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Configuration register

Configuration register. Configuration register. Is located in NVRAM Is inspected during bootstrap process Controls configuration of the router Is a 16-bit register Setting can be inspected using the show version command. Configuration register layout. 0x2102.

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Configuration register

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  1. Configuration register

  2. Configuration register • Is located in NVRAM • Is inspected during bootstrap process • Controls configuration of the router • Is a 16-bit register • Setting can be inspected using the show version command

  3. Configuration register layout 0x2102 • The prefix 0x has no value other than indicating that “the following values are in hex” • The 16-bits are represented as four hex numbers each four bits long

  4. Configuration register composition 0x2102 Hex Binary 2 1 0 2 0010 0001 0000 0010 boot field

  5. Router boot field

  6. Router boot field

  7. Other configuration register bits explained

  8. Standard configuration register settings 0x2102 Binary 0010 0001 0000 0010 Rom boot enable Console speed 9600 baud Break enabled Ignore configuration disabled Boot from Flash memory

  9. Running Startup IOS POST configuration configuration Configuration reads reads BOOTSTRAP register loads IOS loads running configuration Router start-up sequence NVRAM RAM ROM 

  10. Password recovery

  11. Password recovery on 2500 series routers • Restart the router • (type reload or power cycle the router) • Hit Cntl + Break keys within 60 seconds • Note: This will only work in some terminal emulations (e.g. QVT term) • Set the configuration register to 0x2142 • on 2500 routers issue o/r 0x2142 • This will set the configuration register bit 6 and ignore the startup-configuration

  12. Password recovery on 2500 series routers - II • Type i to reload the router • The router will boot and ignore the startup-config • Enter enable mode • Change the enable secret password in the running-config • Change the configuration register to 0x2102 • Copy the running-config into the startup-config • This reverses the earlier setting and the router will use the new enable password in future logins

  13. Password recovery on 2600 series routers • Restart the router • (type reload or power cycle the router) • Hit Cntl + Break keys within 60 seconds • Note: This will only work in some terminal emulations (e.g. QVT term) • The prompt will show ROM monitor mode • Issue the command confreg to change the configuration register settings • Menu-driven process in which [y/n] answers are used to specify “ignore startup-config”

  14. Password recovery on 2600 series routers - II • Type reset to reload the router • The router will boot and ignore the startup-config • Enter enable mode • Change the enable secret password in the running-config • Change the configuration register to 0x2102 • Copy the running-config into the startup-config • This reverses the earlier setting and the router will use the new enable password in future logins

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