Chabot College
ELEC 99.08
IOS Images
CISCO NETWORKING ACADEMY
IOS Image Topics
• Sources of the IOS Image
• Process for finding the IOS
• Ways to modify the source used to
load the IOS
– Configuration register
– Boot system commands
• Copying the IOS to/from a TFTP
server.
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Sources of the IOS Image
• Routers boot Cisco IOS software from:
– Flash
– TFTP server
– ROM (limited version)
• Having more than one option provides
flexibility and fallback alternatives.
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Finding the IOS
1. Examine configuration register.
Look where it directs.
2. If config register points to NVRAM, examine
NVRAM for boot system commands.
Look where those commands say.
3. If instructions fail, follow a fallback sequence:
First, get IOS from FLASH.
If empty, get IOS from TFTP server or from ROM
(depending on hardware platform)
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The Config Register
• A 16-bit number stored in NVRAM.
• Written as a four digit hexadecimal
number:
0x2102
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The Config Register
• The last four bits of the register are
called the boot field. The boot field
determines where the router should
look for the IOS.
• In hexadecimal, the last digit
represents the value of those four bits:
0x2102
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Where can you find it?
Issue the show version command.
The last line of the display shows the
values of the config register:
• now
• after next reload
oak#sh ver
….
….
….
configuration register is 0x2102 (will be 0x2102 at next
reload)
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The Config Register
• Usually, the last digit is 2:
0x2102
• Effects:
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Setting the Config Register
• Global config mode command:
oak(config)#config-register 0x2102
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Boot System Commands
• If the last digit of the config register is 2,
the router examines NVRAM for boot
system commands entered in the config
file:
oak(config)#boot system flash file-name
oak(config)#boot system tftp file-name 192.168.3.10
oak(config)#boot system rom
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Boot System Flash
• To boot the IOS from flash:
oak(config)#boot system flash file-name
• Notes:
– standard boot location
– not vulnerable to network failures that can
occur when loading system images from
TFTP servers
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Boot System TFTP
• To boot the IOS from a TFTP server:
oak(config)#boot system tftp file-name 192.168.3.10
• Notes:
– Purpose: backup
– Purpose: testing of new IOS versions,
before copying to flash
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Boot System ROM
• To boot the IOS from ROM :
oak(config)#boot system rom
• Notes:
– “last resort” fallback option when other boot
instructions fail
– loads only a “mini” IOS, lacking the features
& protocols of the full version
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Boot System Commands
• The commands take effect in the order
they were entered to in the config file.
• These produce different results:
oak(config)#boot system flash file-name
oak(config)#boot system tftp file-name 192.168.4.10
oak(config)#boot system rom
oak(config)#boot system tftp file-name 192.168.4.10
oak(config)#boot system flash file-name
oak(config)#boot system rom
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Boot System Commands
• The commands are read from NVRAM
at boot time.
• You must save them to NVRAM with
copy run start
oak(config)#boot system flash file-name
oak(config)#boot system tftp file-name 192.168.4.10
oak(config)#boot system rom
oak(config)#^Z
oak#copy run start
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Removing Boot System Commands
• Use no
oak(config)#no boot system flash file-name
oak(config)#no boot system tftp file-name 192.168.4.10
oak(config)#no boot system rom
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Review: Locating the IOS Software
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Review: Locating the IOS Software
• In a default config:
– config register is set to 0x---2
– no boot system commands are present
– so the IOS loads from default sources in an
order determined by the hardware platform:
• 2500 Series (per Cisco 2500 documentation)
– Flash
– ROM
• Others (per text & curriculum)
– Flash
– TFTP
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Copying the IOS to/from TFTP
• Prepare
– start tftp server process
– ping
– show flash
• Copy from FLASH to TFTP server to
create a “backup” copy of the IOS
• Copy from TFTP server to FLASH to
upgrade to a new IOS version.
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Preparing for TFTP
1. If necessary, start the TFTP server
process running:
On the desktop of your console PC, double-click the
shortcut labeled "Shortcut to Tftpsrv”
A window opens, verifying that the server process is
now running.
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Preparing for TFTP
2. ping to verify connectivity from router to
TFTP server:
oak#ping 192.168.4.10
Type escape sequence to abort.
Sending 5, 100-byte ICMP Echos to 192.168.4.10,
timeout is 2 seconds:
.!!!!
Success rate is 80 percent (4/5), round-trip
min/avg/max = 1/2/4 ms
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Preparing for TFTP
3. show flash to
– verify size of IOS file
– verify the exact IOS file name
oak#sh flash
System flash directory:
File Length
Name/status
1
6890600 c2500-d-l.120-10
[6890664 bytes used, 1497944 available, 8388608 total]
8192K bytes of processor board System flash (Read ONLY)
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IOS Naming Convention
• 4 parts:
– Hardware platform
– Special features of image
– Area of memory used, compression status
– Version & release number
c2500-d-l.120-10
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IOS Naming Convention
c2500-d-l.120-10
Version
12(10)
Cisco 2500
Platform
Desktop Feature Set
IP, IPX, Appletalk)
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Memory relocatable,
not compressed
IOS Naming Convention
PPPP = Platform
FFFF = Features
MM = Run-time memory and compression format
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Backing Up
oak#copy flash tftp
Source filename []? c2500-d-l.120.10
Address or name of remote host []? 192.168.4.10
Destination filename [c2500-d-l.120-10]?
!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!
!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!
!!!
6890600 bytes copied in 87.788 secs (79202 bytes/sec)
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Upgrading (Don’t try this in our lab!)
oak#copy tftp flash
• Loads a new IOS image into FLASH,
for regular production use.
• Useful for IOS version upgrades.
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Strategies
• Why load the IOS from different places?
Testing. If your router has enough
FLASH memory to hold two copies of the
IOS:
• Use boot system commands to boot the IOS
temporarily from TFTP to test a new version.
• Copy both an old and a new IOS version from
TFTP server to FLASH. Select among them
using boot system commands.
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Summary
• The source for the Cisco IOS software depends
on the settings of:
– configuration register
– boot system commands
– default “fallback sequence”
• The boot system command boot system tftp
filename IPaddress causes the router to boot
IOS from TFTP server.
• copy flash tftp saves the IOS currently
running on the router to a TFTP server.
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