Maybe the operating system was erased, became corrupted, maybe it never existed in the first place, or maybe it can’t load it for some reason. But my conclusion, normally, when I see a device where I’m consoled in, I find myself in the ROMMON mode, my conclusion is, I wasn’t able to load an operating system. So we could have told the device to boot to the ROM monitor mode by setting the boot field to 0. The way that I think about it is, we jump from, “Hey, I can’t find an operating system locally and I haven’t prepared a TFTP server for it.” Then we boot to ROM monitor mode. And then chances are, it’s going to go to ROM monitor mode, okay. There are plenty of white papers on Cisco, I’m sure they’ve got it pretty well documented. We will not teach you how to set that up. I’m not a fan of this feature, but you could use this as a backup. But what happens in the absence of an operating system we can find on the local chassis? We’re going to do that function of booting on a TFTP server.

So let’s say we went through that first step, load that first file in Flash, great. But if it’s 1, immediately look in Flash and find the very first IOS image in Flash and load it. If it’s 2, look at NVRAM and examine the boot system commands sequentially. And when the router examines it, if it’s 0, what does that mean? Load to the ROM monitor, or ROMMON, mode. What’s our default configuration register value? If it hasn’t been changed, it’s 2102, but that has to be examined. Here is a flowchart that visually represents everything we just discussed.

Cisco ios upgrade