Configuration register
© 2002, Cisco Systems, Inc. All rights reserved.
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Configuration register
• Is located in NVRAM
• Is inspected during bootstrap process
• Controls configuration of the router
• Is a 16-bit register
• Setting can be inspected using the show
version command
© 2002, Cisco Systems, Inc. All rights reserved.
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Configuration register layout
0x2102
• The prefix 0x has no value other than
indicating that “the following values are in
hex”
• The 16-bits are represented as four hex
numbers each four bits long
© 2002, Cisco Systems, Inc. All rights reserved.
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Configuration register composition
0x2102
Hex
Binary
2
1
0
2
0010 0001 0000 0010
boot field
© 2002, Cisco Systems, Inc. All rights reserved.
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Router boot field
© 2002, Cisco Systems, Inc. All rights reserved.
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Router boot field
Boot field value Description
0000
(0x0)
Use ROM monitor
mode to boot router
0001
(0x1)
Boot router from ROM
(limited IOS)
0010 – 1111
(0x2 – 0xF)
Boot router using
NVRAM commands
(boots IOS from Flash)
© 2002, Cisco Systems, Inc. All rights reserved.
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Other configuration register bits explained
Register bit Description
15
Diagnostic mode
14
Include network number with IP broadcasts
13
ROM boot enable/disable if network boot fails
12-11
Console speed (usually 9600 baud)
10
IP broadcasts with one’s
8
Break enable/disable
7
OEM enable/disable
6
Ignore configuration enable/disable
3-0
Boot field
© 2002, Cisco Systems, Inc. All rights reserved.
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Standard configuration register settings
0x2102
Binary
0010 0001 0000 0010
Rom boot enable
Console speed 9600 baud
Break enabled
Ignore configuration disabled
Boot from Flash memory
© 2002, Cisco Systems, Inc. All rights reserved.
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Router start-up sequence
NVRAM
ROM
Startup
configuration
POST

BOOTSTRAP
reads
RAM
Configuration
register
IOS
loads IOS
loads running configuration
© 2002, Cisco Systems, Inc. All rights reserved.
Running
configuration
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Password recovery
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Password recovery on 2500 series routers
• Restart the router
 (type reload or power cycle the router)
• Hit Cntl + Break keys within 60 seconds
 Note: This will only work in some terminal emulations
(e.g. QVT term)
• Set the configuration register to 0x2142
 on 2500 routers issue o/r 0x2142
 This will set the configuration register bit 6 and ignore
the startup-configuration
© 2002, Cisco Systems, Inc. All rights reserved.
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Password recovery on 2500 series
routers - II
• Type i to reload the router
• The router will boot and ignore the startup-config
• Enter enable mode
• Change the enable secret password in the
running-config
• Change the configuration register to 0x2102
• Copy the running-config into the startup-config
 This reverses the earlier setting and the router will
use the new enable password in future logins
© 2002, Cisco Systems, Inc. All rights reserved.
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Password recovery on 2600 series
routers
• Restart the router
 (type reload or power cycle the router)
• Hit Cntl + Break keys within 60 seconds
 Note: This will only work in some terminal emulations
(e.g. QVT term)
• The prompt will show ROM monitor mode
• Issue the command confreg to change the
configuration register settings
• Menu-driven process in which [y/n] answers are
used to specify “ignore startup-config”
© 2002, Cisco Systems, Inc. All rights reserved.
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Password recovery on 2600 series
routers - II
• Type reset to reload the router
• The router will boot and ignore the startup-config
• Enter enable mode
• Change the enable secret password in the
running-config
• Change the configuration register to 0x2102
• Copy the running-config into the startup-config
 This reverses the earlier setting and the router will
use the new enable password in future logins
© 2002, Cisco Systems, Inc. All rights reserved.
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