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TCP/IP Tutorial and Technical Overview - IBM Redbooks

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5.6 Open Shortest Path First (OSPF)<br />

The Open Shortest Path First (OSPF) protocol is another example of an interior<br />

gateway protocol. It was developed as a non-proprietary routing alternative to<br />

address the limitations of R<strong>IP</strong>. Initial development started in 1988 <strong>and</strong> was<br />

finalized in 1991. Subsequent updates to the protocol continue to be published.<br />

The current version of the st<strong>and</strong>ard is documented in RFC 2328.<br />

OSPF provides a number of features not found in distance vector protocols.<br />

Support for these features has made OSPF a widely-deployed routing protocol in<br />

large networking environments. In fact, RFC 1812 – Requirements for <strong>IP</strong>v4<br />

Routers, lists OSPF as the only required dynamic routing protocol. The following<br />

features contribute to the continued acceptance of the OSPF st<strong>and</strong>ard:<br />

► Equal cost load balancing: The simultaneous use of multiple paths can<br />

provide more efficient utilization of network resources.<br />

► Logical partitioning of the network: This reduces the propagation of outage<br />

information during adverse conditions. It also provides the ability to aggregate<br />

routing announcements that limit the advertisement of unnecessary subnet<br />

information.<br />

► Support for authentication: OSPF supports the authentication of any node<br />

transmitting route advertisements. This prevents fraudulent sources from<br />

corrupting the routing tables.<br />

► Faster convergence time: OSPF provides instantaneous propagation of<br />

routing changes. This expedites the convergence time required to update<br />

network topologies.<br />

► Support for CIDR <strong>and</strong> VLSM: This allows the network administrator to<br />

efficiently allocate <strong>IP</strong> address resources.<br />

OSPF is a link state protocol. As with other link state protocols, each OSPF<br />

router executes the SPF algorithm (“Shortest-Path First (SPF) algorithm” on<br />

page 177) to process the information stored in the link state database. The<br />

algorithm produces a shortest-path tree detailing the preferred routes to each<br />

destination network.<br />

5.6.1 OSPF terminology<br />

OSPF uses specific terminology to describe the operation of the protocol.<br />

OSPF areas<br />

OSPF networks are divided into a collection of areas. An area consists of a<br />

logical grouping of networks <strong>and</strong> routers. The area can coincide with geographic<br />

or administrative boundaries. Each area is assigned a 32-bit area ID.<br />

196 <strong>TCP</strong>/<strong>IP</strong> <strong>Tutorial</strong> <strong>and</strong> <strong>Technical</strong> <strong>Overview</strong>

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