The process that a Cisco router goes through on boot is not terribly unlike that of any computer. A familiarity with the process will help you to better understand how a router’s various configuration elements relate to one another, and will ultimately be required knowledge in troubleshooting situations.
To begin, we’ll assume that we’re powering up a Cisco 2501 for the first time. At this point, the router should include a valid IOS image in Flash memory, but the router should not yet be configured. The sequence of events that a router goes through during the boot process includes:
- POST. When first powered up, a router will carry out a power-on self-test (POST). Recall that the POST is used to check whether the CPU and router interfaces are capable of functioning correctly.
- Execute bootstrap to load IOS. After a successful POST, the router will execute the Bootstrap program from ROM. The bootstrap is used to search Flash memory for a valid Cisco IOS image. If one is present, the image is loaded. If an image cannot be found, the router will boot the RxBoot limited IOS version found in ROM.
- IOS loads configuration file. Once the IOS image is loaded, it will search for a valid startup configuration in NVRAM. If a valid startup configuration file cannot be found, the router will load the System Configuration Dialog, or what is sometimes called setup mode. This mode allows you to perform the initial configuration of the router.
If these three steps listed seem a little too simple, not to worry – they are the default steps of what I consider to be a “proper” or “good” boot. In Chapter 13 we’ll take a closer look at troubleshooting the boot process when things go wrong.