Cisco ROMMON Recovery

A router is in ROM monitor “ROMMON” mode when you see the "rommon#>" prompt. “rommon” is the same as BIOS for PCs, it is a fallback mechanism for a router when it fails to boot normally due to some reason.

One of the following may be reason for router “rommon” mode:

  • The configuration register “config-reg” is set to boot the router in this mode. Check if the last 4 bits are 0 then it is forcing the router to boot from ROM. For normal boot it should be set to  0x02.
  • Router dynamic RAMDRAM” is bad or faulty.
  • Router may have crashed, may be due to a power outage and is unable to recover.
  • Router IOS image is either corrupted, not valid or not present. Most common cause of a router going into “rommon” or “boot” mode is due to corrupted IOS image resulted due to a failed attempt to upgrade Cisco IOS Software.
  • Similarly, if you are trying to boot your router over the network, check that the TFTP server you are using is reachable when the router comes up.

Here, I described steps for trying to load valid IOS image from the flash.
First of all look for all available devices on your router by giving
 dev command:

rommon 2 >dev 
Devices in device table:
 
idname
 
flash:flash
 
slot0:PCMCIA slot 0
 
slot1:PCMCIA slot 1
 
eprom:EPROM
 
rommon 3 >

Give dir [device ID] command for each Flash or PCMCIA device available, and then look for a valid Cisco IOS software image:

rommon 4 >dir flash: 
         File size           Checksum   File name
 
   35865457 bytes (0x804b4c)   0x6ba0    c3825-adventerprisek9-mz.124-10.bin  
rommon 5 >

Try to boot from the available IOS image in the flash. If the IOS image file is valid, after boot router will resume in normal operation mode:

rommon 6 >boot flash:c3825-adventerprisek9-mz.124-10.bin  
program load complete, entry point: 0x80008000, size: 0x804a30
 
Self decompressing the image : ###################################
 
########...
 

If no valid IOS image is available, then you will need to download a new valid IOS image. A valid Cisco IOS image can be obtained from another similar router, or router with a compatible PCMCIA Flash card file system.

  • If both routers are identical or same series, you can use the Flash card from the other router in order to boot the one you want to recover. Routers run their Cisco IOS software from dynamic RAM (DRAM), so you can remove a PCMCIA card while the router runs.
  • If both routers are different but have a compatible PCMCIA Flash card file system, you can use the other router to load a Cisco IOS Software image into a Flash card, which you can then use with the other router you want to recover.

To copy the image into the PCMCIA card from a working router.

Router#copy flash:c3825-adventerprisek9-mz.124-10.bin disk0:
Insert the PCMCIA card into the router in ROMMON mode and give the boot command:
rommon 5 >boot disk0:c3825-adventerprisek9-mz.124-10.bin  
program load complete, entry point: 0x80008000, size: 0x804a30
 
Self decompressing the image : ###################################
 
########...

When router is up you can copy the image to Flash and set the boot variable so that the router boots through this new image every time it is rebooted.

myrouter#copy disk0:c3845-adventerprisek9-mz.124-10.bin flash:c3825-adventerprisek9-mz.124-10.bin
myrouter#configure terminal
Enter configuration commands, one per line.  End with
CNTL/Z. 
myrouter config)#no boot system
myrouter (config)#boot system flash c3845-adventerprisek9-mz.124-10.bin
myrouter (config)#^Z

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