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Router reloads and boots in ROM Monitor (ROMmon # >) mode unexpectedly with a bad checksum error message


Core Issue

There are two causes for this problem:

  • The image on the flash is corrupted.

    This applies to all router platforms.V
    erify the image size and checksum against information provided in the IOS Upgrade Planner.

  • There is a parity error in transfer from Flash to RAM.

    Memory controllers on c2600, c3600 and c3700 routers do not have support for parity checking. If one or several bits of data are altered during the transfer from Flash to RAM, the router calculates a wrong checksum for the image and aborts the booting process. These are among the possible causes for the parity error:

  • Soft error. If the error happens only once a week, it should be treated as a soft (or transient) error. If there is bad contact between the memory chip and motherboard, reseating the memory chip resolves the problem.

  • A faulty memory chip. In this case, the memory should be replaced.

  • A faulty memory controller or circuitry on the motherboard. In this case, the motherboard should be replaced.


To resolve this issue, perform these steps:

  1. Check the Cisco IOS  Software image checksum and size by issuing the dir flash command in ROMmon mode.

    Compare values with Cisco IOS Software image information in the IOS Upgrade Planner.

  2. Reseat RAM chip(s). Refer to Installing memory in the Cisco 3600 series router.

  3. Replace RAM chip(s).

When Problem Occurs