Thursday, 11 October 2012

IP Routing


Routing refers to the process of transferring data from the source to the destination. It occurs at the network layer of the OSI model. A router selects the appropriate route for the data packets based on its routing tables. Every router maintains a routing table that stores different routes on the network. IP Routing provides a mechanism for the routers to connect networks and transfer data based on the IP header in the data packet.

Routing Fundamentals

Routing is the Process by which a data packet is transported from one location to another. Routers are used in computer networks to route traffic. To be able to route anything, a router, or any other entity that performs the routing, needs to know the following key Information:
  • Destination Address : The destination address of the data pocket that needs to be routed.
  • Identifying sources of Information : The sources (other routers) from which the router can learn the various paths to given destinations.
  • Discovering Router : The initial possible routers, or paths, to the intended destinations.
  • Selecting Routers : The best path to the intended destination.
  • Maintaining Routing Information :  A way of verifying that the known paths to destinations are the most current.


Data is transmitted over the network using protocols. Protocols are a set of rules that define data transfer. The protocols can be classified based on their routing abilities as:

Routing :

Enable routers to determine the available and efficient routes to the destination. Some of the routing protocols include:
  • RIP : The Routing Information Protocol (RIP) is a distance vector protocol that uses hop count as its metric.
  • IGRP  : The Interior Gateway Routing Protocol (IGRP) is a robust protocol for routing within an autonomous system having arbitrarily complex topology and consisting of media with diverse bandwidth and delay characteristics. It was developed by Cisco Systems in the      mid-1980s and is a distance vector interior gateway protocol that uses a combination of metrics to make routing decision.
  • EIGRP : The Enhanced Interior Gateway Routing Protocol (EIGRP) is an modified version of IGRP that was developed by Cisco Systems in the earl 1990s. it uses the same distance vector algorithm and distance information as IGRP and is a distance vector interior getway protocol that has the following features:
  1. Uses a combination of metrics to make routing decisions.
  2. Using the Diffusing Update Algorithm (DUAL) to allow routes to converge quickly.
  3. Sends partial routing table updates.
  4. Implements a neighbour discovery mechanism.
  • OSPF : Open shortest Path First (OSPF) is a link-state routing protocol. It was designed to operate in Transmission Control Protocol/Internet Protocol (TCP/IP) networks and to address the short comings of the Routing Information Protocol (RIP)


Consists of protocols that define routing and addressing. Some of the routed protocols are:
  • Internet Protocol (IP) : The Internet Protocol stack, also known as TCP/IP, is a protocol used in the Internet.
  • Apple Talk (AT) : Apple Talk is a client-server networking system designed by Apple Computer.
  • Novel NetWare Protocol : Novel NetWare is a network operating System (NOS) and protocol suite that provides transparent remote file access and other distributed services.


Consists of protocols that cannot be routed. Some of the Non-Routable protocols are :
  • NetBEUI : Consists of an enhanced version of the NetBIOS Protocol for network operating Systems.
  • Data Link Control (DLC) : Used for mainframe technologies, since it is a transport protocol.
  • Maintenance Operation Protocol (MOP) :  Enables remote communication between hosts and devices on a network.

Routing Metrics

Routing Metrics are values that enable routers decide the best route for the data packet. Metrics consists of network variables that are static for routing protocols, such as RIP and IGRP. The network administrator can assign the metric values for the protocols that do not have metric values assigned by default. Routing protocols have different metrics values, such as Hops,  Bandwidth,   Delay, Load and Cost.


Hops enable to measure the distance traveled by a data packet. The distance is measured in terms of the number of network segment. When a router forwards a data packet to a network segment, it is counted as a single hop. The route with the least number of hops is considered as the ideal routed for the data packet. Consider a network with five routers.


Bandwidth measure the amount of data that can travel over the network segment in a fixed amount of time. The bandwidth is measure is bits per second. The route that sopports higher bandwidth is the best route for the data packet. Consider a network with five routers having different bandwidth.


Delay specifies the amount of time that a router takes to process and transfer a data packet to the next device on the network. The delay is represented with a ยต symbol. Consider a network with five routers having different delay rates.
Load refers to the amount to traffic on a particular link relative to the total capacity of the link. The load value 255 indicates 100% traffic congestion. Lower values indicate moderate traffic on the link. The router sends the data packet over a route that has low traffic. Consider a network with five routers.
Companies who value their operating expenditure more than performance consider the cost factor. These companies send data over their own lines instead of leasing separate lines for minimizing costs even if the delay factor is high.

Routing Mechanisms

Every Router has a routing table based on which the data packets are forwarded to the destinations. The router creates the routing table based on factors, such as, interfaces and route types.

Direct Interfaces

Direct Interfaces consist of routes that are local to the router. For example, If a router is connected to interfaces, the possible router for a data packet get automatically stored in the routing table. The data packets do not need any routing protocols to reach the destination. The router can directly forward the data since the interfaces are connected to the router directly.

Static Routes

Static Routes are the routes that the administrator manually enters in the routing table. These define the IP address of the router that is the next hop of the data packet. Static routes cannot adjust to network changes. For example, If a router or an interface suddenly fails, the routes cannot work. Temporary connections, such as, dial-up networks use static routes.
Static Routers conserve bandwidth, since routers do not generate route update traffic. These routes are time-consuming, as the administrators have to enter the route information in the routing table.
The ip route command enables us to configure static routes. The syntax of the command is:
ip route destination_network_
    [subnet_mark] IP_address_of_next_hop_neighbor
To configure static routes:
  1. Open the CLI.
  2. Switch to the privileged mode.
  3. Switch to the configuration mode.
  4. Enter the following command in the command prompt to configure static route for the IP address    ip route
  5. Press Enter.

Default Routes

Every router requires a default route to which it can send data packets if non to the routes are available in the routing table. The default route can be manually entered in the routing table or the router can dynamically configure it. Default routes enable the router to send data packets to an interface that is not in the local segment. If the Routes use default routing as a last resort when all other routes have been exhausted.
To configure Default routes:
  1. Open the CLI.
  2. Switch to the privileged mode.
  3. Switch to the configuration mode.
  4. Enter the following command in the command prompt to configure default route for the IP address    ip route
  5. Press Enter.

Dynamic Routes

The Router automatically collects routes from the network and stores it in the routing table. These routes are called as dynamic routes. The router first collects routes from its own network. then the router looks for routes running on the some protocol. After collecting all possible routes, the router decides the best possible route for the data. The route information is passed on to the next router on the network. This information goes on extending for each router thus resulting in an updated routing table.
Dynamic Routing uses routing protocols for updating routing tables. Dynamic routing protocols include OSPF, RIP, IGRP, EIGRP.
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