Core Issue
Cisco routers such as the 2500, AS5200, and some 1600 series, run the Cisco IOS from the Flash rather than from RAM. Because of this, Flash is in a Read Only state while the Cisco IOS Software is running on the router. Attempting to write to or erase the Flash results in an error message similar this example:
Router#erase flash:
Erasing the flash filesystem will remove all files! Continue? [confirm]
%dev_open: System flash not writable
%Error erasing flash: (Device is read-only)
Router#
Resolution
If the above error message occurs, perform these steps:
- Establish a console session to the router.
- Put the router into boot mode by issuing the config-register 0x2101 global configuration command as shown in this example:
2500>enable
2500#configure terminal
2500(config)#config-register 0x2101
2500(config)#^Z
!--- Press Ctrl-Z to exit.
Note: Do not save the configuration while in boot mode, as this may result in the loss of configuration data in NVRAM.
- Issue the show version command at the next reload to verify the configuration register setting is correct. Look at the last line of the output, as shown in this example:
2500#show version
Cisco Internetwork Operating System Software
IOS (tm) 2500 Software (C2500-IS-L), Version 11.3(11)T, RELEASE SOFTWARE (fc1)
Copyright (c) 1986-1999 by cisco Systems, Inc.
Compiled Mon 26-Jul-99 18:19 by pwade
Image text-base: 0x0303B008, data-base: 0x00001000
ROM: System Bootstrap, Version 11.0(10c)XB1, PLATFORM SPECIFIC RELEASE SOFTWARE)
BOOTFLASH: 3000 Bootstrap Software (IGS-BOOT-R), Version 11.0(10c)XB1,PLATFORM
2500 uptime is 1 year, 2 weeks, 5 days, 22 hours, 3 minutes
System restarted by power-on
System image file is "flash:c2500-is-l.113-11.T.bin", booted via flash
cisco AS2511-RJ (68030) processor (revision K) with 14336K/2048K bytes of memory.
Processor board ID 22849251, with hardware revision 00000000
Bridging software.
X.25 software, Version 3.0.0.
1 Ethernet/IEEE 802.3 interface(s)
1 Serial network interface(s)
16 terminal line(s)
32K bytes of non-volatile configuration memory.
8192K bytes of processor board System flash (Read Only)
Configuration register is 0x2102 (will be 0x2101 at next reload)
The last line verifies that the configuration register will be0x2101 after the reload. Notice that the Flash shows Read Only.
- Reload the router. You may be prompted to save configuration changes. It is not necessary to save the configuration for the configuration register setting to take effect, as shown in this example:
2500#reload
When the router reloads, the prompt should display (boot), indicating that the router is in boot mode.
- Issue another show version command to ensure the Flash shows Read/Write. The last two lines should now appear like this:
2500(boot)>enable
2500(boot)#show version
8192K bytes of processor board System flash (Read/Write)
Configuration register is 0x2101
At this point, writing to or erasing Flash should be possible.
- After performing the desired Flash procedure, change the configuration register back to 0x2102 in order to boot from the Flash, as shown in this example:
2500(boot)#configure terminal
2500(boot)(config)#config-register 0x2102
2500(boot)(config)#^Z
!--- Press Ctrl-Z to exit.
Reload the router. Do not save the changes. Saving changes while in boot mode may result in a loss of configuration.
2500(boot)#reload
When the router reloads, the router should be running from the Flash and the Flash should show Read Only again.
- If the Flash shows Read Only even after going to boot mode, there may be an issue with the Flash. Power down the router and reseat the Flash Single In-line Memory Modules (SIMMs). If it still shows Read Only, the Flash may need to be replaced.
