A Netonix switch. Single best investment I made for my WISP.
I set them to ping a device on the opposite tower (in a PtP link) so that if the radio (on this side) hangs (but still responds to pings) it won't fool the watchdog into thinking everything is fine. The monitored device must actually pass traffic so that a ping packet can travel to the opposite side, be answered, and then return to the Netonix, thereby resetting the periodic timer.