The Internal Components of a Cisco
Router
In order to configure and troubleshoot a Cisco internetwork, you need
to know the major components of Cisco routers and understand what each
one does. Table as below describes the major Cisco router components.
Component |
Description |
| 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. |
| POST (power-on self-test) |
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. |
| ROM monitor | Stored in the microcode of the ROM, the ROM monitor is used for manufacturing, testing, and troubleshooting. |
| 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. |
| RAM (randomaccess memory) |
Used to hold packet buffers, ARP cache, routing tables, and also the software and data structures that allow the router to function. Running-config is stored in RAM, and the IOS can also be run from RAM in some routers. |
| ROM (read-only memory) |
Used to start and maintain the router. |
| Flash memory | Used on the router to hold the Cisco IOS. Flash memory is not erased when the router is reloaded. It is EEPROM (electronically erasable programmable read-only memory) created by Intel. |
| NVRAM (nonvolatile RAM) |
Used to hold the router and switch configuration. NVRAM is not erased when the router or switch is reloaded. |
| Configuration register |
Used to control how the router boots up. This value can be seen with the show version command and typically is 0x2102, which tells the router to load the IOS from flash memory as well as telling the router to load the configuration from NVRAM. |
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