Cisco 3662 w/ (1) T-1 WIC, multiple FE and E interfaces. Continuous uptime of 1.5 years until this morning around 8:30 AM. The "System" LED on the front and back of the chassis was blinking, indicating that the router was in ROMMON mode. I did not connect via serial to confirm this. A power-cycle of the router brought it right back up. My question is, what causes a router to be in working mode, and suddenly switch to ROMMON? Can a command be issued from the console to do that? The config regsiter is 2102, so if it rebooted itself it should have attempted to load the saved/start-up config. This unit is connected to a very large UPS. I look at other PC's on the same UPS and they were fine. Could a burp (switch to UPS from Main AC) cause the router to reboot, and for some reason not load its startup-config? Thanks.

While 1.5 years sounds good, it isn't really that for your model. Since you didn't say you were using the cisco ups I assume you are using another model or system. All good ups's do a diag and when the battery gets low it could cause power to drop just below what this system needed and yet pass self test. I'd check the batteries on the ups but that would be a wild guess. It could be someone logged in or any number of other things to include RF or other transmissions or even esd unless fiber. I'd just monitor it if your batteries are OK or set the low and hi limit of the ups closer to line. I read it wrong and answer it wrong too. So get off my case you goober.

Received confirmation today that we lost AC over the weekend and the UPS switched to batteries. From there I am not sure what happened. I feel a little bit better knowing that it was an AC problem rather than a router component.