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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)

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RIPng ("RIPv6") Message Format and Features
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OSPF Overview, History, Standards and Versions
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Open Shortest Path First (OSPF)

Interior routing protocols using a distance-vector routing algorithm, such as the Routing Information Protocol (RIP), have a long history and work well in a small group of routers. However, they also have some serious limitations in both scalability and performance that makes them poorly-suited to larger autonomous systems or those with specific performance issues. Many organizations that start out using RIP quickly found that its restrictions and issues made it less than ideal.

To solve this problem, a new routing protocol was developed in the late 1980s that uses the more capable (and more complex) link-state or shortest path first routing algorithm. This protocol is called Open Shortest Path First (OSPF). It fixes many of the issues with RIP and allows routes to be selected dynamically based on the current state of the network, not just a static picture of how routers are connected. It also includes numerous advanced features, including support for a hierarchical topology and automatic load sharing amongst routes. On the downside, it is a complicated protocol, which means it is often not used unless it is really needed. This makes it the complement of RIP and is the reason they both have a place in the spectrum of TCP/IP routing protocols.

In this section I provide a condensed explanation of the concepts and operation behind OSPF. As usual, I begin with an overview of the protocol, discussing how it was developed, its versions and the standards that define them. I describe the concepts behind OSPF, including basic topology and the link state database. I then discuss the more complex optional hierarchical topology of routers, and the roles routers play when this topology is used. I briefly explain the method used for determining routes in OSPF, and the general operation and messaging used in the protocol, including a description of the five OSPF message types. I conclude with an illustration of the formats used for OSPF messages.

Note: The difficult thing about networking is that so many protocols and technologies are so involved that each deserves its own book. This is certainly the case with OSPF itself, which is sufficiently complex that the RFC defining OSPF Version 2 is over 240 pages long. Thus, as you have heard me say before, this section, despite including six topics, can only be considered a high-level description of OSPF.


Quick navigation to subsections and regular topics in this section



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OSPF Overview, History, Standards and Versions
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