Router Elements

What is Router?

Router is an OSI network layer device used to connect different networks. So one can say that router is a WAN device. Cisco is the worldwide leader in providing the hardware and software solutions of WAN. Cisco deals in various models and series of routers.

Cisco Routers

Cisco also defines a three layer hierarchical model as a comparison with OSI layer model. Router basically works at the distribution layer of hierarchical model. Access-list filtering, compression and encryption can be performed at the distribution level of a hierarchical router internetwork. Although Cisco also provides the access-level and core-level routers. Common access-level routers are the Model 1600, 2500 and 2600 routers. Common campus-level distribution routers are Model 4000, 7200 or even 7500 routers with a Fast-Ethernet interface. In 1993, Cisco introduced the Model 7000 router. The Model 7000 router is the direct descendant of the AGS+ and the direct predecessor of the current model 7000 core router. Today, model 7500 routers can be loaded with high performance route switch processors and high performance interface processors such as Versatile Interface Processor (VIP) for optimized routing. Even with the optimized features of the 7500 series routers, many internetwork designers were searching for alternative core routing technologies, such as ATM switching or packet over SONET switching.

Routers Elements

Router is a hardware based device. It means that routers has a motherboard consists of CPU, Memory and some built in chips along with other internal components.

The internal components of Routers are:

Read Only Memory (ROM)
Flash
Random Access Memory (RAM)
Non-Volatile Random Access Memory (NVRAM)
Interfaces

You can verify the status of these components with the router show commands.

ROM

The Read Only Memory (ROM) contains microcode for basic functions. It is basically a firmware that resides some built-in software. ROM consists of four major components. The router command "show version" displays the status of ROM and its component such as Bootstrap.

The four major components of ROM are:

POST: The POST (Power on Self Test) is the first process that runs from ROM after the booting. It checks and verifies the hardware of the devices.

Bootstrap: Every Operating System has two types of file, booting files and the system files. First the booting files load and then booting files load the system files. Bootstrap is the boot loader of the Cisco IOS. It has the path to booting files. After post has been completed, bootstrap loads and runs.

Mini IOS: Automatically boot from ROM. Provides the subset of Cisco IOS.

ROM monitor: You can use ROM monitor mode with manually boot using the b command)

Flash

Flash is a type of memory used for residing the compressed file image of the Cisco IOS. Normally there is only one file resides in the router flash. The size of the flash varies according to the series and model of the router. It can vary from 8 to 64 MB size. Bootstrap loading the IOS from flash in normal booting process. The "show flash" command displays the status and contents of flash.

RAM

The Random Access Memory (RAM) is the current configuration of the router that provides an interface to the end user. It is a type of unsaved and volatile memory. RAM directly affects the performance of the router. The IOS file is decompressed in RAM. It is also known as running configuration. You can verify the status of Random Access Memory (RAM) with the "show running- configuration" command.

NVRAM

The Non Volatile Random Access Memory (NVRAM) is the saved configuration that loads during the start-up process of the Cisco IOS router. That's why it is also known as startup configuration. It is a non volatile type of storage memory that just works like the hard disc of the computer. The router command "show startup-configuration" displays the status of nvram. If you want to reset nvram, execute the privileged mode command "delete nvram". It also has the config register that can be viewed with "show version" command.

Interfaces

Router is just like a junction point that could have multiple interfaces. Each interface connects with a different network and router transmits the data traffic of these networks through its interface. Each interface has its own specific configuration such as logical addressing. There are multiple types of interfaces available on the routers such as Ethernet AUI interfaces for LAN while synchronous serial interfaces and asynchronous interfaces for WAN connections. The type and number of interfaces varies from router model and series. Routers must have at least one Ethernet LAN port and one WAN port normally ISDN BRI.

Some of the router show commands related with interfaces are as follows:

show interface: displays the complete status of all the interfaces in the router

show interface serial 0: displays the complete status of a specific interface

show ip interface brief: displays the short but brief and sufficient details such as the IP addresses and the status of all the interfaces of routers.

Boot Sequence of the Cisco Routers

Run Power on Self Test (POST)

Load and run bootstrap code.

Find the IOS software

Load the IOS software

Find the configuration

Load the configuration

Run

Finding the IOS

After loading the bootstrap find the IOS first, in normal boot the image file of the IOS resides in the flash. If there is no IOS image file in the flash then it attempts net boot, if there is any problem in net boot then RXBOOT mode runs and for the last option ROMMON mode is also available.