Rom contains the necessary firmware to boot up your router and typically has the following four components:
Post (power-on self-test) performs tests on the router’s hardware components.
Bootstrap program brings the router up and determines how the ios image and configuration files will be found and loaded.
Rom monitor (rommon mode) a mini–operating system that allows you to perform low-level testing and troubleshooting, the password recovery procedure.
Mini-ios a stripped-down version of the ios that contains only ip code. This should be used in emergency situations where the ios image in flash can’t be found and you want to boot up your router and load in another ios image. This stripped-down ios is referred to as rxboot mode.
RAM is like the memory in your PC. On a router, it (in most cases) contains the running IOS image; the active configuration file; any tables (including routing, ARP, CDP neighbor, and other tables); and internal buffers for temporarily storing information, such as interface input and output buffers. The IOS is responsible for managing memory. When you turn off your router, everything in RAM is erased.
Flash is a form of nonvolatile memory in that when you turn the router off, the information stored in flash is not lost. Routers store their IOS image in flash, but other information can also be stored here. Note that some lower-end Cisco routers actually run the IOS directly from flash (not RAM). Flash is slower than RAM, a fact that can create performance issues.
NVRAM is like flash in that its contents are not erased when you turn off your router. It is slightly different, though, in that it uses a battery to maintain the information when the Cisco device is turned off. Routers use NVRAM to store their configuration files. In newer versions of the IOS, you can store more than one configuration file here.
Router Boot up Process
A router typically goes through five steps when booting up:
The router loads and runs POST (located in ROM), testing its hardware components, including memory and interfaces.
The bootstrap program is loaded and executed.
The bootstrap program finds and loads an IOS image: Possible locations: – flash, a TFTP server, or the Mini-IOS in ROM.
Once the IOS is loaded, the IOS attempts to find and load a configuration file, stored in NVRAM
After the configuration is loaded, you are presented with the CLI interface. you are placed into is User EXEC mode.
Cisco devices include a feature called Setup mode to help you make a basic initial configuration. Setup mode will run only if there is no configuration file in NVRAM—either because the router is brand-new, or because it has been erased. Setup mode will ask you a series of questions and apply the configuration to the device based on your answers. You can abort Setup mode by typing CTRL+C or by saying “no” either when asked if you want to enter the initial configuration dialog or when asked if you want to save the configuration at the end of the question.
The configuration register is a special register in the router that determines many of its boot up and running options, including how the router finds the IOS image and its configuration file. The configuration register is a four-character hexadecimal value that can be changed to manipulate how the router behaves at bootup. The default value is 0×2102.
The characters “0x” indicate that the characters that follow are in hexadecimal. This makes it clear whether the value is “two thousand one hundred and two” or, as in this case, “two one zero two hexadecimal”.
The fourth character in the configuration register is known as the boot field. Changing the value for this character will have the following effects:
- 0×2100 = Always boot to ROMMON.
- 0×2101 = Always boot to RXBOOT.
- 0×2102 through 0x210F = Load the first valid IOS in flash; values of 2 through F for the fourth character specify other IOS image files in flash.
The third character in the configuration register can modify how the router loads the configuration file. The setting of 0×2142 causes the router to ignore the startup-config file in NVRAM (which is where the password is stored) and proceed without a configuration—as if the router were brand new or had its configuration erased.