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Cisco Router Boot Sequence Tutorial

December 16th, 2011 Go to comments

In this article we will learn about the main components of a Cisco router and how the boot process takes place.

Types of memory

Generally Cisco routers (and switches) contain four types of memory:

Read-Only Memory (ROM): ROM stores the router’s bootstrap startup program, operating system software, and power-on diagnostic test programs (POST).

Flash Memory: Generally referred to simply as “flash”, the IOS images are held here. Flash is erasable and reprogrammable ROM. Flash memory content is retained by the router on reload.

Random-Access Memory (RAM): Stores operational information such as routing tables and the running configuration file. RAM contents are lost when the router is powered down or reloaded.

Non-volatile RAM (NVRAM): NVRAM holds the router’s startup configuration file. NVRAM contents are not lost when the router is powered down or reloaded.

Some comparisons to help you remember easier:

+ RAM is a volatile memory so contents are lost on reload, where NVRAM and Flash contents are not.
+ NVRAM holds the startup configuration file, where RAM holds the running configuration file.
+ ROM contains a bootstrap program called ROM Monitor (or ROMmon). When a router is powered on, the bootstrap runs a hardware diagnostic called POST (Power-On Self Test).

Router boot process

The following details the router boot process:
1. The router is powered on.
2. The router first runs Power-On Self Test (POST)
3. The bootstrap checks the Configuration Register value to specify where to load the IOS. By default (the default value of Configuration Register is 2102, in hexadecimal), the router first looks for “boot system” commands in startup-config file. If it finds these commands, it will run boot system commands in order they appear in startup-config to locate the IOS. If not, the IOS image is loaded from Flash . If the IOS is not found in Flash, the bootstrap can try to load the IOS from TFTP server or from ROM (mini-IOS).
4. After the IOS is found, it is loaded into RAM.
5. The IOS attempts to load the configuration file (startup-config) from NVRAM to RAM. If the startup-config is not found in NVRAM, the IOS attempts to load a configuration file from TFTP. If no TFTP server responds, the router enters Setup Mode (Initial Configuration Mode).


And this is the process we can see on our screen when the router is turned on:


In short, when powered on the router needs to do:

1. Run POST to check hardware
2. Search for a valid IOS (the Operating System of the router)
3. Search for a configuration file (all the configurations applied to this router)

Specify how much RAM, NVRAM and Flash of a router

Also, from the information shown above, we can learn some information about router’s model, RAM, Flash, NVRAM memories as shown below:


Note: The “show version” command also gives us this information.

All the above information is straight-forwarding except the information of RAM. In some series of routers, the RAM information is displayed by 2 parameters (in this case 60416K/5120K). The first parameter indicates how much RAM is in the router while the second parameter (5120K) indicates how much DRAM is being used for Packet memory. Packet memory is used for buffering packets.

So, from the output above we can learn:

Amount of RAM: 60416 + 5120 = 65536KB / 1024 = 64MB
Amount of NVRAM: 239KB
Amount of Flash: 62720KB

Comments (19) Comments
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  1. zoundwave
    August 9th, 2016

    So, from the output above we can learn:

    Amount of RAM: 60416 + 5120 = 65536KB / 1024 = 64MB
    Amount of NVRAM: 239KB
    Amount of Flash: 62720KB

    Question: Why does the amount of RAM be divided by 1024? Is 1024 a constant value?

  2. hopeimright
    August 10th, 2016

    1024 Kb = 1 Mb

  3. vino
    August 12th, 2016

    very useful

  4. Mon
    August 15th, 2016

    @9tut Does the router try to load from TFTP again after there is no config file on the NVRAM ?
    If not then point 5 is incorrect

  5. Anonymous
    August 15th, 2016

    Great!! The router booting process can be illustrated using the booting process of a PC (for easier understanding – POST ==> Hard Drive OS ==> An script or Macro or Apps to load from Startuo folder). Love the explanation.



  6. Anonymous
    October 11th, 2016


  7. theri
    December 2nd, 2016

    pls post CCNA dump for 200-125

  8. Dodgy
    December 9th, 2016

    Can someone help, on Packet Tracer I have configured a routers interfaces, console 0, vty 0-4 passwords plus enable secret then copy running-config startup-config. I have then exited out or logged out but the original config is still present. Is this a time thing or am I doing something wrong????
    When configuring a router if you have any typeing errors ^^^ is this saved when copy running config to startup config ???

  9. Alireza
    February 1st, 2017

    Very useful and resourceful article….

  10. Domain Hosting
    February 24th, 2017

    Thanks so much for giving everyone an update on this matter on your web site. Please be aware that if a completely new post becomes available or when any improvements occur with the current submission, I would be interested in reading a lot more and learning how to make good using of those strategies you discuss. Thanks for your time and consideration of other folks by making this site available.


  11. Kamran
    March 2nd, 2017

    I have a question from the dumps, when a router boots which command is display?

  12. Dalton Howard
    April 18th, 2017

    This is the very best search system in the planet


  13. Deepak
    July 18th, 2017

    The router Ist check boot field value in config register or check boot system command in startup and how it could read boot system command in startup without loading the startup file

  14. Maurice
    August 29th, 2017

    how mad would it be if Septa truly allow for your self sip Whilst driving for the duration of beer 7 days anarchy!


  15. JS
    September 19th, 2017

    Please send me 200-125 dumps and .vce to junurajbanshi @ hotmail .com

  16. javo
    September 21st, 2017

    very good explanation !

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    May 18th, 2018

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