You want to boot using an alternate IOS image.
To specify which IOS image the router should load next time it reboots, use the boot system command:
Router1#configure terminal Enter configuration commands, one per line. End with CNTL/Z. Router1(config)#boot system flash:c3620-jk9o3s-mz.122-7a.bin Router1(config)#boot system flash:c3620-jos56i-l.120-11.bin Router1(config)#boot system slot0:c3620-ik9s-mz.122-13.bin Router1(config)#boot system rom Router1(config)#end
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The router can store as many IOS images in its flash memory as there is space to hold. If there is only one file, it can safely assume that this must be the IOS image to load. However, if the router has several images in its flash storage, you need to specify which one it should load, or the router will simply select one. This is particularly true on routers that have additional flash memory in the form of PCMCIA cards, which can hold many files�not all of them are necessarily IOS images.
With the default configuration register settings, the router will attempt to load the first accessible IOS image it finds in its flash storage. However, loading the first available image might not be appropriate. For instance, in our last recipe we showed that if you have space, you can download a new IOS image without erasing old images. In this case, you probably want the router to load the newer IOS image. It would be better still if the router tried the new image first, then reverted to the old image if the new one failed to load correctly for any reason. The boot system command allows you to specify not only which IOS images to boot from, but also the order in which to try them if the router has trouble booting.
In the example, this router tries a succession of three different IOS images. If they all fail, it resorts to using its boot ROM image.
As we noted earlier, the sequence of the boot system commands is important since the router will attempt to load the IOS images in order of entry. This means that the only way to add a new IOS image to the start of the list is to remove all of the old boot system commands and reenter them again in the order of preference. You can remove all of the boot system commands at once with the following command:
Router1#configure terminal Enter configuration commands, one per line. End with CNTL/Z. Router1(config)#no boot system Router1(config)#end Router1#
Once the old boot system commands have been removed, you can configure a new set in whatever order you require.
In addition to allowing you to boot from IOS images in the router's flash storage, you can also use the boot system command to boot from the IOS image in its ROM storage, or by using the TFTP or remote copy (RCP) protocols across the network. Recipe 1.8 shows an example of booting across the network using TFTP. Table 1-1 shows the options for the boot system command.
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Keyword |
Description |
|---|---|
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On-board flash | |
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PCMCIA flash card in slot0 | |
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PCMCIA flash card in slot1 | |
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Load an image using the MOP protocol | |
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Load bootflash image (not available on all systems) | |
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Load the image from ROM | |
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Load an image using the RCP | |
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Load an image using the TFTP protocol |
In addition to the boot system commands, you can use the router's configuration register to change which image the router boots from. The last octet in the configuration register must be set to "2" or the router will completely ignore the boot system commands. For instance, if the last octet of the configuration register is set to "1", the router will boot from ROM and ignore the boot system commands. In our example, the test router's configuration register was set to 0x2102. The config-register command allows you to set the appropriate configuration register values:
Router1#configure terminal
Enter configuration commands, one per line. End with CNTL/Z.
Router1(config)#config-register 0x2102
Router1(config)#end
Router1#
It is important to remember that, unlike any other configuration command, you don't need to save the running configuration to NVRAM when you change the configuration register setting. It will survive a reload without being saved. In fact, the new setting will not take effect until after the next reload:
Router1#show version Cisco Internetwork Operating System Software IOS (tm) 3000 Bootstrap Software (IGS-RXBOOT), Version 10.2(8a), RELEASE SOFTWAR E (fc1) Copyright (c) 1986-1995 by cisco Systems, Inc. Compiled Tue 24-Oct-95 15:46 by mkamson Image text-base: 0x01020000, data-base: 0x00001000 ROM: System Bootstrap, Version 5.2(8a), RELEASE SOFTWARE Router1 uptime is 2 minutes System restarted by reload Running default software cisco 2500 (68030) processor (revision D) with 16380K/2048K bytes of memory. Processor board serial number 04915359 with hardware revision 00000000 X.25 software, Version 2.0, NET2, BFE and GOSIP compliant. 2 Ethernet/IEEE 802.3 interfaces. 2 Serial network interfaces. 32K bytes of non-volatile configuration memory. 16384K bytes of processor board System flash (Read/Write) Configuration register is 0x2101 (will be 0x2102 at next reload) Router1#
After setting the appropriate boot system commands and reloading the router, you can use the show version command to see which image file the router used to boot:
Router2#show version Cisco Internetwork Operating System Software IOS (tm) 3600 Software (C3620-IK9S-M), Version 12.2(13), RELEASE SOFTWARE (fc1) Copyright (c) 1986-2002 by cisco Systems, Inc. Compiled Tue 19-Nov-02 19:04 by pwade Image text-base: 0x60008930, data-base: 0x61276000 ROM: System Bootstrap, Version 11.1(19)AA, EARLY DEPLOYMENT RELEASE SOFTWARE (fc1) Router2 uptime is 2 hours, 4 minutes System returned to ROM by reload System restarted at 21:13:13 EST Wed Jan 15 2003 System image file is "slot0:c3620-ik9s-mz.122-13.bin" cisco 3620 (R4700) processor (revision 0x81) with 41984K/7168K bytes of memory. Processor board ID 05969532 R4700 CPU at 80Mhz, Implementation 33, Rev 1.0 Bridging software. X.25 software, Version 3.0.0. SuperLAT software (copyright 1990 by Meridian Technology Corp). Basic Rate ISDN software, Version 1.1. 1 Ethernet/IEEE 802.3 interface(s) 1 FastEthernet/IEEE 802.3 interface(s) 1 Serial network interface(s) 1 ISDN Basic Rate interface(s) DRAM configuration is 32 bits wide with parity disabled. 29K bytes of non-volatile configuration memory. 16384K bytes of processor board System flash (Read/Write) 16384K bytes of processor board PCMCIA Slot0 flash (Read/Write) 16384K bytes of processor board PCMCIA Slot1 flash (Read/Write) Configuration register is 0x2102 Router2#
In this case, the router says that it loaded its IOS image from slot0:, as configured. After changing your boot system commands, you should make sure to reboot and verify that the router behaves as expected. You don't want the wrong IOS image to accidentally get loaded the next time the router reboots. If you do have problems with the boot system command, connect to the console and reload the router. This will display any error messages as the router boots. The router does not capture these messages anywhere; this is the only way to see them.
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