What Is Config-register 0x0
RouterX#show startup-config
Using 1359 out of 32762 bytes
Version 12.2
RouterX#show startup-config
Using 1359 out of 32762 bytes
Version 12.2
Configuration Register
The configuration register includes information specifying where to locate the Cisco IOS Software image. You can examine the register with a show command, and you can change the register value with the config-register global configuration command. This topic describes how to display and change the boot information in the configuration register.
Before altering the configuration register, you should determine how the router is currently loading the software image. The show version command will obtain the current configuration register value. The last line of the display contains the configuration register value. This is shown in Example 6-6.
Example 6-6 Output of the show version Command
Cisco IOS Software, 2800 Software (C2800NM-IPBASE-M), Version 12.4(5a), RELEASE SOFTWARE (fc3)
Technical Support: http://www.cisco.com/techsupport Copyright (c) 1986-2006 by Cisco Systems, Inc. Compiled Sat 14-Jan-06 03:19 by alnguyen
ROM: System Bootstrap, Version 12.4(1r) [hqluong 1r], RELEASE SOFTWARE (fc1)
RouterX uptime is 1 week, 5 days, 21 hours, 30 minutes
System returned to ROM by reload at 23:04:40 UTC Tue Mar 13 2007
System image file is "flash:c2800nm-ipbase-mz.124-5a.bin"
Cisco 2811 (revision 53.51) with 251904K/10240K bytes of memory. Processor board ID FTX1013A1DJ 2 FastEthernet interfaces 2 Serial(sync/async) interfaces
DRAM configuration is 64 bits wide with parity enabled. 239K bytes of non-volatile configuration memory. 62720K bytes of ATA CompactFlash (Read/Write)
Configuration register is 0x2102
You can change the default configuration register setting with the config-register global configuration command, as shown in Example 6-7.
Example 6-7 config-register Command
RouterA# configure terminal RouterA(config)# config-register 0x2100
RouterA(config)# end
RouterA#
The configuration register is a 16-bit register. The lowest 4 bits of the configuration register (bits 3, 2, 1, and 0) form the boot field. A hexadecimal number is used as the argument to set the value of the configuration register. The default value of the configuration register is 0x2102. The boot field would be the last digit in the hexadecimal register. The setting of this field determines how the device boots. This is illustrated in Table 6-1.
|
Configuration Register Boot Field Value |
Meaning |
|
0x0 |
Use ROMMON mode (manually boot using the boot command) |
|
0x1 |
Automatically boot from ROM (provides Cisco IOS Software subset) |
|
0x2 to 0xF |
Examine NVRAM for boot system commands (0x2 default if router has flash) |
The guidelines for changing the boot field are as follows:
The guidelines for changing the boot field are as follows:
■ The boot field is set to 0 to enter ROMMON mode automatically. This value sets the boot field bits to 0-0-0-0. In the ROMMON mode, the router displays the ">" or "rommon>" prompt, depending on the router processor type. From the ROMMON mode, you can use the boot command to manually boot the router.
■ The boot field is set to 1 to configure the system to boot the Cisco IOS subset automatically from ROM. This value sets the boot field bits to 0-0-0-1. The router displays the Router(boot)> prompt in this mode.
■ The boot field is set to any value from 0x2 to 0xF to configure the system to use the boot system commands in the startup-config file in NVRAM. The default is 0x2. These values set the boot field bits to 0-0-1-0 through 1-1-1-1.
The show version command is used to verify changes in the configuration register setting.
The new configuration register value takes effect when the router reloads.
In Example 6-6, the show version command indicates that the current configuration register setting of 0x2104 will be used during the next router reload.
NOTE When using the config-register command, all 16 bits of the configuration register are set. Be careful to modify only the bits that you are trying to change, for example, the boot field, and leave the other bits as they are. Remember that the other configuration register bits perform functions that include selecting the console baud rate and determining whether to use the saved configuration in NVRAM.
The show flash command displays the contents of flash memory, including the image file names and sizes. This is illustrated in Example 6-8.
Example 6-8 show flash Command
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