Router– It is a network device that forwards data packets across networks. Routers perform “data traffic directing” service on the Internet. It is a microprocessor-controlled device that is connected to two or more data lines from different networks.
What inside a Router?
- CPU– The CPU executes operating system instructions, such as process routing data.
- RAM– RAM stores the instructions and data needed to be executed by the CPU. Router expand IOS in it upon boot. it hold- Packet buffers, ARP cache, routing tables, Running-config. It also known as DRAM.
- ROM– Cisco devices use ROM to store: The bootstrap instructions, ROM monitor software, Basic diagnostic software, Scaled-down version of IOS. Used to start and maintain the router.
- Flash memory- Stores the Cisco IOS by default. Flash memory is not erased when the router is reloaded. It is EEPROM created by Intel.
- NVRAM (nonvolatile RAM)- Used to hold the routers startup-configuration. NVRAM is not erased when the router or switch is reloaded. The configuration register is stored in NVRAM.
- Mini-IOS- Called the RXBOOT or bootloader by Cisco, the mini-IOS is a small IOS in ROM that can be used to bring up an interface and load a Cisco IOS into flash memory. The mini-IOS can also perform a few other maintenance operations.
- POST- Stored in the microcode of the ROM, the POST is used to check the basic functionality of the router hardware and determine which interfaces are present.
- Bootstrap- Stored in the microcode of the ROM, the bootstrap is used to bring a router up during initialization. It will boot the router and then load the IOS.
- ROM monitor- Stored in the microcode of the ROM, the ROM monitor is used for manufacturing, testing, and troubleshooting.
- Configuration register- Used to control how the router boots up. Default value is set to 0x2102 (#sh version). The default setting tells the router to load the IOS from flash memory as well as to load the configuration from NVRAM.
Booting Process of a router
A router typically goes through five steps when booting up-
- POST- First the router runs POST (located in ROM) to detect and initialize The routers hardware components, including memory and interfaces.
- Bootstrap loading- The bootstrap program is loaded and executed.
- Load IOS- The bootstrap program finds and loads an IOS image, present in Flash memory, TFTP server, or the Mini-IOS in ROM.
- Search NVRAM data- Once the IOS is loaded, it find and load a configuration file from NVRAM. If there is no configuration file, then it starts up the System Configuration Dialog.
- After the configuration is loaded, you are presented with the CLI interface.