Router Components

Processor: Cisco router has a CPU that execute the IOS (Inter-networking Operating System) commands.
for examples:
700 Series Intel 80386SL
800 Series Motorola PowerQUICC 8xx PowerPC core
1000 Series Motorola Dragonball
1600 Series Motorola Dragonball
2500 Series Motorola 680EC30
3100 Series Motorola 680EX30

ROM Monitor: It is used for manufacturing, testing and troubleshooting. It is a non-volatile memory. It stores bootstrap program.
                    Bootstrap program loads the IOS image for the router with the help of configuration register, this configuration register is a file that decide the boot mode for the IOS image, the register value is a set of 4 hexadecimal digit, the last hexadecimal digit decide the boot up process. The value for the image are:

• 0*0  - boots the router into ROM monitor mode.
• 0*1 - boots the router using the mini-IOS.
• 0*2 - boots the router using the default boot sequence.

 RAM: It holds packet buffer, ARP cache, routing table, software and data structure that allows the router to function. It stores running-config of the router and it also stores decompressed IOS in later router models.

Flash Memory: It is an Electronically Erasable and Re-Programmable memory chip. The Flash memory contains the full Operating System Image (IOS, Internetwork Operating System). This allows you to upgrade the OS without removing chips. Flash memory retains content when router is powered down or restarted.

NVRAM: NVRAM (Non-volatile Random Access Memory) is used to store the startup configuration. This is the configuration file that IOS reads when the router boots up. It is extremely fast memory and retains its content when the router is restarted.

Mini-IOS: The mini-IOS is not present in every router. The mini-IOS components provides an alternative file for the router boot up. It can also perform a few other maintenance operations.

Router Boot Sequence

1. Router is powered on.

2. The bootstrap program (ROMmon) is loaded from ROM.

3. The bootstrap runs POST. 
               POST (Power On Self Test) checks for the basic functionality of router hardware and determines which interface are present or not. The POST is a series of 14 tests that runs in the reverse numerical order.

4. The bootstrap attempts to load the IOS from Flash.
          a) If the IOS is not found in the Flash, the bootstrap loads into RAM the basic IOS stored in ROM
          b) If the IOS is found in Flash, it is loaded into RAM.

5. The IOS attempts to load the startup-config file from NVRAM
          a) If the startup-config is not found in NVRAM, the IOS attempts to load a configuration file from TFTP.
          b) If no TFTP server responds, the router enters Initial Configuration Mode.
          c) If startup-config is found in NVRAM, it is loaded into RAM.

6. The startup-config becomes the running-config in RAM.