About GRUB Mode and the Configuration Register
The Cisco CSR 1000V/ISRv has a 16-bit configuration register in NVRAM. Each bit has value 1 (on or set) or value 0 (off or clear), and each bit setting affects the router behavior upon the next reload power cycle. The GRUB mode supports a subset of configuration register options compared to ROMMON options on other Cisco routers.
You can use the configuration register to:
-
Force the router to boot into the GRUB (bootstrap program)
-
Select a boot source and default boot filename
-
Recover a lost password
The table below describes the configuration register bits.
BitNumber |
Hexadecimal |
Meaning |
---|---|---|
00–03 |
0x0000–0x000F |
Boot field. The boot field setting determines whether the router loads an operating system and where it obtains the system image. See the table "Boot Field Configuration Register Bit Descriptions" for details. |
06 |
0x0040 |
Causes the system software to ignore the contents of NVRAM. This can be used for password recovery. |
Note |
Entering the GRUB mode for Cisco CSR1000V running on cloud solutions depends on the console access capabilities of the cloud provider. If the cloud provider provides limited access to console, you cannot access the GRUB mode for password recovery. |
The next table describes the boot field, which is the lowest four bits of the configuration register (bits 3, 2, 1, and 0). The boot field setting determines whether the router loads an operating system.
Boot Field(Bits 3, 2, 1, and 0) |
Meaning |
---|---|
0000 (0x0) |
At the next power cycle or reload, the router boots to the GRUB (bootstrap program). In GRUB mode, you must manually boot the system image or any other image by using the boot command. |
0001 - 1111 (0x01 - 0x0F) |
At the next power cycle or reload, the router sequentially processes each boot system command in global configuration mode that is stored in the configuration file until the system boots successfully. If the no boot system commands are stored in the configuration file, or if these commands are not executed successfully, GRUB dictates the image that needs to be booted. |
Note |
Use the 0x000 setting to configure the router to automatically enter GRUB mode when the router reboots. |