Router Basics

What is a Router?

Network router is a device or a piece of software in a computer that forwards and routes data packets along networks. A network router connects at least two networks, commonly two LANs or WANs or a LAN and its ISP network. A router is located at any where one network meets another. A router has two key jobs: 

- The router ensures that information doesn't go where it's not needed. This is crucial for keeping large volumes of data from clogging the network.

- The router makes sure that information does make it to the intended destination. 

In performing these two jobs, a router joins the two networks, passing information from one to the other and, in some cases, performing translations of various protocols between the two networks. It also protects the networks from one another, preventing the traffic on one from unnecessarily spilling over to the other. This process is known as routing.

Routing is a function associated with the Network layer (layer 3) in the Open Systems Interconnection (OSI) model. Routers use network layer protocol headers, such as IP header where the source and destination addresses are included, and routing tables to determine the best path to forward the packets. For the communication among routers and decide the best route between any two hosts, routing protocols such as ICMP are used.   

One of the tools a router uses to decide which path a packet should go is a routing table. Information in the routing tables can be static (with routes manually entered by the network administrator) or dynamic (where routers communicate to exchange connection and route information using various routing protocols). A routing table can be as simple as a few lines in the smallest routers, but can grow to massive size and complexity in the very large routers that handle the bulk of Internet messages. As the number of networks attached to one another grows, the routing table for handling traffic among them grows, and the processing power of the router is increased. 

Protocol address = IP address, Physical address = MAC address. This figure shows the path taken by a packet to go from Source PC to Destination PC through three routers.

The two common series of router that we learn in CISCO are 2500 and 2600.

2500 Series -

- AUI & Serial are called Interfaces

- AUX & Console are called Ports

- Interfaces are used to connect two routers

- Ports are used to configure a Router

- A console port is used to directly configure a router

- AUX (Auxiliary) port is used to configure a router remotely. This is done using an external modem

- AUI (Attachment Unit Interface) is used to connect with LAN

- Serial Interfaces are used to connect a router to another router. The cable used for this is V.35 cable (it is a 60 pin cable)

2600 Series -

1600 Series -

To see all the series of router access the link below;

http://www-tss.cisco.com/eservice/common/hardware/02cards_modules/router/chassis_intf_info.html

Internal parts of a Router 

Processor

Motherboard

SMPS (Switch Mode Power Supply)

RAM (aka Running Configurations)

NVRAM (Nonvolatile RAM, aka Startup Configurations)

ROM (handle functions such as POST, BSL, Mini IOS)

Flash (IOS, Internetwork Operating System)

Note: POST (Power On Self Test), BSL (Boot Strap Loader)

All the configurations that you do on a router are stored in RAM. Once the router is restarted these configurations are lost and you will have to re-configure the router again. NVRAM is like a saved RAM, where all the configurations that you do on a router can be saved permanently, so even if it is restarted you don't loose it. You will learn about this in details when you do Backup and Restore.

Router Connections

2500 Series Router Connection

 

2600 Series Router Connection