Chapter 10 began with a look at the boot process of a Cisco router, including an overview of the different environments into which a Cisco router can be booted – ROM Monitor mode, the boot image stored in ROM, or a normal IOS image stored in Flash memory or on a TFTP server. The break sequences to access ROM Monitor mode for different applications were also discussed, as were the boot system commands that can be added to a router’s startup configuration file to control the startup environment.
This was followed by a look at a router’s configuration register value, and the impact that this value has on how a router behaves during the boot process. A variety of different elements were looked at in this section, including how to enable or disable the break sequence, bypass the startup configuration file and, most importantly, the values associated with the boot field.
The act of changing a router’s configuration register was looked at next. This included a look at the commands to change the register value from the regular IOS image, as well as from ROM Monitor mode, on both Cisco 2500 and 2600 routers. While both routers accomplish the same goal in two steps, the commands used in ROM Monitor mode are different.
The chapter ended with a look at troubleshooting, both for the purpose of password recovery and restoring a missing or corrupted IOS image. The configuration registers played a key role in both processes, allowing the startup configuration file to be bypassed in the case of changing a password, while the boot image was used to download a new IOS image from a TFTP server.