Rebooting and Reloading - Configuring Image Loading Characteristics

On all platforms except Class A Flash file system platforms:

• If the configuration register is set to ignore NVRAM, the router enters setup mode.
• If the configuration register is not set to ignore NVRAM,
  • The startup software checks for configuration information in NVRAM.
  • If NVRAM holds valid configuration commands, the Cisco IOS software executes the commands automatically at startup.
  • If the software detects a problem with NVRAM or the configuration it contains (a CRC checksum error), it enters setup mode and prompts for configuration.

On Class A Flash file system platforms:

• If the configuration register is set to ignore NVRAM, the router enters setup mode.
• If the configuration register is not set to ignore NVRAM,
  • The startup software uses the configuration pointed to by the CONFIG_FILE environment variable.
  • When the CONFIG_FILE environment variable does not exist or is null (such as at first-time startup), the router uses NVRAM as the default startup device.
  • When the router uses NVRAM to start up and the system detects a problem with NVRAM or the configuration it contains, the router enters setup mode.

Problems can include a bad checksum for the information in NVRAM or an empty NVRAM with no configuration information.

For more information on environment variables, refer to the "Setting the BOOTLDR Environment Variable " section.

When a router is powered on or rebooted, the following events happen:

• The ROM monitor initializes.
• The ROM monitor checks the boot field (the lowest four bits) in the configuration register.
  • If the last digit of the boot field is 0 (for example, 0x100), the system does not boot. Instead the system enters ROM monitor mode and waits for user intervention. From ROM monitor mode, you can manually boot the system using the boot or b command.
  • If the last digit of the boot field is1 (for example, 0x101), the boot helper image is loaded from ROM. (On some platforms, the boot helper image is specified by the BOOTLDR environment variable.)
  • If the last digit of the boot field is 2 through F (for example, 0x102 through 0x10F), the router boots the first valid image specified in the configuration file or specified by the BOOT environment variable.

Note

The configuration register boot field value is expressed in hexadecimal. Because the boot field only encompasses the last four bits (represented by the last hexadecimal digit) of the configuration register value, the only digit we are concerned with in this discussion is the last digit. Consequently, 0x1 (0000 0001) is equivalent to 0x101 (1 0000 0001) in discussions of the boot field, because in both cases the last four bits are 0001.

When the boot field is 0x102 through 0x10F, the router goes through each bootsystem command in order until it boots a valid image. If bit 13 in the configuration register is set, each command will be tried once (bit 13 is indicated by the position occupied by b in the following hexadecimal notation: 0xb 000). If bit 13 is not set, the bootsystem commands specifying a network server will be tried up to five more times. The timeouts between each consecutive attempt are 2, 4, 16, 256, and 300 seconds.

If the router cannot find a valid image, the following events happen:

• If all boot commands in the system configuration file specify booting from a network server and all commands fail, the system attempts to boot the first valid file in Flash memory.
• If the "boot-default-ROM-software" option in the configuration register is set, the router will start the boot image (the image contained in boot ROM or specified by the BOORLDR environment variable).
• If the "boot-default-ROM-software" option in the configuration register is not set, the system waits for user intervention at the ROM monitor prompt. You must boot the router manually.
• If a fully functional system image is not found, the router will not function and must be reconfigured through a direct console port connection.

Note	Refer to your platform documentation for information on the default location of the boot image.

When looking for a bootable file in Flash memory:

• The system searches for the filename in Flash memory. If a filename is not specified, the software searches through the entire Flash directory for a bootable file instead of picking only the first file.
• The system attempts to recognize the file in Flash memory. If the file is recognized, the software decides whether it is bootable by performing the following checks:
  • For run-from-Flash images, the software determines whether it is loaded at the correct execution address.
  • For run-from-RAM images, the software determines whether the system has enough RAM to execute the image.

The figure below illustrates the basic booting decision process.

Figure 1

Booting Process

The lowest four bits of the 16-bit configuration register (bits 3, 2, 1, and 0) form the boot field. The following boot field values determine if the router loads an operating system and where it obtains the system image:

• When the entire boot field equals 0-0-0-0 (0x0), the router does not load a system image. Instead, it enters ROM monitor or "maintenance" mode from which you can enter ROM monitor commands to manually load a system image. Refer to the "Manually Booting from Flash Memory in ROMMON " section for details on ROM monitor mode.
• When the entire boot field equals 0-0-0-1 (0x1), the router loads the boot helper or rxboot image.
• When the entire boot field equals a value between 0-0-1-0 (0x2) and 1-1-1-1 (0xF), the router loads the system image specified by bootsystem commands in the startup configuration file. When the startup configuration file does not contain bootsystem commands, the router tries to load a default system image stored on a network server.

When loading a default system image from a network server, the router uses the configuration register settings to determine the default system image filename for booting from a network server. The router forms the default boot filename by starting with the word cisco and then appending the octal equivalent of the boot field number in the configuration register, followed by a hyphen (-) and the processor type name (cisconn-cpu). See the appropriate hardware installation guide for details on the configuration register and the default filename.

You modify the boot field from either the hardware configuration register or the software configuration register, depending on the platform.

Most platforms have use a software configuration register. Refer to your hardware documentation for information on the configuration register for your platform.

The hardware configuration register can be changed only on the processor card with dual in-line package (DIP) switches located at the back of the router. For information on modifying the hardware configuration register, refer to the appropriate hardware installation guide.

Because many platforms can boot images from several locations, these systems use special ROM monitor environment variables to specify the location and filename of images that the router is to use. In addition, Class A Flash file systems can load configuration files from several locations and use an environment variable to specify startup configurations:

• BOOT Environment Variable
  
• BOOTLDR Environment Variable
  
• CONFIG_FILE Environment Variable
  
• Controlling Environment Variables
  

If your router does not find a valid system image, or if its configuration file is corrupted at startup, or the configuration register is set to enter ROM monitor mode, the system enters ROM monitor mode. From this mode, you can manually load a system image from the following locations:

• Internal Flash memory or a Flash memory PC card
• A network server file
• ROM
• A local or remote computer, using the Xmodem or Ymodem protocol (Cisco 1600 series and Cisco 3600 series routers only)

You may only boot from a location if the router can store an image there. Therefore, not all platforms can manually load from these locations.

You can also enter ROM monitor mode by restarting the router and then pressing the Break key or issuing a "send break" command from a telnet session during the first 60 seconds of startup.

The ROM monitor supports command aliasing modeled on the aliasing function built into the Korn shell. The alias command is used to set and view aliased names. This allows the user to alias command names to a letter or word. Aliasing is often used to shorten command names or automatically invoke command options.

Aliases are stored in NVRAM and remain intact across periods of no power. These are some of the set aliases:

• b --boot
• h --history
• i --intialize/reset
• r --repeat
• k --stack
• ? --help

The following example shows a pre-aliased menu-type list for ROMMON commands:

> ?
$ state      Toggle cache state (? for help)
B [filename] [TFTP Server IP address | TFTP Server Name]
             Load and execute system image from ROM or from TFTP server
C [address]  Continue execution [optional address]
D /S M L V   Deposit value V of size S into location L with modifier M
E /S M L     Examine location L with size S with modifier M
G [address]  Begin execution
H            Help for commands
I            Initialize
K            Stack trace
L [filename] [TFTP Server IP address | TFTP Server Name]
             Load system image from ROM or from TFTP server, but do not
             begin execution
O            Show configuration register option settings
P            Set the break point
S            Single step next instruction
T function   Test device (? for help)
Deposit and Examine sizes may be B (byte), L (long) or S (short).
Modifiers may be R (register) or S (byte swap).
Register names are: D0-D7, A0-A6, SS, US, SR, and PC

If your options appear in the above menu-type format, you can use the listed aliased commands. To initialize the router or access server, enter the i command. The i command causes the bootstrap program to reinitialize the hardware, clear the contents of memory, and boot the system. To boot the system image file, use the b command.

The ROM monitor software characteristics will vary depending on your platform. For further details on ROM monitor mode commands, refer to the appropriate hardware installation guide, or perform a search on Cisco.com.

1.    enable

2.    show bootvar

3.    more nvram:startup-config

4.    show version

1.    enable

2.    show version

3.    configure terminal

4.    config-register value

5.    end

6.    show version

7.    copy running-config startup-config

8.    reload

• Examples
  

To set the BOOTLDR environment variable, complete the tasks in this section:

• Examples
  

To schedule a reload of the system image to occur on the router at a later time (for example, late at night or during the weekend when the router is used less) or to synchronize a reload network-wide (for example, to perform a software upgrade on all routers in the network), complete the task in this section.

Note	A scheduled reload must take place within approximately 24 days.

1.    enable

2.    reload in [hh:]mm [text ]

3.    reload at hh : mm [month day | day month ] [text ]

• Examples
  

1.    enable

2.    show reload

1.    enable

2.    reload cancel

• Examples
  

1.    enable

2.    reload

3.    Press the Break key during the first 60 seconds while the system is booting.

4.    ?

• What to Do Next
  

1.    enable

2.    reload

3.    Press the Break key during the first 60 seconds while the system is booting.

4.   Do one of the following:

• Examples
  

1.    enable

2.    reload

3.    Press the Break key during the first 60 seconds while the system is booting.

4.    boot filename [ip-address ]

• Examples
  

1.    enable

2.    reload

3.    Press the Break key during the first 60 seconds while the system is booting.

4.    boot

• Examples
  

1.    enable

2.    reload

3.    Press the Break key during the first 60 seconds while the system is booting.

4.    boot system mop filename [mac-address ] [interface ]

• Examples
  

To return to EXEC mode from the ROM monitor, you must continue loading from the default system image. To exit ROMMON mode and resume loading, use the following command in ROM monitor mode:

                
                  continue
                
              

ROMMON > continue