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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Thanks so much, i never knew that the IOS loads into the RAM, I used to think it runs from the flash memory. Kudos
want to be clear ON meaning and function of REGISTER 0×2102 & 0×2142 ……
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For the RAM you mentioned that there are 2 parameters (in this case 60416K/5120K). The 60416k is the “available” RAM, while the 5120K is the RAM that is currently being used right?
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In the last paragraph of this guide: “….(in this case 60416K/5120K). The first parameter indicates how much RAM is in the router….”
Should it be “The first parameter indicates how much DRAM is in the router….”
Is it DRAM?? If so then is this a correct way to think:
RAM size = amount of DRAM + amount of DRAM being used for Packet memory ?
I know DRAM = Dynamic RAM, but do we need to worry about the difference between DRAM & RAM??
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Why does the 7th line statement “By default, routers look here first for an Internetwork Operating System (IOS) file during boot.” of “Types of memory” paragraph is not reflected on the flowchart or diagram?
I think the flowchart needs revision…
In which of the 4 memories does the configuration register located?
What is the essence of looking first in RAM for IOS during boot?
From my understanding, RAM is initially blank during boot.
Thumbs UP!!!
Isn’t startup-config file stored in the RAM? I dont see this in the diagram above. Good Job 9tut for providing valuable info. Thanks.
RICS – my thought exactly- But if RAM is initially blank IOS is loaded from Flash and IOS is not found in flash – Try to locate IOS in TFTP, If is not found in TFTP Server it runs ROM again to start the same process to locate IOS. What are you views people?
Its Easy to understand…