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CISCO Router and Switch Internal Components

Each IOS device has two main components; hardware and software. Every IOS based router used the same hardware and firmware components to assist during the boot up process. Router/switch have the following component.

  • ROM (Read only memory): The software in ROM can’t be changed unless you actually swap out the ROM chip on our router. ROM is non-volatile. When you turn off your device the contents of ROM is not erased. ROM contains the firmware necessary to boot up your router and typically has the following four components.

  • POST (Power on self test): The post is used to check the basic functionality of the router hardware and determine which interfaces are present.

  • Bootstrap: The bootstrap is used to bring a router up during initialization. It will boot the router and then load the IOS.

  • ROM Monitor (ROMMON Mode): A mini-operating system that allow you to perform low level testing and trouble shooting. For instance ROMMON is used during the password recovery procedure. To abort the router’s normal boot up procedure of loading the IOS, use the (ctrl+Break) key within 60 second of boot up.

  • Mini IOS: Called the RXBOOT or boot loader by Cisco, the mini IOS is a stripped-down version of IOS in ROM that can be used to bring up an interface and load the Cisco IOS in flash memory. The mini IOS can also perform a few other maintenance operation.

  • Configuration Register: The configuration register is store in NVRAM. It is used to control how the router boots up. The value can be found at the last line of the show version command output and by default is set to 0x2102. Which tell the router to load the IOS from flash memory as swell as to load the configuration from NVRAM.

  • Flash: It stores the Cisco IOS by default. It is not volatile memory because flash memory is not erased when the router is reloaded. It is EEPROM (Electronically erasable programmable read only memory). It is created by Intel. It also contains other information like secondary configuration file. The flash size start forms 4 MB.

  • NVRAM (Non-volatile RAM): it is used to hold the router and switch configuration file. NVRAM is not erased when the router or switch is reloaded. The configuration register is store in NVRAM. It uses a battery to maintain the information when the Cisco device is turn off. The size of NVRAM is 32 KB to 64 KB.

  • RAM: It is used to hold down packet buffers, ARP cache, routing table and also the software and data structures that allow the router to function. Running-config file is store in RAM and most routers expand the IOS from flash into RAM upon boot up. The RAM size starts from 16 MB.

  • Processor: Cisco router and switch used Motorola and Intel processor. The power of Motorola processor is 70 MHz and used RISC based processor (Reduced instruction set computer). The power of inter processor is 300 MHz to 400 MHZ. Basically Intel processor is used in ISR router.

    BOOTING PROCESS OF ROUTER AND SWITCH:

  • When you bring up a new Cisco router it performs a power in self test. It check the all hardware component, including memory and interfaces.
  • If post is successfully pass then router execute boot strap program.
  • The boot strap program finds and loads an IOS image possible locations of the IOS image include flash, TFTP server or the mini IOS in ROM.
  • Once the valid IOS is loaded, the bootstrap program attempts to find and load a valid configuration file, which is normally store in NVRAM. If the IOS can’t find a configuration file, it start up the system configuration dialog box.
  • After the configuration is loaded and running properly then a preconfiguration file is copy from startup-config to RAM and finally you get the CLI prompt.


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