Understanding Routing Table

Routing Table

We all know, that the router’s primary function is to forward a packet towards its destination address. A router have physical interfaces which connects to other network devices. 

The routing table contains a list of specific routing destinations or pair of IP addresses in its memory. Sometimes it is also called routing information base of RIB. When the router receives a packet of data, it references the routing table to know where to send that data. The device kernel reads the routing table. Each entry of row in the routing table defines a route. This route is in two types such as network routes and host routes. So the router does not have to send any query or to wait for a routing input from user to send packets each time.

Network routes says, how to reach to a specific network ID within the internetwork. Whereas the host route provides information to reach a particular host on a network.

All IP enabled devices use routing table. Means the routing table keeps the information about the entire topology of the network such as IP address, gateway address, port number, subnet mask, interface, metric.

Metrics: if in doubt about metrics, it is a number describes the best route or it helps in router to choose best route among multiple routes to the destination. It helps dynamic routing protocols like, RIP, OSPF, EIGRP to decide which is shortest and best path to reach the destination. 

Static routing uses a routing table which can preconfigured manually, where all entries will remain the same unless they are changed manually. It can be best used for small networks, hosts. For many small office work space, people use window system. If you want to add a static route to the table, the below syntax can be used:

Route ADD destination_network MASK subnet mask gateway_ip metric_cost

In case of Dynamic routing, protocols allow routers to get information from other (peer) routers on the network such as the routers can be configured to learn IP destinations from other routers in order to update routing table entries without user intervention. For more information in dynamic routing please refer to EIGRP, OSPF, BGP and RIP post. Dynamic routing is suitable for larger internetworks.

A sample routing table provided as below:

Network Destination	Subnet Mask	Gateway	Interface	Metric
101.X.X.0	255.255.255.0	10.X.X.2	Eth01	1
Default	0.0.0.0	10.X.X.3	Eth02	0
127.X.X.9	255.255.255.0	10.X.X.4	S0	291
10.X.X.8/16	255.255.255.0	10.X.X.1	GE1/0/0	11

-DR

Basics of Router, IOS and Gateway

Router

A router is a Computer or a Device used for connecting multiple networks. This means it has multiple interfaces that each belong to a different IP network.

It also have components like RAM,ROM,PROCESSOR like computer CPU.

Basically when a router receives a IP packet on one interface, it determines which interface to use for forwarding the packets to which destination address.

• Used for packet switching and packet filtering.
• It also determines the best path to send packets.
• It never forward broadcast by default.

The router will receive a Packet that is encapsulated in one type of datalink frame such as ethernet, and while forwarding the packet it encapsulate to different frame like PPP (Point to point protocol).

Many vendors/OEMs (Original Equipment Manufacturer) are manufacturing Routers now a days.

CISCO, D-LINK, DATASYS, HUAWEI etc.

A sample Cisco Router

Router Interfaces

There are many interfaces in a router, the physical interface is known as WIC-2T  (Wan Interface Card).

In case of Leased Line connections, A modem can be connected through this interface by the serial V.35 cable, DTE smart DB60 cable serial as shown below in pic. 

WIC Card

Serial Cable

A serial cable used at serial port of router is to use as administrative terminal access which can be used for configuring the router.

There are many types and series of routers. For higher series of routers i.e. at core routers  another interface used is STM card. STM-1, STM-4, STM-16 like this for Fiber connectivity .

  

: Fig :STM-1 CARD  

Typically A router can be represented in a network with a symbol as shown below.:

IOS

Internetwork operating System, is the OS used in Routers. Basically IOS manages the hardware and software resources of the router including memory allocation, Processes, Security and file systems.

• It provides the privilege levels for access to different commands.
• It provides the interface between user and hardware enabling user to execute commands to   configure and manage the devices.
• It provides Connectivity, Modularity, Reliability in network redundancy, Security policies.
• The IOS is stored in Flash Memories and can be updates accordingly when needed.
• It carries Network protocols and Functions. 
• Connecting high speed traffic between devices.
• Routers are Network Layer ( L3) Devices. 

 

Flash Memory Card of Router

NVRAM: Non Volatile RAM. 

The "enable" password or the "enable−secret" password is stored in the startup configuration file in the nonvolatile RAM (NVRAM). The password recovery procedure involves booting the router while ignoring the startup configuration file in the NVRAM.

Routers come in different series to Market as per the requirement and sizing of the network.

Below are some sample series; 

Configuring Router (Cisco) for example:

If you need to configure your router from CLI (Command Line Interface) mode, You need to enable command first.

Router>enable (Enter)

Output:

Router#

Then Type

Router# Config t

or

Router# Configure terminal

If you type a question mark (?) at the prompt, you will get all the commands available.

Basic Routing:

• Routing is the process of transferring data from one local area network to another local area network.
• Bridge is a network connection that connects two or more network segments and shares traffic as necessary according to hardware addresses. A bridge is a layer two device (data link).
• Router is a device that receives and forwards traffic according to software addresses. A router is a layer three device according to OSI model.
• Network interface is a software object that connects to a physical device such as modem or network card, Lan Card.
• Demand dial interfaces - these are interfaces such as VPN, persistent dial-up connection and PPPoE connection. New demand dial interfaces are added through Network Interfaces node.
• Windows includes software router called Routing and Remote access service. This is a multiprotocol router capable of LAN to LAN, LAN to WAN, VPN, NAT routing through IP networks. It also supports routing futures such as IP multicasting, demand-dialing, packet filtering, DHCP relay, build in support for RIP 2 and OSPF.
• NAT stands for network address translation and is a service that is part of a router in which the header information in IP data grams is modified by the router before being sent out. This allows many computer with private addresses to share a single public IP and still be able to surf the net.
• Switching: Lan switching is a form of packet switching in local area network. It is hardware based method and packets are sent where it is need. It is of collision free and reliable technique. There are many kinds like layer 2, layer 3,layer 4 and multi-layer switching [combination of all layers].

Basic router used for Routing Network

Gateway:

A gateway is a network node or an interface of a router in a computer network. It uses packet switching technique for data transmission.

Network gateways, or protocol translation gateways can perform protocol conversions to connect networks with different network protocol technologies.

It is responsible for the data for communication by routing to another network. It can stop the traffic at its interface also. 

It operates at layer 3 of OSI model. All the data inflows and outflows are managed by it. The gateway gives us access into different networks for which we can send email, browse at web pages, buy online, chat online, etc. 

On basis of data flow and directions, it can be further divided as unidirectional gateway and bidirectional gateway for either one direction or both directions.

Sometimes we have heard as default gateway. The default gateway set to router address as xx.xxx.xx.1 at many places. In larger networks, a network gateway typically acts as a proxy server and a firewall.

A router have many algorithms to work. Those routing algorithms help the routers to work in such a way its intended.

Below are the routing algorithms;

Static and Dynamic: Static routes are manually configured and modified whereas dynamic routes dynamically maintain the routing table as per the network and the changes occur within the network.

Flat and Hierarchical: In a flat routing system, the routers are peers of all other routers. In a hierarchical routing system, some routers form a routing backbone or area.

Host-intelligent and Router-intelligent: In host intelligent, routing algorithms allow the source end system determines the entire route to a destination (source routing). Most routing algorithms assume that hosts know nothing about network, and the path determination process is done by the routing algorithms called router intelligent.

-DR