Cisco Systems 78-16019-02 manual Software Configuration Register Settings

Chapter 3 Initial Configuration

About the Software Configuration Register

•Enable booting from a TFTP server

•Recover a lost password

•Boot the system manually using the boot command at the bootstrap program prompt.

•Force the system to boot automatically from the system bootstrap software (boot image) or from its default system image in onboard Flash memory, using any boot system commands stored in the startup configuration file in NVRAM

Software Configuration Register Settings

Table 3-4describes each of the software configuration register bits.

Caution To avoid confusion and possibly halting the system, remember that valid configuration register settings might be combinations of settings and not just the individual settings listed in Table 3-4. For example, the value of 0x0101 is a combination of settings (bit 8 is 0x0100 and bits 00 through 03 are 0x0001).

Table 3-4 Software Configuration Register Bits

Bit Number	Hexadecimal	Description
00 to 03	0x0000 to 0x000F	Controls the system boot behavior (also known as the boot field)
06	0x0040	Causes system software to ignore NVRAM contents
07	0x0080	Enables the OEM bit
08	0x0100	Disables the break function
09	0x0200	Uses secondary bootstrap during system boot
10	0x0400	Uses an IP broadcast with all zeros
11 to 12	0x0800 to 0x1000	Sets the console line speed (default is 9600 baud)
13	0x2000	Boots the default Flash software if network boot fails
14	0x4000	Uses IP broadcasts without network numbers
15	0x8000	Enables diagnostic messages and ignores the NVRAM contents

Bit 8 controls the console break function. Setting bit 8 (the factory default) causes the system to ignore the console break key. Clearing bit 8 causes the system to use the break key or break signal as a command to force the system into the bootstrap monitor (ROMMON), thereby halting normal operation.

Regardless of the setting of the break enable bit, a break causes a return to the ROMMON during the first few seconds (approximately five seconds) of booting.

Bit 9 controls the secondary bootstrap program function. Setting bit 9 causes the system to use the secondary bootstrap. Clearing bit 9 (the factory default) causes the system to ignore the secondary bootstrap. The secondary bootstrap program is used for system debugging and diagnostics.

Bit 10 controls the host portion of the IP broadcast address. Setting bit 10 causes the system to use all zeros. Clearing bit 10 (the factory default) causes the system to use all ones. Bit 10 interacts with bit 14, which controls the network and subnet portions of the IP broadcast address.

Table 3-5shows the combined effect of bits 14 and 10.

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3-26	78-16019-02, Cisco IOS Release 12.2(18)SV2