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Short for Open Shortest Path First

Short for Open Shortest Path First, a routing protocol developed for IP networks based on the shortest path first or link-state algorithm.

Routers use link-state algorithms to send routing information to all nodes in an Internetwork by calculating the shortest path to each node based on topography of the Internet constructed by each node. Each router sends that portion of the routing table (keeps track of routes to particular network destinations) that describes the state of its own links, and it also sends the complete routing structure (topography).

The advantage of shortest path first algorithms is that they result in smaller more frequent updates everywhere. They converge quickly, thus preventing such problems as routing loops and Count-to-Infinity (when routers continuously increment the hop count to a particular network). This makes for a stable network.

The disadvantage of shortest path first algorithms is that they require a lot of CPU power and memory. In the end, the advantages out weigh the disadvantages. OSPF Version 2 is defined in RFC 1583. It is rapidly replacing RIP on the Internet.


If you want to know more about OSPF, you can visit the links that I've given.

OSPF Home Page:

In this page you can find all the information to implement this routing protocol. It gives links to various RFCs that are required for implementing it. If you are new to OSPF, this is not the starting point. I recommend go thru some tutorials.

OSPF-2 Protocol Overview:

This site gives good introduction to OSPF Version 2. This is latest version of OSPF.

To RIP or To OSPF?

That is the question: should you use the older, more established RIP (Routing Information Protocol) or the new-kid-on-the-block OSPF (Open Shortest Path First) IGP (Interior Gateway Protocol)? This article can help you make the right decision--it describes the pro's and con's for using either protocol and provides some specific recommendations.

Mailing List:


OSPF Books:

Free routing software

If you want to want to learn practically about OSPF, you can make your linux PC to a OSPF (RIP/BGP) router. Here is the more information about the free routing software Zebra.

GNU Zebra is free software (distributed under GNU Generic Public License) that manages TCP/IP based routing protocols. It supports BGP-4 protocol as described in RFC1771 (A Border Gateway Protocol 4) as well as RIPv1, RIPv2 and OSPFv2. Unlike traditional, Gated based, monolithic architectures and even the so-called "new modular architectures" that remove the burden of processing routing functions from the cpu and utilize special ASIC chips instead, Zebra software offers true modularity.

The best way to learn OSPF is system testing it. I am giving link to the OSPF testing suits, which describes how to test OSPF and also gives test plans. The test plan becomes your guide to learn OSPF behavior under various conditions.

OSPF Test Suits:


This test suit is not for free. But it is very good one. This test suit is called ANVL. This can simulate number of routers in number of complex topologies. These topologies are very useful for testing OSPF protocol. Here is what they say about ANVL.

OSPF (Open Shortest Path First) is a link-state routing protocol intended to support large numbers of subnetworks within a single Autonomous System. ANVL's automated conformance test product allows network equipment manufacturers to verify their device's compliance to the OSPF standard.

This test suite includes over 300 tests based on RFC 1583 and RFC 2328. Network equipment manufacturers can use ANVL to quickly determine OSPF implementation errors by running ANVL's high log level (pass/fail). More detailed results (including packet decodes) can also be viewed by running the test suite's medium and low log levels.

Miscellaneous Links:

Open Shortest Path First IGP (ospf) Charter (Links to Draft and RFC)


Multicast Extenstions to OSPF (mospf) Charter. ( links to drafts and RFC)

Research paper describing OSPF usage and functionality

OSPF: Frequently Asked Questions