Do they support active passive failover. And can you just use one sensor and set it to fail open?
The is no statefull failover for the Cisco IPS Sensors. You can set the sensors to fail open, but only the 4260 has a hardware failopen capability. This means that the other sensors have to fail in such a way that they KNOW that they've failed in order to pass traffic around the sensor. In my experience this is not a reasonable assumption to make and you would be better serviced making other failopen arraingements with an eaternal switch.