Router components | ram |rom | nvram |explained

Brief overview of each major components
CPU
The CPU executes operating system instructions .
Cisco routers use Cisco IOS as the operating system.
It also does the computing,For Ex-: it executes algorithms such as Dijkstra's Shortest Path First Algorithm for OSPF routing protocol.
a lot of things are done by the CPU.
Router Memory Components
1.RAM (Random access memory) -
RAM is volatile memory and loses its content when the router is powered down or restarted.
RAM stores the instructions and data needed to be executed by the CPU.
RAM is used to store following components:
Operating System: The Cisco IOS (Internetwork Operating System) is copied into RAM during boot-up.
Running Configuration File: This is the configuration file that stores the configuration commands that the router IOS is currently using. With few exceptions, all commands configured on the router are stored in the running configuration file, known as running-config.
IP Routing Table: This file stores information about directly connected and remote networks. It is used to determine the best path to forward the packet.
ARP Cache: This cache contains the IPv4 address to MAC address mappings, similar to the ARP cache on a PC. The ARP cache is used on routers that have LAN interfaces such as Ethernet interfaces.
Packet Buffer: Packets are temporarily stored in a buffer when received on an interface or before they exit an interface.
2.NVRAM
It stands for Non-volatile RAM,
By "non-volatile", we mean that the contents of NVRAM are not lost when the router is powered down or reloaded.
NVRAM is used to store the router's startup configuration/Backup config file
NVRAM is used by the Cisco IOS as permanent storage for the startup configuration file (startup-config)
so that way there we can return to the previous configuration we had before the reboot. We're going to look in NVRAM for that, we find that startup configuration file, and we load it.
The router looks for the configuration information in the NVRAM unless the router is configured to load the configuration file from some other source.
If NVRAM is empty when the router reloads, you will be prompted to enter setup mode.
3.ROM
ROM stands for Read-Only Memory.
ROM is a form of permanent storage.
Non-Volatile
ROM does not lose its contents when the router loses power or is restarted.
ROM is used to start and maintain the router.
Cisco routers use ROM to store:
Bootstrap startup program-:
power-on diagnostic test programs (the POST) -:
Mini-IOS/Limited IOS-:
Flash memory can be erased or overwritten, thus making the Cisco IOS upgradeable.
If the bootstrap cannot find the IOS in Flash, a stripped-down version of the IOS that will be loaded from ROM instead.
Provides a limited backup version of the OS, in case the router cannot load the full featured IOS.
The contents of ROM cannot be altered or erased; the entire ROM chip must be replaced if an upgrade/repair is necessary.
4.Flash Memory
Flash memory is non-volatile computer memory
It is used as permanent storage for the IOS and other system related files.
This is a type of EEPROM.
It can be electrically stored and erased thus making the Cisco IOS upgradeable.
Power Supply: A power supply is a component that supplies power to the router.
The power supplies can be AC or DC.
Configuration Register:
The configuration register is what decides from where the router is going to boot the IOS image from.
By default, the output is set to 0x2120, which tells the router to load the IOS from flash memory and to load the configuration from NVRAM.
This value can be found at the last line of the show version command.
External Components
Management ports
These include the console and auxiliary ports used to configure, manage and troubleshoot the router.
Console ports -
Console port is used to access the administration of the initial configuration and the command line interface (CLI).
AUX port -:
An RJ-45 port for remote administration access, similar to the console port. It is now considered an old port, as it was used to support dial up modems.
LAN interfaces -: includes 2 integrated 10/100/1000 Ethernet ports
Generally, they are used to provide LAN access by connecting with switches and users, or interconnect to another router.
USB interfaces : 
Two USB host ports are included for additional storage space.
Enhanced high-speed WAN interface card (EHWIC) slots - Two slots that support different types of interface modules, including Serial, digital subscriber line (DSL), switch port, and wireless.
Storage Slots
The Cisco 1941 ISR also has storage slots to support expanded capabilities.By default, only slot 0 is populated with a compact flash card from the factory, and it is the default boot location.
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