Configuration registers for Cisco routers

1. Foreword

The configuration register is a 16-bit virtual register that specifies the order in which to start the router, interrupt parameters, and set the console baud rate. The value of the register is usually expressed in hexadecimal.

The configuration Command Config register allows you to change the value of the configuration register.

2. Start Order

Configure the last 4 bits of the register, specifying the location of the boot file that the router must use when it starts:

R>l 0x0000 designated router into ROM monitoring mode

L 0x0001 Specifies to boot from ROM

L 0X0002-0X000F values are referenced in the order specified in the NVRAM configuration file in command boot system

If the configuration file does not have the boot system command, the router attempts to start with the first file in the system flash memory, and if it fails, the router attempts to load a file with a default filename from the network with TFTP (determined by the value of the boot field, such as cisco2-4500), If it fails, the system loads the boot from the boot flash.

The default file name consists of the word Cisco, the value of the startup bit, and the router type or the name of the processor. For example, the boot field on one 4500 is set to 3, and the default boot file name is cisco3-4500.

Take the MC3819 (CPU model, mostly Motorola) router startup sequence as an example, the following is the four stages of startup:

1. System Bootstrap

2. Start loading (Read configuration information and start the minimum function of the Flash file system)

3. Launch System iOS image file

4. Interface initialization/system reboot

3. Configuration Registers

3.1. What do you mean by

Table 1 Configuration Registers What do you mean, 1 configuration registers, what do you mean?

Number of register digits	Hexadecimal	Function description
0-3 (starting order)	0x0000-0x000f	Start field: 0000-stay at the boot prompt (> or Rommon >) 0001-Boot from ROM,
4	－	Not used
5	－	Not used
6	0x0040	Configuration system ignores configuration information in NVRAM
7	0x0080	Start OEM bit
8	0x0100	After setting, the Suspend key is not available when the system is running, and if it is not set, the system enters the boot monitor mode (rommon>)
9	-
10	0x0400	All 0 is the broadcast address.
11-12	0x0800 to 0x1800	Console line speed, the default is 00 or 9600bps
13	0x2000	If startup fails, the system starts with the default ROM software
14	0x4000	－
15	0x8000	This setting enables diagnostic messages and ignores the contents of NVRAM

For example:

The show version command allows you to see the value of the router configuration register, which by default is 0x2102. Each of these four numbers has important implications. Below is a one by one introduction from low to high.

The first 2, reduced to binary 0010, which is the boot field, plays a vital role in the start-up of the router iOS, and when any of the values in the Boot field is 2-15, the router is normal to boot, when this value is 0 o'clock After the router starts to enter Rommon mode, this value is 1 o'clock, the router enters into rxboot mode (2500 router's flash in the configuration register value is 2102 when the property is read only, if want to upgrade iOS must change the register value to 2101)

0, to restore the binary to 0000, the four-bit, the key role is the third (that is, the entire register bit 7), the value of 0, when the router will be launched from the NVRAM inside the configuration file into RAM to run, the value of 1, The configuration of the NVRAM is ignored after the router is started (this is why we changed the value of the register to 2142 in the password recovery)

1, to restore the binary value of 0001, we pay attention to BIT8, the value of 0 o'clock, the router in normal operation mode CTRL + BREAK is invalid; value 1, the router in any running mode as long as the CTRL + Break will immediately enter the Rommon mode.

The second 2, reduced to binary to 0100, where BIT13, the value of 0 o'clock, the router if the network boot will try infinitely many times. When the value is 1 o'clock, the router makes a maximum of 5 network boot attempts.

Register digit hex Function description

0-3 (starting order) 0x0000-0x000f boot field: 0000-stay at the boot prompt (> or Rommon > Under) 0001-Boot from ROM

4-Not used

5-Not used

6 0x0040 configuration system ignores configuration information in NVRAM

7 0x0080 start OEM bit

8 0x0100 is set, the Pause key is not available when the system is running, and if it is not set, the system enters the boot monitor mode (rommon>)

9-

All 0 of 0x0400 are broadcast addresses.

11-12 0x0800 to 0x1800 console line speed, the default is 00 or 9600bps

0x2000 If startup fails, the system starts with the default ROM software

0x4000-

0x8000 This setting enables diagnostic messages and ignores the contents of NVRAM

Typical parameters

L 0X2102: The interrupt key is blocked during the run, the router will view the contents of the NVRAM configuration to determine the boot order, if the boot failure will be initiated with the default ROM software.

L 0x2142: Use when restoring password. Ignores NVRAM configuration information and goes to the initial configuration dialog mode