How Cisco Routers Boot IOS Software?
In short, Cisco routers can boot Cisco IOS software from:
1. Flash memory
2. TFTP server
3. ROM (not full Cisco IOS)
Normally it’s booted from Flash RAM. But if it fails, it tries to load IOS from tftp server.
If this also fails, it boots a “small” ios which is in ROM.
1. NVRAM (check config-register)
2. FLASH (load IOS)
3. TFTP (boot IOS from tftp if FLASH fails)
4. ROM (If TFTP fails, load “mini” IOS from ROM)
Booting up the router and locating the Cisco IOS
1. POST (power on self-test)
2. Bootstrap code executed
3. Check Configuration Register value (NVRAM) which can be modified using the config-register command
0 = ROM Monitor mode
1 = ROM IOS
2 – 15 = startup-config in NVRAM
4. Startup-config file: Check for boot system commands (NVRAM)
If boot system commands in startup-config
a. Run boot system commands in order they appear in startup-config to
locate the IOS
b. If boot system commands fail, use default fallback sequence to locate
the IOS (Flash, TFTP, and ROM)
If no boot system commands in startup-config use the default fallback
sequence in locating the IOS:
a. Flash (sequential)
b. TFTP server (netboot)
c. ROM (partial IOS) or keep retrying TFTP depending upon router model
5. If IOS is loaded, but there is no startup-config file, the router will use the default fallback sequence for locating the IOS and then it will enter setup mode or the setup dialogue.
6. If no IOS can be loaded, the router will get the partial IOS version from ROM
Default (normal) Boot Sequence
Power on Router – Router does POST – Bootstrap starts IOS load – Check configuration register
to see what mode the router should boot up in (usually 0×2102 to read startup-config in NVRAM / or 0×2142 to start in “setup-mode”) – check the startup-config file in NVRAM for boot-system commands – load IOS from Flash.
Boot System Commands
Router(config)# boot system flash IOS filename – boot from FLASH memory
Router(config)# boot system tftp IOS filename tftp server ip address – boot from a TFTP server
Router(config)# boot system rom – boot from system ROM
When a router starts up it needs to know which software to load and which configuration file to use. The way it determines these two things is by looking at its configuration register setting and its startup configuration in nonvolatile RAM (NVRAM).
When a router is booting up, it searches its configuration register to determine where to find the Cisco IOS Software. After the Cisco IOS Software has been loaded, the router attempts to load the configuration file, if one exists. This file contains all the user-specified configuration information for the router. Of course, it’s not the only way; now, let’s go through to the Cisco routers booting process.
1. Flash memory
2. TFTP server
3. ROM (not full Cisco IOS)
Normally it’s booted from Flash RAM. But if it fails, it tries to load IOS from tftp server.
If this also fails, it boots a “small” ios which is in ROM.
1. NVRAM (check config-register)
2. FLASH (load IOS)
3. TFTP (boot IOS from tftp if FLASH fails)
4. ROM (If TFTP fails, load “mini” IOS from ROM)
Booting up the router and locating the Cisco IOS
1. POST (power on self-test)
2. Bootstrap code executed
3. Check Configuration Register value (NVRAM) which can be modified using the config-register command
0 = ROM Monitor mode
1 = ROM IOS
2 – 15 = startup-config in NVRAM
4. Startup-config file: Check for boot system commands (NVRAM)
If boot system commands in startup-config
a. Run boot system commands in order they appear in startup-config to
locate the IOS
b. If boot system commands fail, use default fallback sequence to locate
the IOS (Flash, TFTP, and ROM)
If no boot system commands in startup-config use the default fallback
sequence in locating the IOS:
a. Flash (sequential)
b. TFTP server (netboot)
c. ROM (partial IOS) or keep retrying TFTP depending upon router model
5. If IOS is loaded, but there is no startup-config file, the router will use the default fallback sequence for locating the IOS and then it will enter setup mode or the setup dialogue.
6. If no IOS can be loaded, the router will get the partial IOS version from ROM
Default (normal) Boot Sequence
Power on Router – Router does POST – Bootstrap starts IOS load – Check configuration register
to see what mode the router should boot up in (usually 0×2102 to read startup-config in NVRAM / or 0×2142 to start in “setup-mode”) – check the startup-config file in NVRAM for boot-system commands – load IOS from Flash.
Boot System Commands
Router(config)# boot system flash IOS filename – boot from FLASH memory
Router(config)# boot system tftp IOS filename tftp server ip address – boot from a TFTP server
Router(config)# boot system rom – boot from system ROM
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