Which bits represent the boot field in the configuration register value?

The Cisco IOS configuration register is a 16-bit software setting that controls the router's startup behavior. The lowest four bits (bits 0, 1, 2, and 3) are collectively known as the "boot field." This field dictates the source from which the router attempts to load the operating system image. For example, a boot field value of 00 (0x0) instructs the router to boot into ROM monitor (ROMMON) mode, while a value of 02-0F (0x2-0xF) instructs it to use the boot system commands stored in the startup configuration in NVRAM. The default setting is typically 0x2102, where the last digit '2' represents the boot field value.

A. The highest bits (e.g., bit 13) are used for other functions, such as booting from the ROM software image if the network boot fails.
B. This is incomplete. The boot field is composed of four bits (0-3), not just the lowest two, to allow for 16 different boot behaviors.
C. The highest four bits (12-15) are not used for the boot field; they control functions like console line speed and other diagnostic settings.

1. Cisco Systems, Inc., "Use of the Configuration Register on All Cisco Routers," Document ID: 13715. In the section "Configuration Register Bits," the document states, "The lowest four bits of the configuration register (bits 3, 2, 1, and 0) form the boot field." The accompanying table, "Boot Field Values," details the function of each value from 0000 to 1111.

2. Cisco Systems, Inc., "Cisco IOS Configuration Fundamentals Command Reference," Release 12.2. Under the config-register command description, the documentation specifies the syntax config-register value and explains the meaning of the bits. The section on the boot field explicitly details that bits 00-03 control the boot source.

3. Cisco Networking Academy, "CCNA Routing and Switching: Scaling Networks v6.0 Companion Guide." Chapter 2, "LAN Redundancy," in the section "Router and Switch Startup," describes the POST and bootup process. It details the role of the configuration register and explicitly identifies the last hexadecimal digit (representing the lowest four bits) as the boot field that determines the IOS image source.