Easy Cisco Router Boot Sequence

The Cisco Router Boot Sequence

When a router boots up, it performs a series of steps, called the Boot Sequence, to test the hardware and load the necessary software and files. The boot sequence consists of the following steps:


1. The router performs a POST. The POST tests the hardware to verify that all components of the device are operational and present. For example, the POST checks for the different interfaces on the router. The POST is stored in and run from ROM (read-only memory).

2. The bootstrap looks for and loads the Cisco IOS software. The Bootstrap is a program in ROM that is used to execute programs. The bootstrap program is responsible for finding where each IOS program is located and then loading the file. By default, the IOS software is loaded from flash memory in all Cisco routers. It can also be loaded from other locations like a TFTP server or router’s ROM (the “mini IOS”).

3. The IOS software looks for a valid configuration file stored in NVRAM. This file is called startup-config and is only there if an administrator copies the running-config file into NVRAM, a brand new router will not have such file.

4. If a startup-config file is in NVRAM, the router will load and run this file. The router is now operational. If a startup-config file is not in NVRAM, the router will start the setup-mode configuration upon bootup.