Join Lazaro Diaz for an in-depth discussion in this video Understanding the router boot sequence, part of Cert Prep: Cisco Certified Entry Networking Technician (100-101).
- Okay, now that we've looked at the different Router components, I want to see how the Router actually boots. What is the sequence? The order in which the Router boots up. Well, the first one that boots up or that gets activated is ROM. In there the POST, the Power On Self Test is performed. All right, and then once it does that, this Bootstrap that exists, its job is to locate the IOS, the Operating System on the Router.
Which it, normally, it'll find it in FLASH. Now, we'll go through the normal operating procedure as far as boot up is concerned. So, POST is performed, all the checks went well, the Bootstrap initiates, it starts looking for the IOS which is in FLASH. If the FLASH is found, then the IOS looks for the Startup configuration. Where it's located? It's located in NVRAM. Now if there is a Startup configuration in NVRAM, once you get there then it's loaded into RAM.
And that's what you see on your string. Once you hit enter and you're in user mode or then you start, get into privileged mode, or you may be prompted to login, what have you. That Startup configuration, once the IOS finds it in NVRAM it will load it into RAM. All right, so that's why it says you may be asked to Login. So if it's a blank configuration you won't be. Where there is a configuration, then you may be asked. But what if something goes wrong? Let's say the POST goes well. The Bootstrap is locating for the IOS but it cannot find it.
Then normally IOS is found in FLASH but if not, it doesn't find it, it will try to go to TFTP, a TFTP server and locate it there. And if it fails and it can't locate because it doesn't have an IP address or where to go, then it will search for an IOS in ROM which it's not going to find one. What's going to happen is you're going to boot up into something called ROMmon and you don't want to be there. ROMmon means that something got corrupted and your FLASH cannot be found and that's why you see yourself there.
So definitely if something's going wrong, that you see yourself on a prompt that says R-O-M-M-O-N, ROMmon, ROM monitor mode and you find yourself there, your FLASH is corrupt. More than likely, that's what it is. Again, if it all goes well from that step, the IOS is found, then it goes into NVRAM. But if there's no configuration, all right, the IOS looks for a configuration file in NVRAM and finds no configuration, it'll again, try and locate it using a TFTP.
And if it doesn't find it there you boot up into something called Setup mode which basically is just a Wizard. It says you you want to enter the initial configuration dialog and then that's where you answer yes or no. If you answer yes, meaning you type yes or just the letter Y and you hit Enter, what's going to happen is it's going to take you through a Wizard. A step–by–step Wizard on putting the Routers host name, IP address, basic configs. That is Setup mode.
Definitely, you will want to answer no and do everything from scratch. You don't want to go through the Setup Wizard. And for your examination, not that you'll have to do this, but you're going to be doing things from scratch anyway. You'll be putting in an IP address, putting in the command for an IP address. If you have to use a host name then you'll put it in, by putting in the host name in the appropriate prompts. But this is the Boot order. Just know that, if all goes well POST, Bootstrap finds the IOS, it's in FLASH. The IOS then looks for the Startup config file in NVRAM.
From NVRAM, if there is any configuration file there it will load it. And then it loads it into RAM, and then you will go ahead, and then Login and start doing your work. If not, you can see what could happen. There's many scenarios of what could happen but this is the Boot Sequence of a Router. If asked, which sometimes you are, it's a very basic topic that they want you to know how the Boot Sequence of a Router is. So you know where the problem in your certification. So ROM, FLASH, NVRAM, and then finally RAM.
- The basics of networking
- The TCP/IP model vs. OSI model
- Understanding the Cisco three-layer model
- Collision and binary domains
- Converting binary to decimal and hexadecimal
- IP addressing
- Diagramming summarization
- Working Cisco IOS
- Managing Cisco internetworks
- IP routing
- Security with ACLs
- Configuring and verifying NAT
- IPv6 addressing
Skill Level Intermediate
Q. What is the latest exam relevant to this topic?
A. This course is for an older version of the ICND1 exam: exam #100-101. The #100-101 exam was retired by Cisco in August 2016, and replaced with a new exam, exam #100-105. This course is scheduled to be retired June 30, 2017 and will be removed from the library. For the most up to date course, please see Cert Prep: Cisco Certified Entry Networking Technician (100-105) by Todd Lammle.