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Table Of Contents  The TCP/IP Guide
 9  TCP/IP Lower-Layer (Interface, Internet and Transport) Protocols (OSI Layers 2, 3 and 4)
      9  TCP/IP Internet Layer (OSI Network Layer) Protocols
           9  TCP/IP Routing Protocols (Gateway Protocols)
                9  TCP/IP Interior Routing Protocols (RIP, OSPF, GGP, HELLO, IGRP, EIGRP)
                     9  Open Shortest Path First (OSPF)

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Open Shortest Path First (OSPF)
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OSPF Basic Topology and the Link State Database (LSDB)
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OSPF Overview, History, Standards and Versions
(Page 2 of 2)

Overview of OSPF Operation

The fundamental concept behind OSPF is a data structure called the link-state database (LSDB). Each router in an autonomous system maintains a copy of this database, which contains information in the form of a directed graph that describes the current state of the autonomous system. Each link to a network or another router is represented by an entry in the database, and each has an associated cost (or metric). The metric can be made to include many different aspects of route performance, not just a simple hop count as is used in RIP.

Information about the autonomous system moves around the autonomous system in the form of link-state advertisements (LSAs), messages that let each router tell the others what it currently knows about the state of the AS. Over time, the information that each router has about the autonomous system converges with that of the others, and they all have the same data. When changes occur to the internetwork, routers send updates to ensure that all the routers are kept up-to-date.

To determine actual routes, each router uses its link-state database to construct a shortest-path tree. This tree shows the links from the router to each other router and network, and allows the lowest-cost route to any location to be determined. As new information about the state of the internetwork arrives, this tree can be recalculated, so the best route is dynamically adjusted based on network conditions. When more than one route with an equal cost exists, traffic can be shared amongst the routes.

OSPF Features and Drawbacks

In addition to these obvious benefits of the link-state algorithm, OSPF includes several other features of value especially to larger organizations. It supports authentication for security, and all three major types of IP addressing (classful, subnetted classful and classless). For very large autonomous systems, OSPF also allows routers to be grouped, and arranged into a hierarchical topology. This allows for better organization and improved performance through reduced link-state advertisement traffic.

Naturally, the superior functionality and many features of OSPF do not come without a cost. In this case, the primary cost is that of complexity. Where RIP is a simple and easy-to-use protocol, OSPF requires more work and more expertise to properly configure and maintain. This means that even though OSPF is widely considered better than RIP technically, it's not for everyone. The obvious role for OSPF is as a routing protocol for larger or higher-performance autonomous systems, leaving RIP to cover the smaller and simpler internetworks.

Key Concept: Open Shortest Path First (OSPF) was developed in the late 1980s to provide a more capable interior routing protocol for larger or more complex autonomous systems that were not being served well by RIP. It uses the dynamic shortest path first or link state routing algorithm, with each router maintaining a database containing information about the state and topology of the internetwork. As changes to the internetwork occur, routers send out updated state information, which allows each router to dynamically calculate the best route to any network at any point in time. OSPF is a complement to RIP in that RIP is simple but limited, where OSPF is more capable but more complicated.



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Open Shortest Path First (OSPF)
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OSPF Basic Topology and the Link State Database (LSDB)
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Version 3.0 - Version Date: September 20, 2005

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