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Cisco router and any other computer don’t have a very different set of processes for booting if they are not very similar. An understanding of the processes that would help in setting up the configuration of a router and its various elements can lead to relating the booting process of the router to that of any other computer. Both router and a simple computer have a sequence of events that lead to the booting process.
Beginning with a computer, the BIOS (Basic Input/ Output System) is used for ensuring three main functions that make way for booting up of the system. The BIOS provides a set of machine code subroutines. The codes are called by the operating system and various hardware components of the computer are being accessed. The BIOS then causes the initiation of the boot sequence and allows the third process of changing the low-level setup options. The BIOS code is later burned onto a Flash EPROM memory chip installed on the motherboard (Mossywell). In a Cisco router (for example Cisco 2501 router), the Flash memory contains a valid IOS image which is similar to the BIOS of a simple computer. The router is yet to be configured and here also the sequence of events goes through during the completion of the boot process (DiNicolo, 2006).
The boot process in the router begins with POST (Power-on self-test). The router carries out a POST after being powered up. The purpose of this POST is to check the capability of CPU and router interfaces about their ability to function properly. Next in the line is the execution of bootstrap to load IOS (Input/ Output System). The successful outcome of POST, the router executes the Bootstrap program already burned previously in ROM. The bootstrap searches the Flash memory for a valid Cisco IOS image and is then loaded after the positive completion of the search. In case of non-availability of any IOS image, the router gets booted with the RxBoot limited IOS version found in ROM. IOS loading is basically the loading of the configuration file. The IOS image upon loading searches the NVRAM for a valid startup configuration. In case if there is no valid startup configuration file, the router then looks for System Configuration Dialog which is also called setup mode. This mode also enables the user to perform the initial configuration of the router (DiNicolo, 2006).
The simpler computers begin their processes after the Reset signal is issued. Various components of the motherboard are made to undergo the checking process. The CPU is allowed to execute codes only after the Reset signal is turned off. Testing and initialization of the hardware are as per the Power on Self Test (POST) protocol and upon successful completion of the same, the BIOS initiates the boot sequence from the hard disk or any other device specified in the BIOS setup. The process begins with ROM BIOS which initiates Power on Self Test. The BIOS discovers the boot device. Upon discovery, the BIOS loads the contents of the very first physical sector of the disk into memory and then instructs the CPU to execute the code. The machine code routines are the most basic part of BIOS with the size of each of the routines differing with different BIOS and are accessed through issuing software interrupts to the CPU. The BIOS loads the interrupt vector table and thereby enables the mapping process between the interrupt number and the corresponding routine’s memory location (Mossywell).
Reference
DiNicolo, D. (2006), Cisco Router Boot Process Posted in CCNA Study Guide Chapter 07. Web.
Mossywell, Computer Boot Sequence. Web.
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