This tutorial explains the fundamental steps of the Cisco IOS boot sequence process. Besides, you will learn how Cisco routers select the OS image to load and how the running configuration file gets initialized.
What is Cisco IOS Boot Sequence?
Cisco IOS boot sequence is a set of tasks Cisco routers perform when they get powered on or reloaded (rebooted). These tasks include a series of procedures and tests to verify router hardware components before selecting a router operating system to load into RAM.
What are The Steps in The Cisco IOS Boot Sequence?
When you power on or reboot a Cisco router, the router tries to go through four steps, called the Cisco IOS boot sequence. The Cisco IOS boot process consists of four steps in the following order:
Step 1. The router runs the POST process, which stands for power-on self-test, to detect hardware components installed on the router and verify that they are in good working condition.
Step 2. A copy of the bootstrap program, stored in ROM, gets placed in RAM and then executed from RAM.
If one of these first two steps does not succeed, the boot process won’t continue, and the router initialization phase fails.
Step 3. The bootstrap program selects a Cisco IOS image among those stored in flash memory or on network file servers and loads it into RAM. In some cases, the bootstrap process may load the ROMMON OS instead of an IOS image file. Next, the currently loaded OS takes control of the router from the bootstrap program.
ROMMON is a Cisco router operating system used for troubleshooting tasks like password recovery. In this router mode, you can also download and install a Cisco IOS image from a TFTP server.
Step 4. When the bootstrap program decides to load and run a Cisco IOS image file, Cisco IOS looks for the startup configuration file in NVRAM and copies it into RAM as the running configuration file. If the starting configuration file does not exist in NVRAM, you can instruct the router to download one from a network file server or through the console port.
Related Lessons to Cisco IOS Boot Sequence
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