A router typically goes through five steps when booting up:
1. The router loads and runs POST (located in ROM), testing its hardware components, including memory and interfaces.
2. The bootstrap program is loaded and executed.
3. The bootstrap program finds and loads an IOS image: Possible locations of the IOS image include flash, a TFTP server, or the Mini-IOS in ROM.
4. Once the IOS is loaded, the IOS attempts to find and load a configuration file, which is normally stored in NVRAM if the IOS cannot find a configuration file, it starts up the System Configuration Dialog.
5. After the configuration is loaded, you are presented with the CLI interface (remember that the first mode you are placed into is User EXEC mode.
If you are connected to the console port, you’ll see the following output as your router boots up:
System Bootstrap, Version 11.0(10c), SOFTWARE
Copyright (c) 1986-1996 by cisco Systems
2500 processor with 6144 Kbytes of main memory
F3: 5593060+79544+421160 at 0×3000060
Cisco Internetwork Operating System Software
IOS ™ 2500 Software (C2500-I-L), Version 12.0(5)
Copyright (c) 1986-1999 by cisco Systems, Inc.
Compiled Tue 15-Jun-99 19:49 by phanguye
Image text-base: 0×0302EC70, data-base: 0×00001000
<–output omitted–>
cisco 2501 (68030) processor (revision N) with 6144K/2048K bytes of memory.
Processor board ID 18086269, with hardware revision 00000003
Bridging software.
X.25 software, Version 3.0.0.
Basic Rate ISDN software, Version 1.1.
1 Ethernet/IEEE 802.3 interface(s)
2 Serial network interface(s)
32K bytes of non-volatile configuration memory.
16384K bytes of processor board System flash (Read ONLY)
00:00:22: %LINK-3-UPDOWN: Interface Ethernet0, changed state to up
00:00:22: %LINK-3-UPDOWN: Interface Serial0, changed state to up
00:00:22: %LINK-3-UPDOWN: Interface Serial1, changed state to up
00:00:23: %LINEPROTO-5-UPDOWN: Line protocol on Interface Ethernet0, changed state to up
00:03:13: %LINK-5-CHANGED: Interface Serial0, changed state to administratively down
00:03:13: %LINK-5-CHANGED: Interface Serial1, changed state to administratively down
Cisco Internetwork Operating System Software
IOS ™ 2500 Software (C2500-I-L), Version 12.0(5)
Copyright (c) 1986-1999 by cisco Systems, Inc.
Compiled Tue 15-Jun-99 19:49 by phanguye
Press RETURN to get started!
There are a few things to point out here. First, notice that the router is loading the bootstrap program (System Bootstrap, Version 11.0(10c)) and then the IOS image (IOS ™ 2500 Software (C2500-I-L), Version 12.0(5)). During the bootup process, you cannot see the actual POST process.
However, you will see information about the interfaces going up and/or down this is where the IOS is loading the configuration and bringing up those interfaces that you previously activated.
Sometimes, if the router has a lot of interfaces, the Press RETURN to get started! message is mixed in with the interface messages. Once the display stops, just hit ENTER to access User EXEC mode. This completes the bootup process of the router.
Bootstrap Program
As you saw in the bootup code example, the bootstrap program went out and found the IOS and loaded it. The bootstrap program goes through the following steps when trying to locate and load the IOS image:
1. Examine the configuration register value. This value is a set of four hexadecimal digits. The last digit affects the bootup process. If the last digit is between 0×2 and 0xF, then the router proceeds to the next step. Otherwise, the router uses the values shown in Table 6-1 to determine how it should proceed next.
2. Examine the configuration file in NVRAM for boot system commands, which tell the bootstrap program where to find the IOS. These commands are shown in the following paragraph.
3. If no boot system commands are found in the configuration file in NVRAM, use the first valid IOS image found in flash.
4. If there are no valid IOS images in flash, generate a TFTP local broadcast to locate a TFTP server (this is called a netboot and is not recommended because it is very slow and not very reliable for large IOS images).
5. If no TFTP server is found, load the Mini-IOS in ROM (RXBOOT mode).
6. If there is Mini-IOS in ROM, then the Mini-IOS is loaded and you are taken into RXBOOT mode; otherwise, the router either retries finding the IOS image or loads ROMMON and goes into ROM Monitor mode.
As below table contains the three common configuration register values in the fourth hex character of the configuration register that are used to influence the bootup process. The values in the configuration register are represented in hexadecimal, the register being 16 bits long.
For step 2 of the bootup process described in the last paragraph, here are the boot system commands that you can use to influence the order that the bootstrap program should use when trying to locate the IOS image:
Router(config)# boot system flash name_of_IOS_file_in_flash
Router(config)# boot system tftp IOS_image_name IP_address_of_server
Router(config)# boot system rom
The boot system flash command tells the bootstrap program to load the specified IOS file in flash when booting up.
Note that, by default, the bootstrap program loads the first valid IOS image in flash. This command tells the bootstrap program to load a different image. You might need this if you perform an upgrade and you have two IOS images in flash the old one and new one.
By default, the old one still loads first unless you override this behavior with the boot system flash command or delete the old IOS flash image.
You can also have the bootstrap program load the IOS from a TFTP server this is not recommended for large images, since the image is downloaded via the UDP protocol, which is slow.
And last, you can tell the bootstrap program to load the Mini-IOS in ROM with the boot system rom command. To remove any of these commands, just preface them with the no parameter.
1. The router loads and runs POST (located in ROM), testing its hardware components, including memory and interfaces.
2. The bootstrap program is loaded and executed.
3. The bootstrap program finds and loads an IOS image: Possible locations of the IOS image include flash, a TFTP server, or the Mini-IOS in ROM.
4. Once the IOS is loaded, the IOS attempts to find and load a configuration file, which is normally stored in NVRAM if the IOS cannot find a configuration file, it starts up the System Configuration Dialog.
5. After the configuration is loaded, you are presented with the CLI interface (remember that the first mode you are placed into is User EXEC mode.
If you are connected to the console port, you’ll see the following output as your router boots up:
System Bootstrap, Version 11.0(10c), SOFTWARE
Copyright (c) 1986-1996 by cisco Systems
2500 processor with 6144 Kbytes of main memory
F3: 5593060+79544+421160 at 0×3000060
Cisco Internetwork Operating System Software
IOS ™ 2500 Software (C2500-I-L), Version 12.0(5)
Copyright (c) 1986-1999 by cisco Systems, Inc.
Compiled Tue 15-Jun-99 19:49 by phanguye
Image text-base: 0×0302EC70, data-base: 0×00001000
<–output omitted–>
cisco 2501 (68030) processor (revision N) with 6144K/2048K bytes of memory.
Processor board ID 18086269, with hardware revision 00000003
Bridging software.
X.25 software, Version 3.0.0.
Basic Rate ISDN software, Version 1.1.
1 Ethernet/IEEE 802.3 interface(s)
2 Serial network interface(s)
32K bytes of non-volatile configuration memory.
16384K bytes of processor board System flash (Read ONLY)
00:00:22: %LINK-3-UPDOWN: Interface Ethernet0, changed state to up
00:00:22: %LINK-3-UPDOWN: Interface Serial0, changed state to up
00:00:22: %LINK-3-UPDOWN: Interface Serial1, changed state to up
00:00:23: %LINEPROTO-5-UPDOWN: Line protocol on Interface Ethernet0, changed state to up
00:03:13: %LINK-5-CHANGED: Interface Serial0, changed state to administratively down
00:03:13: %LINK-5-CHANGED: Interface Serial1, changed state to administratively down
Cisco Internetwork Operating System Software
IOS ™ 2500 Software (C2500-I-L), Version 12.0(5)
Copyright (c) 1986-1999 by cisco Systems, Inc.
Compiled Tue 15-Jun-99 19:49 by phanguye
Press RETURN to get started!
There are a few things to point out here. First, notice that the router is loading the bootstrap program (System Bootstrap, Version 11.0(10c)) and then the IOS image (IOS ™ 2500 Software (C2500-I-L), Version 12.0(5)). During the bootup process, you cannot see the actual POST process.
However, you will see information about the interfaces going up and/or down this is where the IOS is loading the configuration and bringing up those interfaces that you previously activated.
Sometimes, if the router has a lot of interfaces, the Press RETURN to get started! message is mixed in with the interface messages. Once the display stops, just hit ENTER to access User EXEC mode. This completes the bootup process of the router.
Bootstrap Program
As you saw in the bootup code example, the bootstrap program went out and found the IOS and loaded it. The bootstrap program goes through the following steps when trying to locate and load the IOS image:
1. Examine the configuration register value. This value is a set of four hexadecimal digits. The last digit affects the bootup process. If the last digit is between 0×2 and 0xF, then the router proceeds to the next step. Otherwise, the router uses the values shown in Table 6-1 to determine how it should proceed next.
2. Examine the configuration file in NVRAM for boot system commands, which tell the bootstrap program where to find the IOS. These commands are shown in the following paragraph.
3. If no boot system commands are found in the configuration file in NVRAM, use the first valid IOS image found in flash.
4. If there are no valid IOS images in flash, generate a TFTP local broadcast to locate a TFTP server (this is called a netboot and is not recommended because it is very slow and not very reliable for large IOS images).
5. If no TFTP server is found, load the Mini-IOS in ROM (RXBOOT mode).
6. If there is Mini-IOS in ROM, then the Mini-IOS is loaded and you are taken into RXBOOT mode; otherwise, the router either retries finding the IOS image or loads ROMMON and goes into ROM Monitor mode.
As below table contains the three common configuration register values in the fourth hex character of the configuration register that are used to influence the bootup process. The values in the configuration register are represented in hexadecimal, the register being 16 bits long.
| Value in Last Digit | Bootup Process |
| 0×0 | Boot the router into ROMMON mode |
| 0×1 | Boot the router into RXBOOT mode using the Mini-IOS |
| 0×2-0xF | Boot the router using the default boot sequence |
Router(config)# boot system flash name_of_IOS_file_in_flash
Router(config)# boot system tftp IOS_image_name IP_address_of_server
Router(config)# boot system rom
The boot system flash command tells the bootstrap program to load the specified IOS file in flash when booting up.
Note that, by default, the bootstrap program loads the first valid IOS image in flash. This command tells the bootstrap program to load a different image. You might need this if you perform an upgrade and you have two IOS images in flash the old one and new one.
By default, the old one still loads first unless you override this behavior with the boot system flash command or delete the old IOS flash image.
You can also have the bootstrap program load the IOS from a TFTP server this is not recommended for large images, since the image is downloaded via the UDP protocol, which is slow.
And last, you can tell the bootstrap program to load the Mini-IOS in ROM with the boot system rom command. To remove any of these commands, just preface them with the no parameter.
