Inside a Router

a router is a computer. like a PC, a router also includes CPU, RAM and ROM. the components in a router are:

  • Central Processing Unit (CPU). the CPU executes operating system instructions, such as system initialization, routing fuinctions and switching functions.

  • Random-Access Memory (RAM). RAM is volatile memory and it loses its content when the router is powered down or restarted. RAM stores the instructions and data needed to be executed by the CPU. RAM is used to store these components:

    • Operating System: the Cisco IOS is copied into RAM during bootup.

    • Running Configuration File: this is the file that stores the configuration commands that the router is currently using. with few exceptions, configuration commands are directly stored to the running configuration, known as running-config.

    • IP Routing Table: this file stores information about directly connected and remote networks. it is used to determine the best path to forward the packet.

    • ARP Cache: this is similar to the ARP cache on a PC, this contains the IPv4 address to MAC address mappings. this is used on routers that have LAN interfaces such as Ethernet.

    • Packet Buffer: packets are temporarily stored in a buffer when received on an interface or before they exit an interface.


  • Read-Only Memory (ROM). ROM does not lose its content when the router is powered down or restarted. cisco devices use ROM to store the bootstrap instructions, basic diagnostic software and scaled-down version of IOS. ROM uses firmware (firmware includes software that does not normally need to be modified or upgraded, such as the bootup instructions), which is software that is embedded inside the integrated circuit.

  • Flash Memory. Flash is a nonvolatile computer memory that can be electrically stored or erased. flash is used to store the router operating system, Cisco IOS. the IOS is copied to the RAM during bootup process, where it can be executed by the CPU (some old routers run the IOS directly from flash). flash consists of SIMMs and PCMCIAs cards, which can be upgraded to increase the amount of flash memory.

  • Nonvolatile RAM (NVRAM). unlike most kind of RAMs, NVRAM does not lose its content when power is turned off. Cisco IOS use this permanent storage to store the startup configuration file (startup-config). to save the changes implemented to the router in case the router is powered down or restarted, the running-config must be copied to NVRAM as the startup-config.


Router boot-up process
there are four major phases to the bootup process:

  • performing the Power-On Self Test (POST). POST is a common process done by almost every computer during bootup. when the router is powered on, the software on the ROM chip perform POST to test the router hardware. during this process, the router executes diagnostics from ROM on several hardware components including the CPU, RAM and NVRAM. after the process is done, the router executes the bootstrap program.

  • loading the bootstrap program. after the POST process, the router copies the bootstrap program from ROM into RAM. once in RAM, the CPU executes the instructions in the bootstrap program. the main purpose of the bootstrap program is to locate the Cisco IOS and load it into RAM. (at this point, if you have a console connection to the router, you will begin to see output).

  • locating and loading Cisco IOS. usually, the IOS is located in the flash memory, but can also be stored remotely, such as a TFTP server (a TFTP server can be used as a central storage for IOS images or as a backup server for IOS). if a full IOS image can not be located, a scaled-down version of the IOS is copied from ROM into RAM. this version of IOS is used to help diagnose any problems and can be used to load a complete version of the IOS into RAM. once the IOS begins to load, you may see a string of pounds signs (#), while the image decompresses.

  • locating and loading the configuration file. after the IOS is loaded, the bootstrap program searches for the startup configuration file (startup-config) in the NVRAM. if the file exists, it is copied into RAM as the running configuration file (running-config). if the file does not exists, the router may serach for a TFTP server. if the router detects that it has an active link to another configured router, it sends a broadcast searching for a configuration file across the active link. this condition will cause the router to pause. if the startup configuration file can not be located, the router prompts the user to enter setup mode. setup mode is a series of questions prompting the user for basic configurations information (setup mode is not to be used to enter complex configuration and is not commonly used by network administrators). you can terminate the setup process by pressing the Ctrl-C at any time. if the setup mode is not used, the IOS creates a default running-config. this file is a basic configuration file (this file does not contain any interface addresses, routing information, passwords or other specific configuration information).


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