content rule.
Like this, when all your servers are down, the CSS will just forward the packets to the default gateway.
Gilles.
Gilles.