When you bring up a Cisco IOS device, just like a computer, it will go through a boot-up process. Let me give you just a brief heads up regarding what happens to a Cisco IOS device that has not been configured upon bootup:
- It goes through a Power-On Self-Test (POST), once the machine goes through this step.
- It will then look for the IOS in flash memory and it expands the IOS into RAM and, yes, the IOS is compressed. Don't worry about how it's compressed; just know it is. This is very important, because you need enough RAM on the router for it to be able to expand.
- Then, the IOS needs to decide whether this device has any configurations—where does it look for this information? The answer to that is Non-Volatile Random-Access Memory (NVRAM); it is looking for the startup-configuration file. Once found, it will load any configurations...
