Implementation of OSPF (Open Short Path First) Routing Protocol using Dijkastra Algorithm.

Open Shortest Path First (OSPF) is a standard routing protocol that’s been used the world over for many years. Supported by practically every routing vendor, as well as the open source community, OSPF is one of the few protocols in the IT industry you can count on being available just about anywhere you might need it.

Enterprise networks that outgrow a single site will often use OSPF to interconnect their campuses and wide area networks (WANs).

If you’re considering a dynamic routing protocol because your network has outgrown static routes, OSPF might seem a little daunting. It’s not quite as easy to set up as EIGRP so the temptation might be to simply use EIGRP and avoid the intimidating terminology that comes along with a complete understanding of OSPF.

My recommendation is not to let OSPF scare you. It’s true that OSPF in large implementations can be complex. However, an OSPF configuration supporting smaller networks can be comparatively simple.

In this post, I’ll discuss some of OSPF’s major principles, and then follow up with a simple configuration that brings up OSPF between two Cisco routers and exchange routes.

The work of node is to communicate information between nodes assigned by routing protocol. It also helps in passing information and data packets between the nodes. The nodes have information about the network directly attached and with the help of routing protocol, it is aware of the entire structure of the network. There are different types of protocol, but we discuss about protocols used in IP network.

The IP network is classified into two categories:

1. Interior Gateway Protocol
2. Exterior Gateway Protocol

Interior Gateway protocol communicates routing data within a single routing domain. It is divided into Link State Routing protocol and Distance Vector Routing protocol. The link state routing protocol maintains the full structure of the network on each router connected to its network. For example, Routing Information Protocol (RIP) and Open Shortest Path First (OSPF). In the Distance Vector Routing protocol, the route information is periodically shared in the entire network. For example, Interior Gateway Routing Protocol (IGRP). An exterior Gateway protocol communicates routing information with independent systems.For example,

 Exterior Gateway Protocol (EGP)

 Border Gateway Protocol (BGP) 

Open Shortest Path First (OSPF) is a routing protocol developed for Internet Protocol (IP) networks by the interior gateway protocol (IGP) working group of the Internet Engineering Task Force (IETF). The working group was formed in 1988 to design an IGP based on the shortest path first (SPF) algorithm for use in the Internet. Similar to the Interior Gateway Routing Protocol (IGRP), OSPF was created because in the mid-1980s, the Routing Information Protocol (RIP) was increasingly unable to serve large, heterogeneous internetworks.

OSPF is a classless routing protocol, which means that in its updates, it includes the subnet of each route it knows about, thus, enabling variable-length subnet masks. With variable-length subnet masks, an IP network can be broken into many subnets of various sizes. This provides network administrators with extra network-configuration flexibility.These updates are multicasts at specific addresses (224.0.0.5 and 224.0.0.6).

OSPF has two primary characteristics:

1) The protocol is open (non proprietary), which means that its specification is in the public domain. The OSPF specification is published as Request For Comments (RFC) 1247.

2) The second principal characteristic is that OSPF is based on the SPF algorithm, which sometimes is referred to as the Dijkstra algorithm, named for the person credited with its creation.

OSPF is a Link State routing protocol that calls for the sending of link-state advertisements (LSAs) to all other routers within the same hierarchical area. Information on attached interfaces, metrics used, and other variables is included in OSPF LSAs. As OSPF routers accumulate link-state information, they use the SPF algorithm to calculate the shortest path to each node.

As a Link State routing protocol, OSPF contrasts with RIP and IGRP, which are Distance Vector routing protocols. Routers running the Distance Vector algorithm send all or a portion of their routing tables in routing-update messages to their neighbors.

Additional OSPF features include equal-cost, multipath routing, and routing based on upper-layer type-of-service (TOS) requests. TOS-based routing supports those upper-layer protocols that can specify particular types of service. An application, for example, might specify that certain data is urgent. If OSPF has high-priority links at its disposal, these can be used to transport the urgent datagram.

OSPF supports one or more metrics. If only one metric is used, it is considered to be arbitrary, and TOS is not supported. If more than one metric is used, TOS is optionally supported through the use of a separate metric (and, therefore, a separate routing table) for each of the eight combinations created by the three IP TOS bits (the delay, throughput, and reliability bits). If, for example, the IP TOS bits specify low delay, low throughput, and high reliability, OSPF calculates routes to all destinations based on this TOS designation.

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