ROURING ALGORITHM: LINK STATE COMPUTER NETWORKS ROURING ALGORITHM: LINK STATE Group Memers:- Mehul Sharma(9914103192) Karan Saraswat(9914103193) Harshit(9914103194) Simran Singhla(9914103196) Shubham Kesarwani(9914103198)

INTRODUCTION Link-state routing protocols are one of the two main classes of routing protocols used in packet switching networks for computer communication the other being distance-vector routing protocols Examples of link-state routing protocols include Open Shortest Path First (OSPF) and intermediate system to intermediate system (IS-IS).

The link-state protocol is performed by every switching node in the network .The basic concept of link-state routing is that every node constructs a map of the connectivity to the network, in the form of a graph, showing which nodes are connected to which other nodes. Each node then independently calculates the next best logical path from it to every possible destination in the network. The collection of best paths will then form the node's routing table

Open Shortest Path First (OSPF) is a routing protocol for Internet Protocol (IP) networks. It uses a link state routing (LSR) algorithm and falls into the group of interior routing protocols, operating within a single autonomous system (AS). It is defined as OSPF Version 2 in RFC 2328 (1998) for IPv4. Intermediate System to Intermediate System (IS- IS) is a routing protocol designed to move information efficiently within a computer network, a group of physically connected computers or similar devices. It accomplishes this by determining the best route for datagrams through a packet-switched network.

Dijkstra’s Algorithm All link-state routing protocols apply Dijkstra’s algorithm to calculate the best path route. The algorithm is commonly referred to as the shortest path first (SPF) algorithm.

Working of Link state Link-state routing protocols are also known as shortest path first protocols and are built around Edsger Dijkstra’s shortest path first (SPF) algorithm.

Link-State Updates Link-state updates (LSUs) are the packets used for OSPF routing updates. This is how OSPF exchanges LSUs to discover the best routes.

Link-State Routing Process Each router learns about its own links and its own directly connected networks. This is done by detecting that an interface is in the up state. Link-state routers do this by exchanging Hello packets with other link-state routers on directly connected networks and neighbours respond to this by sending hello packets back to routers.

Link-State Routing Process Each router builds a link-state packet (LSP)containing the state of each directly connected link. This is done by recording all the pertinent information about each neighbor, including neighbor ID, link type, and bandwidth. Each router floods the LSP to all neighbors. Those neighbors store all LSPs received in a database. They then flood the LSPs to their neighbors until all routers in the area have received the LSPs. Each router stores a copy of each LSP received from its neighbors in a local database. Each router uses the database to construct a complete map of the topology and computes the best path to each destination network

Advantages of link state routing Link-state protocols use cost metrics to choose paths through the network. The cost metric reflects the capacity of the links on those paths.  Link-state protocols use triggered updates and LSA floods to immediately report changes in the network topology to all routers in the network. This leads to fast convergence times.  Each router has a complete and synchronized picture of the network. Therefore, it is very difficult for routing loops to occur.  Routers use the latest information to make the best routing decisions. 

Advantages of link state routing The link-state database sizes can be minimized with careful network design. This leads to smaller Dijkstra calculations and faster convergence.  Every router, at the very least, maps the topology of its own area of the network. This attribute helps to troubleshoot problems that can occur. 

Disadvantages of link state routing They require more memory and processor power than distance vector protocols. This makes it expensive to use for organizations with small budgets and legacy hardware.  They require strict hierarchical network design, so that a network can be broken into smaller areas to reduce the size of the topology tables.  They require an administrator who understands the protocols well.  They flood the network with LSAs during the initial discovery process. This process can significantly decrease the capability of the network to transport data. It can noticeably degrade the network performance. 

Disadvantages of dijkastra The major disadvantage of the algorithm is the fact that it does a blind search  there by consuming a lot of time waste of necessary resources.  Another disadvantage is that it cannot handle negative edges. This leads to  acyclic graphs and most often cannot obtain the right shortest path.