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One free scan finds every outdated or missing driver and matches the right update for your exact hardware.Free scan · exact hardware matchconfig-register 0x2101 is a legacy Cisco configuration-register value used mainly on classic IOS routers and older platforms. In Cisco’s common router interpretation, it selects bootstrap/default-ROM boot behavior, ignores BREAK during boot, falls back to ROM if the first boot attempt fails, and uses a 9600-baud console. It is not automatically an error, but it is not the usual production value on many classic IOS devices either. On modern IOS XE switches, especially Catalyst 9000 models, the displayed register may be legacy information; Cisco provides separate boot and ROMMON controls instead.
Always identify the hardware and software family before changing the value. Cisco’s platform guidance is the authority for the device you are operating.
What the Cisco configuration register controls
On classic IOS platforms, the configuration register is a hexadecimal control value stored in nonvolatile memory. Its bit fields influence how the device starts and how the console behaves, including:
- Which boot field and image-selection method the device uses.
- Whether the startup configuration in NVRAM is loaded.
- Whether a BREAK signal is accepted during boot.
- The console baud rate.
- Whether the device falls back to ROM or bootstrap software after a failed boot.
Cisco describes the traditional register and its bit meanings in Understand Configuration Register Usage on all Routers. Those meanings are not a universal contract for every IOS XE platform.
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How to decode 0x2101
The following is the common classic-IOS interpretation. Individual hardware generations can implement the boot field differently, so treat “first image in flash” as legacy/platform-dependent behavior rather than a promise for every router.
| Hex portion | Common meaning |
|---|---|
0x0001 |
Boot field 1: use default ROM/bootstrap behavior or the first available IOS image in flash, depending on the platform. |
0x0100 |
Disable BREAK during boot. |
0x2000 |
Boot the default ROM software if the initial boot attempt fails. |
| Combined value | Cisco’s common table describes 0x2101 as booting into bootstrap, ignoring BREAK, falling back to ROM, and using 9600 baud. |
“Boots into bootstrap” does not mean “always enters ROMMON.” ROMMON is the lower-level ROM Monitor environment. Bootstrap can be an intermediate stage that locates and launches an IOS image, so a router with 0x2101 may still reach IOS normally.
0x2101 versus 0x2102 and 0x2142
| Value | Typical classic-IOS behavior | Operational use |
|---|---|---|
0x2101 |
Boot field 1; bootstrap/default-ROM or first-image behavior, BREAK ignored, ROM fallback, commonly 9600 baud. | Legacy or platform-specific boot procedures; not usually the standard production choice. |
0x2102 |
Boot field 2; commonly processes configured boot system commands, loads an IOS image from flash, and loads the startup configuration. |
Factory-default or normal production setting on many classic IOS platforms. |
0x2142 |
Sets the bit that causes NVRAM startup configuration to be ignored. | Password recovery or controlled recovery only; unsafe as a permanent production value. |
0x2101 does not normally mean “ignore the configuration.” Cisco associates that behavior with values such as 0x2142. An apparently empty configuration can instead result from an ignored startup file, an empty file, a different supervisor or stack member, or a failed boot sequence.
0x2102 is common, not universal. Use the documented value for the exact platform and release rather than changing every device to it automatically.
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From privileged EXEC mode, run:
Router# show versionNear the end of the output, look for a line such as:
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Configuration register is 0x2101Some platforms also show a pending value:
Configuration register is 0x2101 (will be 0x102 at next reload)The first value is active now; the parenthetical value is scheduled for the next reload. Resolve that pending state before rebooting. Cisco documents this output pattern in its router guidance and in the Cisco 800 Series Product Overview.
Restore a normal boot setting on classic IOS
If the platform documentation identifies 0x2102 as the normal value and the router has been left at 0x2101 unintentionally, use this sequence:
- Capture the current state:
Router# show version Router# show running-config | include boot system|config-register Router# show startup-config - Confirm the platform, IOS family, available image files, and configured boot statements. Verify that the intended image exists and that you have console or out-of-band access.
- Set the value in global configuration mode:
Router# configure terminal Router(config)# config-register 0x2102 Router(config)# end Router# show version - Check the resulting “will be … at next reload” text and confirm the running configuration is the one you intend to preserve.
- Save only after validation:
Router# copy running-config startup-config - Reload during an approved maintenance window:
Router# reload
A wrong register, missing image, invalid boot statement, or damaged flash can still lead to ROMMON or an unsuccessful boot. Changing the register alone does not diagnose those conditions.
Set the value from ROMMON
On platforms whose ROMMON supports the command, enter:
rommon 1 > confreg 0x2101
rommon 2 > resetconfreg is a ROMMON command; config-register is normally an IOS configuration-mode command. The new ROMMON value takes effect after reset or a power cycle. Cisco documents this procedure for 800-series devices in the Cisco 800 Series ROMMON Appendix. ROMMON syntax and available fields vary by hardware.
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What to do when a device unexpectedly shows 0x2101
- Record
show version, the running and startup configurations, and any pending register value. - Identify whether the device is classic IOS, IOS XE, or a platform-specific variant.
- Inspect image files,
boot systemstatements, boot variables, and storage health. - Compare the current value with the platform’s documented production setting. On many classic routers this is
0x2102, but that is not a universal repair. - If you change the value, retain console or out-of-band access, verify the image and configuration, save deliberately, and reload only in a maintenance window.
Possible boot-failure causes include a missing or corrupt IOS image, bad flash metadata, invalid boot variables, a platform expecting bootstrap behavior, ROMMON-specific settings, or supervisor/stack complications. The register is a clue, not proof of the root cause.
Why the console may be unreadable
Cisco’s common table associates 0x2101 with a 9600-baud console. A mismatch between that rate and the terminal emulator produces garbled characters. Confirm the device’s documented console rate and match the terminal settings. Other register values or platform-specific implementations can select different rates.
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Do not apply classic-router register advice wholesale to IOS XE switches. On Catalyst 9000 platforms, Cisco documents explicit controls for boot and ROMMON behavior:
Switch(config)# no boot manual
Switch(config)# boot manual
Switch(config)# [no] boot enable-break
Switch(config)# system ignore startupconfig
Switch# show romvarOn some Catalyst 9000 releases, show version displayed a traditional register value even though the legacy config-register command was ineffective or misleading. Cisco says that incorrect display behavior was removed beginning with IOS XE 16.12.4 and 17.3.1. Use Cisco’s Configure Register Equivalent Parameters in IOS XE for the exact release and model, and inspect show romvar rather than assuming a router-style register controls the switch.
Common symptoms and their interpretation
The router shows 0x2101 but still boots IOS
That can be normal on older platforms: bootstrap follows its legacy or default image-selection process and launches IOS. Observe the actual boot sequence instead of equating bootstrap with ROMMON.
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The device enters ROMMON after a change
Check for a valid image, correct boot variables, readable flash, and platform-specific ROMMON requirements. A register change may expose an existing image or storage problem rather than create one.
The startup configuration appears to have disappeared
Check the register for 0x2142 or another NVRAM-ignore setting. Then verify that the startup file exists and that you are examining the intended supervisor or stack member.
config-register has no effect
On a Catalyst 9000 IOS XE switch, use the documented equivalent commands and show romvar. On classic IOS, verify that the command was entered in configuration mode and that the pending value shown by show version matches your intent.
Should you leave 0x2101 in place?
Leave it unchanged when the platform documentation supports it, the device boots reliably, and the organization intentionally uses its bootstrap or first-image behavior. Consider changing it when the device should follow deterministic production boot statements, has been left in a recovery or lab state, or the documented normal value for that platform is 0x2102. Never use 0x2142 as a routine fix for a password problem without a recovery plan, because it bypasses the saved configuration.
Frequently Asked Questions
Does 0x2101 erase the startup configuration?
No. Cisco commonly associates NVRAM-ignore behavior with 0x2142, not 0x2101. Verify the register, startup file, and active supervisor before concluding that configuration was lost.
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Does 0x2101 always enter ROMMON?
No. It commonly selects bootstrap/default-ROM behavior, and bootstrap may locate and launch IOS. ROMMON entry also depends on image, storage, boot-variable, and platform conditions.
Should every Cisco device be changed to 0x2102?
No. 0x2102 is common on many classic IOS platforms, but the correct value is platform- and release-dependent. Modern IOS XE switches may use equivalent boot controls instead.
Why does show version show another value for the next reload?
The current register and the pending next-reload register differ. Treat the parenthetical value as the setting that will be used after reboot and correct it before reloading.
What value is used for password recovery?
0x2142 is the common classic-IOS recovery value because it ignores the NVRAM startup configuration. Restore the documented production setting afterward.
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