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CISCO NETWORKING ACADEMYCISCO NETWORKING ACADEMY

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ELEC 99.08ELEC 99.08IOS Images


IOS Image TopicsIOS 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.


Sources of the IOS ImageSources 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.


Finding the IOSFinding the IOS1. 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)


The Config RegisterThe Config Register

• A 16-bit number stored in NVRAM.

• Written as a four digit hexadecimal number:

0x2102


The Config RegisterThe 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


Where can you find it?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)


The Config RegisterThe Config Register• Usually, the last digit is 2:

0x2102• Effects:


Setting the Config RegisterSetting the Config Register• Global config mode command:

oak(config)#config-register 0x2102


Boot System CommandsBoot 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-nameoak(config)#boot system tftp file-name 192.168.3.10oak(config)#boot system rom


Boot System FlashBoot 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


Boot System TFTPBoot 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


Boot System ROMBoot 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


Boot System CommandsBoot System Commands

oak(config)#boot system flash file-nameoak(config)#boot system tftp file-name 192.168.4.10oak(config)#boot system rom

• The commands take effect in the order they were entered to in the config file.

• These produce different results:

oak(config)#boot system tftp file-name 192.168.4.10oak(config)#boot system flash file-nameoak(config)#boot system rom


Boot System CommandsBoot System Commands

oak(config)#boot system flash file-nameoak(config)#boot system tftp file-name 192.168.4.10oak(config)#boot system romoak(config)#^Zoak#copy run start

• The commands are read from NVRAM at boot time.

• You must save them to NVRAM with copy run start


Removing Boot System CommandsRemoving Boot System Commands

oak(config)#no boot system flash file-nameoak(config)#no boot system tftp file-name 192.168.4.10oak(config)#no boot system rom

• Use no


Review: Locating the IOS SoftwareReview: Locating the IOS Software


Review: Locating the IOS SoftwareReview: 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


Copying the IOS to/from TFTPCopying 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.


Preparing for TFTPPreparing for TFTP1. 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.


Preparing for TFTPPreparing 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


Preparing for TFTPPreparing 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)


IOS Naming ConventionIOS Naming Convention

• 4 parts:– Hardware platform– Special features of image– Area of memory used, compression status– Version & release number

c2500-d-l.120-10


IOS Naming ConventionIOS Naming Convention

c2500-d-l.120-10

Cisco 2500Platform

Desktop Feature SetIP, IPX, Appletalk)

Memory relocatable, not compressed

Version12(10)


IOS Naming ConventionIOS Naming ConventionPPPP = Platform FFFF = Features

MM = Run-time memory and compression format


Backing UpBacking Up

oak#copy flash tftpSource filename []? c2500-d-l.120.10Address or name of remote host []? 192.168.4.10Destination filename [c2500-d-l.120-10]? !!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!6890600 bytes copied in 87.788 secs (79202 bytes/sec)


Upgrading Upgrading (Don’t try this in our lab!)(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.


StrategiesStrategies• 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.


SummarySummary

• 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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