Module 5
Managing Cisco IOS
Software
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Default sequence for Booting the IOS Image
Do not confuse the IOS image with the IOS configuration that boots from
NVRAM
Boot System Commands – globally entered and stored in
NVRAM in the configuration file
Flash memory
Tftp Server
ROM (limited image)
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Locating and Loading the IOS Image
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Boot Options
Remember multiple
images can be stored,
so specify name
Specify name of image,
plus IP address of
server
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Boot System Commands
• To find out if there are boot system commands
configured on a router, enter any one of the following
commands:
– show running-config
– show config
– show startup-config
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Configuration Register
• The order in which the router looks for system bootstrap
information depends on the boot field
• Setting is in the configuration register. The config register
number is in NVRAM
• Default is usually 0x2102 or 0x102 for routers with built-in
flash
• Router command “show version” will tell you what the Hex
number is
Router(config)#config-register 0x101 (to boot from ROM)
Router(config)#config-register 0x102 (or 0x2102 to boot from
NVRAM)
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When the router doesn’t boot properly
• Could be:
– Config file is missing
– Incorrect boot system statement in the
configuration file
– Incorrect config-register setting
– Flash image itself could be corrupted
– Hardware failure
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IOS File System
• Routers require both a configuration file and an operating
system file in order to work properly
– The configuration file resides in NVRAM
• This startup file is copied into RAM when the router boots
• Now the config becomes the “running config”.
– The IOS resides in flash memory
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Backing Up Configuration Files
• It is a good idea to use a TFTP server to back up the
configuration files to, and then to copy them back to startup or
running configuration.
• To copy to the tftp server
Router(config)#copy running-config tftp
• To copy from the tftp server
Router(config)#copy tftp running-config
• Before doing this, you should erase the backup (startup) config
in NVRAM and reboot so that there is no configuration in either
place.
Router(config)#erase startup-config
Router(config)#reload
• You can also copy and paste the configuration files onto a
floppy or other disk so that they will be available.
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Using TFTP to Upload an IOS
Before using TFTP to upload an IOS:
• Check memory first (show flash)
• Ping the IP address of your TFTP server
• Know the name of the file you are copying from, make
sure it is in the default directory
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Backing Up the IOS to a TFTP Server
It is a good idea to back up the current IOS before copying
a new one, in case the copy fails - flash is erased BEFORE
the actual copy begins.
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Upgrading the IOS from TFTP
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X-MODEM
• If the IOS image in flash has been erased or corrupted, the IOS
may need to be restored from the ROM monitor mode
(ROMmon).
– First, find out why the image didn’t load with:
dir flash: command
– If there is an image found, use:
boot flash: (then the name of the image)
– If it boots this time, check the config-register value with the
show version command first.
– If the register is correct, use show running-config to
see if there are boot system commands present.
• If you determine that a new image has to be loaded, you can
use the X-Modem method
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X-Modem
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