Boot Sequence of Cisco Router & Switch

Cisco perform some specific steps while booting or turning on device. It is necessary to understand boot sequence of router / switch because this helps you for troubleshooting purpose. When a Cisco device boots up, it performs a series of steps, called the boot sequence, to test the hardware and load the necessary software.

The boot sequence comprises the following steps:

1. At the very beginning POST operation is executed, which tests the hardware to verify that all components of the device are present and operational. The POST takes stock of the different interfaces on the switch or router, and it’s stored in and runs from read-only memory (ROM). If this fails then device gets stopped else it will proceed to next bootloader step.
2. The bootstrap in ROM then locates and loads the Cisco IOS software by executing programs responsible for finding where each IOS program is located. Once they are found, it then loads the proper files. By default, the IOS software is loaded from flash memory in all Cisco devices. The default order of an IOS loading from a Cisco device begins with flash, then TFTP server, and finally, ROM. If it fails to locate IOS file then device will go into ROMMON mode.
3. The IOS software then looks for a valid configuration file stored in NVRAM. This file is called startup-config (Startup configuration) and will be present only if an administrator has copied the running-config file into NVRAM means this startup-config file is only created when we save running configuration.
4. If a startup-config file is found in NVRAM, the router or switch will copy it in RAM, and name the file the running-config. The device will use this file to run, and the router/switch should now be operational.
5. If no startup-config file is found in NVRAM, the router/switch reacts by broadcasting out any interface that detects carrier detect (CD) to locate a TFTP host in its search for a configuration.
6. When that fails, the device will show message of the Initial Configuration Dialog (ICD) mode.
7. In ICD mode if we enter Yes then auto configuration done by device based on few questions & ans. like hostname, IP Address etc. In ICD mode if we enter No then device will use Default Configuration file to run, and the router/switch should now be operational.

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