When a Cisco router powers up, it first runs a series of POSTs (Power-On Self Test). A POST is a series of diagnostic tests designed to verify the basic operation of the network interfaces, memory, and the CPU.
Depending on the model or router of switch you're using, you can actually see some of these tests being passed. Here, I've reloaded a Cisco 2950 switch, and you can see some of the POSTs being run and passed at the very beginning of the bootup process.
Initializing flashfs...
flashfs[1]: 79 files, 3 directories
flashfs[1]: 0 orphaned files, 0 orphaned directori
flashfs[1]: Total bytes: 7741440
flashfs[1]: Bytes used: 5980672
flashfs[1]: Bytes available: 1760768
flashfs[1]: flashfs fsck took 7 seconds.
flashfs[1]: Initialization complete.
Done initializing flashfs.
POST: System Board Test : Passed
POST: Ethernet Controller Test : Passed
ASIC Initialization Passed
POST: FRONT-END LOOPBACK TEST : Passed
POSTs are particularly effective at detecting major problems early in the boot process, such as a broken fan. If the POST detects such a problem (usually called an "environmental factor") that would cause the router or switch to overheat after booting, the POST will fail, give you a clear message as to why the POST failed, and will then stop the boot process.
But let's speak positively here!
After the router passes the POST, it looks for a source from which to load a valid Internetwork Operating System (IOS). The router has three sources from which it can load an IOS image, and it's a good idea to know these sources and the order in which the router will look in each for the IOS image:
1. Flash memory (the default).
2. A TFTP server. (Trivial File Transfer Protocol)
3. Read-Only Memory (ROM)
To change that order, a change must be made to the configuration register, and we'll talk about that later in the course. It's similar to the Microsoft Registry in that you should never change this value unless you are sure of the result.
Once the IOS is found, the router looks for a valid startup configuration file. By default, the router will look for the startup configuration file in Non-volatile RAM (NVRAM).
If no valid startup configuration file is found, the router enters setup mode, where the router runs the system configuration dialogue, a series of questions involving basic router setup.
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