Configuration Registers

The configuration register is a 16-bit number that defines how a router operates on a power cycle. These options include if the IOS will be loaded from Flash or ROM. Configuration registers advise the CPU to load the configuration file from the NVRAM or to ignore the configuration file stored in memory, for example. The default configuration register is displayed as 0x2102. Table 4-1 displays the binary conversion from 0x2102.

Table 4-1 0x2102 Binary Conversion

Bit Number

Value

15

0

14

0

13

1

12

0

11

0

10

0

9

0

8

1

7

0

6

0

5

0

4

0

3

0

2

0

1

1

0

0

The bits are numbered from right to left. In the preceding example, the value is displayed as 0x2102 (0010.0001.0000.0010). The function of the configuration register bits is determined by their position, as follows:

• Bits 0 through 3—Determines the boot option whether the router loads the IOS from the Flash (binary value is 010) or from ROM (binary value is 000).

• Bit 6—Tells the router to load the configuration from NVRAM if set to 1 and to ignore the NVRAM if set to 0.

• Bit 7— Referred to as the OEM (OEM = original equipment manufacturer) bit in Cisco documentation and is not used.

• Bit 8—Specifies whether to enter ROM mode without power cycling the router. If bit 8 is set to 1 and the break key is issued while the router is up and running normally, the router will go into ROM mode. This is a dangerous scenario because if this occurs, your router immediately stops functioning.

• Bit 10—Specifies the broadcast address to use, where 1 equals the use of all 0s for broadcast at boot (in conjunction with bit 14). Bit 10 interacts with bit 14.

• Bits 11 and 12—Set the console port's baud rate. For example, if bits 11 and 12 are set to 00, the baud rate is 9600 bps. A baud rate of 4800 bps can be set when these bits are set to 01. 10 sets the baud rate to 2400 bps, and 11 sets the baud rate to 1200 bps.

• Bit 13—Tells the router to boot from ROM if the Flash cannot boot from a network, such as a TFTP server. If bit 13 is set to 0 and no IOS is found, the router will hang. If bit 13 is set to 1 and no IOS is found, the router boots from ROM.

• Bit 14—Interacts with Bit 10 to define broadcast address.

• Bit 15—Specifies to enable diagnostics display on startup and ignore the NVRAM. To view the current configuration register, use the show version IOS command.

Example 4-4 displays the configuration register of a router, R1.

Example 4-4 (Truncated) show version Command

R1>show version

Cisco Internetwork Operating System Software

IOS (tm) 2500 Software (C2500-AJS40-L), Version 11.2(17)

, RELEASE SOFTWARE (fc1)

Copyright (c) 1986-1999 by Cisco Systems, Inc. Compiled Tue 05-Jan-99 13:27 by ashah Image text-base: 0X030481E0, data-base: 0x00001000 ROM: System Bootstrap, Version 5.2(8a), RELEASE SOFTWARE

continues

Example 4-4 (Truncated) show version Command (Continued)

BOOTFLASH: 3000 Bootstrap Software (IGS-RXBOOT), Version 10.2(8a), RELEASE SOFTWARE R1 uptime is 6 days, 1 hour, 36 minutes System restarted by reload

System image file is "flash:c2500-ajs40-l.112-17.bin", .. ..booted via flash cisco 2520 (68030) processor (revision E) with 8192K/2048K byte Processor board ID 02956210, with hardware revision 00000002 Bridging software.

SuperLAT software copyright 1990 by Meridian Technology Corp. X.25 software, Version 2.0, NET2, BFE and GOSIP compliant. TN3270 Emulation software. Basic Rate ISDN software, Version 1.0.

1 Ethernet/IEEE 802.3 interface(s)

2 Serial network interface(s)

2 Low-speed serial(sync/async) network interface(s)

1 ISDN Basic Rate interface(s)

32K bytes of non-volatile configuration memory.

16384K bytes of processor board System flash (Read ONLY)

Configuration register is 0x2102

The output from Example 4-4 displays the configuration register as 0x2102. The show version command also displays other useful router information, such as the router's uptime, the IOS image in use, and the hardware configuration. To change the configuration register, use the global configuration command, configure-register register-value. When a configuration register is changed, use the show version command to ensure that the register has been changed to the new value.

Table 4-2 displays common configuration register values you can use in day-to-day troubleshooting of Cisco IOS routers.

Table 4-2 Common Registers and Descriptions

Register Value

Description

0x2100

Boots the router using the system bootstrap found in ROM.

0x2102

Boots the router using Flash and NVRAM. This is the default setting.

0x2142

Boots the router using Flash and ignores NVRAM. This value is used to recover passwords or modify configuration parameters.

0 0

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