Cisco Router Boot Sequence Tutorial
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. By default, routers look here first for an Internetwork Operating System (IOS) file during boot.
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
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Simple explanation yet very understandable.
There is something I don’t understand.
In step 3 you say “the router first looks for “boot system” commands in startup-config file” but then you say that the startup config is loaded only in step 5.
So at step 3 the startup config file is not loaded yet so the device can’t read it..
Any explanation ? thx
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Any one that can clear this up please??
There is something I don’t understand.
In step 3 you say “the router first looks for “boot system” commands in startup-config file” but then you say that the startup config is loaded only in step 5.
So at step 3 the startup config file is not loaded yet so the device can’t read it..
Any explanation ? thx
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Cisco Router Boot Sequence tutorial it’s mind blowing.now i’m fully understand how the router power on self test in the 1st position. for latest dumps help me please {debojitganguly2@gmail.com}……..
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Any one that can clear this up please??
There is something I don’t understand.
In step 3 you say “the router first looks for “boot system”……
Dear.
1- firstly when router power on, POST test will be run to check hardware status.
2- Secondly now is time to check on which location our valid IOS located , it depends which Confg register value we set on router normally by default 0×2102 is set on router.
“the router first looks for “boot system” commands in startup-config file” this line means that might be we set manually commands that our router should firstly find IOS on FTP then others.
Example: Router(config)#boot system tftp: C1841-ADVIPSERVICESK9-M 10.1.1.1
if not then router will boot up by default .
1- firstly Check flash if not found in flash then in FTP .
2- If not found in FTP then it will go in roman mode.
.
After loading IOS from flash then router will search valid configuration file in NVram.
if this not in Nvram than router will find out on FTP .
or if also not located in FTP then it will show Setup wizard
thank you! well explained..
ahtasham,
I think, you already answer your question.
You can change the configuration register value to modify how the router boots and runs. These are the main reasons you would want to change the configuration register:
>To force the system into the ROM monitor mode
>To select a boot source
>To enable or disable the Break function
>To control broadcast addresses
>To set the console terminal baud rate
>To load operating software from ROM
>To enable booting from a Trivial File Transfer Protocol (TFTP) server
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I found this tutorial little confusing. Read Todd-Lammle, 7th edition page no. 280. It explains the boot sequence properly.
@rahman: Could you please tell us what is wrong in the tutorial above?
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