1. Chabot College
ELEC 99.08
IOS Images

2. 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.

3. 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.

4. 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)

5. The Config Register 0x2102 A 16-bit number stored in NVRAM.
Written as a four digit hexadecimal number:
0x2102

6. 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

7. 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)

8. The Config Register
Usually, the last digit is 2:
0x2102
Effects:

9. Setting the Config Register
Global config mode command:
oak(config)#config-register 0x2102

10. 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
oak(config)#boot system rom

11. Boot System Flash To boot the IOS from flash: Notes:
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

12. Boot System TFTP To boot the IOS from a TFTP server: Notes:
oak(config)#boot system tftp file-name
Notes:
Purpose: backup
Purpose: testing of new IOS versions, before copying to flash

13. Boot System ROM To boot the IOS from ROM : Notes:
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

14. 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
oak(config)#boot system rom
oak(config)#boot system tftp file-name
oak(config)#boot system flash file-name
oak(config)#boot system rom

15. 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
oak(config)#boot system rom
oak(config)#^Z
oak#copy run start

16. Removing Boot System Commands
Use no
oak(config)#no boot system flash file-name
oak(config)#no boot system tftp file-name
oak(config)#no boot system rom

17. Review: Locating the IOS Software

18. 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)
TFTP

19. 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.

20. 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.

21. Preparing for TFTP
2. ping to verify connectivity from router to TFTP server:
oak#ping
Type escape sequence to abort.
Sending 5, 100-byte ICMP Echos to , timeout is 2 seconds:
.!!!!
Success rate is 80 percent (4/5), round-trip min/avg/max = 1/2/4 ms

22. 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
c2500-d-l
[ bytes used, available, total]
8192K bytes of processor board System flash (Read ONLY)

23. IOS Naming Convention c2500-d-l.120-10 4 parts: Hardware platform
Special features of image
Area of memory used, compression status
Version & release number
c2500-d-l

24. Memory relocatable, not compressed
IOS Naming Convention
c2500-d-l
Version
12(10)
Cisco 2500
Platform
Desktop Feature Set
IP, IPX, Appletalk)
Memory relocatable, not compressed

25. IOS Naming Convention
PPPP = Platform FFFF = Features MM = Run-time memory and compression format

26. Backing Up oak#copy flash tftp Source filename []? c2500-d-l.120.10
Address or name of remote host []?
Destination filename [c2500-d-l ]?
!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!
bytes copied in secs (79202 bytes/sec)

27. 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.

28. 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.

29. 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.