The Supervisor Engine III hardware allows backplanes in the Catalyst switches to operate independently. The Supervisor Engine III boots differently from the earlier supervisor engines. The Supervisor Engine III boot process uses two images: ROM monitor and Catalyst family supervisor engine software. The ROM monitor is the first software to run when the switch is powered up or reset. Based on how the ROM monitor has been configured, the switch either stays in ROM monitor mode or loads the supervisor engine software. A configuration register allows you to configure the ROM monitor, and the BOOT environment variable allows you to specify the location and filename of images to boot.