IP Routing

                                          IP Routing

Forwarding:

It refers to datagram transfer. It is performed by host or router. It uses routing table.

Routing:

It refers to propagation of routing information. It is performed by routers. It inserts or changes values in routing table.

Two Forms of Internet Routing:

Static Routing:

In Static routing, the table is initialized when system boots and there is no further changes. It is used by most Internet hosts. The typical routing table has two entries. For the local network it has direct delivery and for the communication to some other network it follows the nearest default route.

Dynamic Routing:

In dynamic routing the table is initialized when system boots. It includes routing software which learns routes and updates table. In this way continuous changes are possible due to routing software. It is used by IP routers. It requires special software which continuously updates the routing information. Each router communicates with neighbors. It passes routing information and uses Route Propagation Protocol to exchange the information with other routers.

Autonomous System Concept:

An autonomous system can be thought of as a set of networks and routers under one administrative authority. The term is flexible. It can be or correspond to an entire intuition or a single corporation. It is needed because no routing protocol can scale to entire Internet. Each Autonomous System chooses a routing protocol to exchange routing information which is summarized before being passed to another group.

Classification of Internet Routing Protocols:
There are two broad classes of Internet Routing Protocol:
Interior Gateway Protocols (IGPs):
It is used among routers within autonomous system. The destinations lie within IGP.
Exterior Gateway Protocols (EGPs):
It is used among autonomous systems. The destinations lie throughout Internet

Internet Routing Protocols:

Following are the Internet Routing Protocols.

1. Border Gateway Protocol (BGP)
2. Routing Information Protocol (RIP)
3. Open Shortest Path First Protocol (OSPF)

1. Border Gateway Protocols:

It is most popular Exterior Gateway Protocol in Internet.
Characteristics:

• Provides routing among autonomous systems (EGP).
• Provides policies to control routes advertised.
• Uses reliable transport (TCP).
• Gives path of autonomous systems for each destination.
• EGP is the of choice in the Internet.
• Current version is four (BGP-4).
• Provides facilities for Transit Routing.

2. Routing Information Protocols (RIP):

Characteristics:

• Used for routing within an autonomous system (IGP).
• Hop Count Metric: Routing Information Protocols measures distance in network hops, where each network between the source and destination counts as a single hop.
• Uses UDP for all message transmissions.
• Routing Information Protocols is used over LAN. Version 1 of Routing Information Protocols uses hardware broadcast and version 2 allows delivery via multicast.
• Used to advertise default route propagation. An organization can use Routing Information Protocols to install a default route in each router.
• It uses distance vector algorithm.
• Routing Information Protocols allows hosts to listen passively and update its routing table.

3. The Open Shortest Path First Protocol (OSPF):

As the internet grew in size, so did organizations. In particular, large ISPs appeared. To satisfy demand for a routing protocol that can scale to large organizations, the IETF devised an IGP known as the Open Shortest Path First Protocol (OSPF).

Characteristics:

• Designed as an Interior Gateway Protocol used to pass routing information among routers within an autonomous system.
• Open Shortest Path First includes a 32-bit address mask with each address, which allows the address to be classful, classless, or subnetted.
• A pair of routers using Open Shortest Path First can authenticate each message to ensure that messages are only accepted from a trusted source.
• Open Shortest Path First allows a router to introduce routes learned from another means (e.g., from BGP).
• Open Shortest Path First uses link-state routing.
• Traditional link state routing is inefficient across a multi-access network, such as an Ethernet, because all routers attached to the network broadcast link status. OSPF optimizes by designing a single router to broadcast on the network.

Written By: Asad Hussain