Analyzing the Boot Process of a Cisco IOS Router

Posted on January 7, 2009 by David Davis in Networking with

You can learn a lot about a router by watching it power-up / boot-up. When the router boots, you can learn how fast it is, how much RAM it has, how much storage it has, how many interfaces it has, what type of interfaces, and much more. Many administrators don’t think about the boot-up process because they may not boot-up a router every day. In this article, we will boot a router and analyze the boot process. By knowing what to look for, you will quickly know what is normal and what is not.

Router Boot-up Part 1

You have just turned on the power switch of your router. You are connected to the console power with a terminal emulator. The screenshot below shows the first part of the router boot-up (power-on). In this first part of the boot process, I’d like to point out three things. First, you see that this router has a “2500 processor”. In other words, this router is part of the Cisco 2500 series of routers. Next, it says “with 16384 Kbytes of main memory”. This means that this router has 16Mb (16384Kb) of RAM. The RAM is where the operating system (the IOS) is loaded in when the router is running. The RAM is also used to process packets. Just like a PC, RAM is a very necessary thing, and the more you have the better. Lastly, I’d like to point out that this is IOS 2500 Software, series C2500-IS-L. More specifically, it is Version 12.3, patch level 12. From this small portion of the boot-up process, you learned your router’s architecture, amount of RAM, and operating system version. This is very important information for any administrator to know.

Router Boot-up Part 2

Now the router boot-up process continues. The information keeps scrolling. Here is a screenshot of the next part of the process: In this part of the process, we learn a few more things about this router. First, this router is a 2509 and it has a 68030 processor. This processor is a Motorola processor that is built into the motherboard. Again, you are told how much RAM the router has. In the next section, you learn some very important information about the router’s interfaces:

Ethernet Interfaces: this router has 1 Ethernet interface. This isn’t a Fast Ethernet interface, just a regular 10Base-T Ethernet interface. Ethernet interfaces are used to connect to the local area network (LAN).

Serial Interfaces: this router has 2 Serial network interfaces. These interfaces usually connect to a CSU, and would be used to connect to a WAN circuit.


Terminal Lines: this router has 8 terminal lines. These are serial lines used to connect to modems, dumb terminals, printers, or the console port of other devices. These lines make this router able to be a “terminal server”. After you learn about your interfaces, you learn more about the router’s storage:

Non-Volatile Memory: this router has 32Kb of non-volatile RAM. This is also called NVRAM. NVRAM is where your configuration files (such as your startup-configuration) are stored. NVRAM holds what is stored on it when the router is turned off. However, it is only large enough to store your configuration files.

Flash Memory: this router has 16384Kb of flash memory. This equates to 16Mb of flash. The flash memory is similar to the hard disk on a PC or server. In other words, when your router is powered off, the contents of the flash memory are not lost. The Flash memory is generally where the IOS (operating system) is stored.

Router Boot-up Part 3

Now the router boot-up process continues. The information keeps scrolling. Here is a screenshot of the next part of the process: After you see the information on the interfaces, NVRAM, and flash, you are on the lookout for the message “Press RETURN to get started!” This means that the boot-up process has completed. At this point, console messages from the boot-up process begin scrolling across the screen. You will see some of the same messages you saw in the boot-up process (but not all the same messages). You will also see some new messages scroll by. For example, notice the message that says “ChicagoRouter is undergoing a cold start”. This means that the router was actually powered off and powered back on using the power switch. The router could have also restarted because it panicked or because it was reloaded (warm start). Note that this router must have had a configuration file because it did not enter setup mode. Setup mode is where the router asks you questions about its initial configuration.


Understanding the boot-up process of a router, or switch, is a critical skill. Different router, or switch, models will have slightly different power-up processes. However, by understanding the boot-up process of one model, you will better be able to understand them all. You should be familiar with the good and bad messages that you may experience in the power-up process. By knowing what to look for, you can quickly diagnose a problem, and make a resolution. More information on router power-up, and router architecture, can be found at the Cisco Modular Access Routers documentation site.

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