Cisco Router Boot up Sequence
All Cisco devices run on IOS. The knowledge of boot up sequence of a router helps in troubleshooting a router.
The event sequence that takes once the router is turned ON helps the router hardware to load the operating system. The Operating system in case of a router is IOS (Internetwork Operating System). All Cisco devices run on IOS. The knowledge of boot up sequence helps in troubleshooting a router.
The sequences of events that occur when a router is turned are as follows
1. Power On Self Test (POST)
2. Loading and executing Bootstrap
3. Finding operating systems
4. Loading operating systems
5. Finding the configurations
6. Loading the configurations
Power on Self Test:
Power-on self-test is abbreviated as POST. This event in boot sequence verifies and ensures that all the components present in the router are working properly. During POST, the router determines all the components that are present on the router. POST makes use of ROM monitor code present in Read Only Memory
Loading and executing bootstrap:
POST is followed by loading and executing bootstrap in the sequence. Bootstrap is stored in ROM memory. It plays a key role in finding and loading Internetwork Operating System. It checks for the register values where an operating system is present. By default, the register value is 2012 in hexadecimal. Once operating system is loaded, bootstrap becomes idle and not executed until next power cycle.
Finding Operating System:
Bootstrap finds the location of operating system with the help of value present in the configuration register. The operating system is generally located in the Flash memory of the router. If the router does not have flash memory, the router looks for TFTP server to load IOS image. If the router did not find Internetwork Operating System in any one of the locations, bootstrap fails and runs ROM MON code present in Read Only Memory of the router.
Loading Operating System:
After finding the Operating System, bootstrap loads operating system into Random Access Memory present in the router. Some routers instead of loading the operating system into RAM, they directly execute operating system directly from flash.
Finding the configurations:
Once operating system is loaded, it starts searching for configurations that are made. By default, all the configurations are stored in NVRAM of the router. We can even configure a router to fetch the configuration from a remote TFTP server.
Loading the configurations:
Once the configuration is found, IOS executes the same. If No configuration is found in the NVRAM or in TFTP server, the router enters into configuration mode. All configurations done in this mode are stored in Random Access Memory, a temporary storage. Once the configuration is done, it must be saved into Startup-config file or it will be erased when we restart a router.
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