CISCO AFFIRMS SAFETY OF LIEBERT UPS/ROUTER INTERFACE
Voicemail software in Cisco routers requires a graceful shutdown in the event an
outage exceeds UPS battery capacity. Liebert UPSs use a different approach to signal
a low battery condition to the router than competitive units, which may require a
custom interface to prevent damage to the router. Only Liebert’s approach has been
documented as safe by Cisco.
Cisco Unity Express voicemail is basically a server that needs to be gracefully shutdown in the event
of a power failure to avoid the possibility of file system corruption. When the router senses a signal
on the appropriate pin of its AUX port it will issue an autocommand to gracefully shutdown the
CUE module and cleanly dismount the file system.
Liebert UPSs signal a low battery condition to the router in a way that is subtly, yet significantly,
different than competitive units. When the Liebert UPS senses a low battery condition it closes a
set of contacts across two pins in the RS232 cable. It receives a voltage from the router on one pin
and sends the same voltage back to the router on another pin, eliminating the possibility that a
surge from the UPS power supply could damage the router.
In contrast, competitive units supply a voltage from the power supply in the UPS to the router. This
creates the possibility that UPS failure damages the router.
Cisco engineers have tested and approved the interface between a Liebert UPS and its routers.
This interface was documented in the Liebert UPS/Cisco Unity Express Shutdown Application Note
published by Cisco and is reproduced on the following pages.
According to Cisco, there is not currently a safe interface between Cisco routers with Cisco
Unity Express software and competitive UPSs. Cisco engineers recommend an as-yet-undeveloped
custom cable assembly be used, which includes varistors or zener diodes to limit the voltage
applied to the RS232 port, when interfacing competitive units to