1 2 Previous Next 20 Replies Latest reply: Mar 17, 2014 10:04 AM by Lukas RSS

    Router Boot Sequence ?


      Hi guys,

      can you explain the Router boot sequence in detail ?

      where is the start-up config, running config, mini IOS, IOS located ?


      Thank you


        • 1. Re: Router Boot Sequence ?

          Hi Gopal,


          Here i will try my best to tell you on boot sequence,


          first of all any router to start working will require operating system, in case of CISCO router we call its operating system as IOS, which is stored in Flash drive bydefaults comes with any router. Second thing it requires is startup-config which is stored in NVRAM, built-in in cisco router, which indicates router what function it has to run, and the third one is running-config which it require in real time basis stored in RAM memory of Router, which is same as our Personal computer's RAM. Now keeping all this fact in mind,


          Now boot sequence would be,


          1. On power on cisco router first will perform the POST( Power on self test).  The POST tests the hardware to verify that all components of the device are operational and present. For example, the POST checks for the different interfaces on the router. The POST is stored in and run from ROM (read-only memory).

          2. The bootstrap looks for and loads the Cisco IOS software. The bootstrap is a program in ROM that is used to execute programs. The bootstrap program is responsible for finding where each IOS program is located and then loading the file. By default, the IOS software is loaded from flash memory in all Cisco routers.

          3. The IOS software looks for a valid configuration file stored in NVRAM. which is called as startup-config.

          4. If a startup-config file is in NVRAM, the router will load and run this file. The router is now operational. If a startup-config file is not in NVRAM, the router will start the setup-mode configuration upon bootup.

          5. Any further modification on running router will be stored on RAM, where you need to manually execute command copy running-config startup-config to make your current configuration as a startup-config, every time you boot your router.




          Hope This will help you out.





          • 2. Re: Router Boot Sequence ?

            Hi Gopal,


            Hardick has explained most of the things but IOS loading depends upon Configuration Register value which is 0x2102 by default which means that by default load IOS from flash but we can load it from TFTP server as well depending upon boot sytem global configuration commands


            As far as Mini IOS is concerned that was used in old 2500 series routers not in new ISRs and was kept in ROM.


            For simplicity i am not mentioning Boot Field process now.





            • 3. Re: Router Boot Sequence ?

              Hi Zohaib,


              Hey thanks for your comments, i really missed that point. actually that is the bigboss who decides the boot sequence.






              • 4. Re: Router Boot Sequence ?

                Thank You guys

                • 5. Re: Router Boot Sequence ?

                  Just an additional question to add to this, the router boot sequence is set out as above and easily explained.  However....the switch boot sequence defines the first action as the boot loader being loaded from ROM then defines that it is the boot loader that preforms the POST, does this mean the sequence is different for switches or is there a mistake in course literature?

                  • 6. Re: Router Boot Sequence ?

                    hi , i want to ask a question


                    i have a router cisco 7206

                    i performed two commands as below:

                    Bras#sh bootflash:

                    -#- ED ----type---- --crc--- -seek-- nlen -length- ---------date/time--------- name

                    1   .. image        31E6C0CD  80D9F8   28  7919992 Jan 1 2000 04:24:55 +03:00 c7200p-kboot-mz.124-4.XD.bin


                    57615880 bytes available (7920120 bytes used)

                    Bras#sh dis

                    Bras#sh disk2:

                    -#- --length-- -----date/time------ path

                    1     37962496 Aug 04 2011 00:00:14 c7200p-advipservicesk9-mz.124-24.T4.bin

                    3         3480 Jan 23 2001 04:04:10 accouting

                    4            0 Aug 07 2011 17:43:24 backup

                    5         9255 Jul 08 2012 08:44:14 backup/Bras-backup-198

                    6         9308 Jul 10 2012 08:44:26 backup/Bras-backup-199

                    7         9310 Jul 11 2012 08:25:44 backup/Bras-backup-200

                    8         9310 Jul 11 2012 08:44:32 backup/Bras-backup-201

                    9         9529 Jul 13 2012 08:44:44 backup/Bras-backup-202

                    12        9256 Jul 04 2012 08:43:50 backup/Bras-backup-194

                    13        9308 Jul 05 2012 21:08:48 backup/Bras-backup-195

                    14        9308 Jul 07 2012 08:44:06 backup/Bras-backup-196

                    15        9255 Jul 07 2012 19:12:32 backup/Bras-backup-197

                    19        9529 Jul 14 2012 08:44:52 backup/Bras-backup-203


                    218361856 bytes available (38096896 bytes used)





                    my question is , does disk2 is considered as a flash which is didicated for loading the ios and sh bootflash is considered a rommon place which holds the bootstrap or bootloader ???


                    i just want to fix my info and nail it down



                    with my best regards

                    • 7. Re: Router Boot Sequence ?

                      Hi Ahmed,

                                           sh bootflash: ==> Shows the  bootloader file and is used during the intial bootup process.  Its small in size.

                                           sh disk2:  ==> Shows the router IOS image file loadaed after the intial startup process completed . Its size is big compare to bootloader fiel.



                      • 8. Re: Router Boot Sequence ?

                        Hi Matt,


                        The boot process for Routers and switches are same.



                        • 9. Re: Router Boot Sequence ?

                          Hi Hardik,


                          I am studing for CCNA and during my practice exams I got the boot sequence question wrong by answering  POST -> Config -> IOS instead of POST -> IOS -> Config.  I can see from your post that I was wrong but I still do not understand this based on the boot process details.  In my reference material I see the following description which indicates that the config is read before the IOS is loaded:


                          Assuming that there are no critical errors in the POST, the bootstrap checks the startup-config file in NVRAM for boot system commands. These commands may have been entered by the router admin to over ride the default behavior, perhaps to load a different IOS for test purposes.


                          Assuming there are no boot system commands, the router loads the first valid IOS image it finds in flash memory.


                          This clearly indicates that the startup-config is read BEFORE the IOS is loaded.  Perhaps it can be argued that the startup-config is not applied until after the IOS is loaded, but clearly it is looked for and found before the IOS is looked for.


                          Can anyone explain this to me?




                          • 10. Re: Router Boot Sequence ?

                            This maybe a little late, but I'll just post my reply about the Router boot sequence anyway, and be corrected if I'm wrong.

                            When  the router boots up, the bootstrap program is run. This program will then run POST (Power On Self Test). When everything is okay, it will then check the Configuration Register Value (0x2102 which refers to the FlashMemory by default) to determine where the IOS will be loaded from. Once the IOS has been loaded (decompressed to RAM), it will then look into NVRAM for the startup-config. If a startup-config is not found, the router will automatically go to setup mode.

                            If what I recall is correct - from Todd Lammle's book - a non ISR (Integrated Services Router) will broadcast out all its active interfaces looking for a TFTP (Trivial File Transfer Protocol) Server to ask for the startup-config IF one is not found in NVRAM.

                            • 11. Re: Router Boot Sequence ?

                              the sequence is:

                              Power on self test (post)

                              Load and run bootstrap code

                              Find IOS software

                              Load IOS software

                              Find Configuration file

                              Load Configuration file



                              So the short version is: POST --> Boostrap --> IOS --> Config --> Run


                              PBICR (pretty boys invent cars for racing) ;-) whatever works right

                              • 12. Re: Router Boot Sequence ?

                                In my experience the investment in the Cisco study materials from Cisco press or your preferred source are a wise use of funds available. (some used book stores carry them from time to time)


                                If you are hung up on the sequence go to You Tube and search Cisco Router Boot Sequence...there are a few videos there on the topic.

                                • 13. Re: Router Boot Sequence ?

                                  check this link...has correct answer and additional info that is helpful



                                  • 14. Re: Router Boot Sequence ?

                                    Hey Guys, i m bit confused abt this question.


                                    "During normal operation, from which loaction most Cisco swithces and routers run the IOS?"

                                    Options are:




                                    Disk Drive


                                    Please help me with quiz. Thanks alot

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