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.

Configuration register

Configuration register

5.1.4

This page will explain how a router uses the configuration register. Students will also learn how to change the boot field. The order in which the router looks for system bootstrap information depends on the boot field setting in the configuration register. The default configuration register setting can be changed with the global configuration mode command config-register. Use a hexadecimal number as the argument for this command. The configuration register is a 16-bit register in NVRAM that is represented as 4 hexadecimal digits. The lowest four bits of the configuration register form the boot field. To ensure that the upper 12 bits are not changed, first use the show version command to retrieve the current values of the configuration register. Then use the config-register command and change only the value of the last hexadecimal digit. To change the boot field in the configuration register, follow these guidelines:  To enter the ROM monitor mode, set the configuration register value to 0xnnn0, where nnn represents the previous value of the non-boot field digits. This value sets the boot field bits to 0000 binary. After a reload or power cycle, the device will boot to ROM monitor prompt. Use the b command to boot the operating system manually. To boot from the first image in Flash or to boot to the IOS in ROM (platform dependant), set the configuration register to 0xnnn1, where nnn represents the previous value of the non-boot field digits. This value sets the boot field bits to 0001 binary. Older platforms, such as Cisco 1600 and 2500 routers, will boot to a limited IOS in ROM. Newer platforms, such as Cisco 1700, 2600 and high end routers, will boot from the first image in Flash. To configure the system to use the boot system commands in NVRAM, set the configuration register to any value from 0xnnn2 to 0xnnnF, where nnn represents the previous value of the non-boot field digits. These values set the boot field bits to a value between 0010 and 1111 binary. The router sequentially processes each boot system command in NVRAM until the process is successful or the end of the list is reached. If there are no boot system commands in the startup configuration file, the router attempts to boot the first file in flash memory. The Lab Activity will require students to change the boot process of a router. The next page will teach students how to troubleshoot IOS boot failures.

Using the boot system command

Using the boot system command

5.1.3

This page will explain how boot system commands are used to specify the fallback boot-up sequence for Cisco IOS software.
The three figures show boot system entries. These entries specify that a Cisco IOS software image will load first from flash memory, then from a network server, and finally from ROM:

• Flash memory - A system image can be loaded from flash memory. Information stored in flash memory is not vulnerable to network failures that can occur when system images are loaded from TFTP servers. 
• Network server - If flash memory is corrupted, a system image can be loaded from a TFTP server. 
• ROM - The final bootstrap option is to boot from ROM. However, a system image in ROM is usually a subset of the Cisco IOS that lacks the protocols, features, and configurations of the full Cisco IOS. Also, if the software has been updated, a router may have an older version stored in ROM.  

The command copy running-config startup-config saves the commands in NVRAM. The router will execute the boot system commands as needed in the order in which they were originally entered into configuration mode.
In the Lab Activities, students will use boot system commands to display information about the Cisco IOS image and boot procedure of a router.
The next page will describe the router configuration register.