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Troubleshooting IOS boot failure

Troubleshooting IOS boot failure
5.1.5 
This page will explain why a router may not boot properly and show students what to do when this occurs.
There are several reasons that a router may not boot properly:
  • Configuration file has missing or incorrect boot system statement
  • Incorrect configuration register value
  • Corrupted flash image
  • Hardware failure
When a router boots, it looks in the startup configuration file for a boot system statement. This boot system statement can force the router to boot from another image instead of the IOS in flash. Use the show version command to look for the line that identifies the boot image source.
Use the show running-config command and look for a boot system statement near the top of the configuration. If the boot system statement points to an incorrect IOS image, use the no version of the command to delete the statement.
If the configuration register setting is incorrect, the IOS cannot load from flash. The value in the configuration register tells the router where to get the IOS. To confirm this, use the show version command and look at the last line for the configuration register. The correct value varies for different hardware platforms. A part of the documentation of the internetwork should be a printed copy of the show version output. If that documentation is not available, there are resources on the Cisco documentation CD or Cisco website to identify the correct configuration register value. To correct this, change the configuration register and save this as the start-up configuration.
If there is still a problem, the router may have a corrupted flash image file. If this is the case, an error message should be displayed during boot. That message may take one of several forms. Some examples are as follows:
  • open: read error...requested 0x4 bytes, got 0x0
  • trouble reading device magic number
  • boot: cannot open "flash:"
  • boot: cannot determine first file name on device "flash:"
If the flash image is corrupt, a new IOS should be uploaded into the router.
If none of the above appears to be the problem, the router could have a hardware failure. If this occurs, contact the Cisco Technical Assistance Center (TAC). Although hardware failures are rare, they do occur.
The value of the configuration register is not displayed by the show running-config or show startup-config commands.
Students can use the Lab Activities to troubleshoot IOS boot failure and document configuration register settings.
This page concludes this lesson. The next lesson will show students how to manage the Cisco IOS File System. The first page will give an overview of the IOS File System.

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