Open Shortest Path First

Open Shortest Path First
O
pen Shortest Path First (OSPF)
is an open standards routing protocol. It is important to recognize
that Cisco’s implementation of OSPF is a standards-based version. This means that Cisco
based its version of OSPF on the open standards. While doing so, Cisco has also added features
to its version of OSPF that may not be found in other implementations of OSPF. This becomes
important when interoperability is needed.
OSPF has become one of the most widely used routing protocols in existence today because of
the ability to implement it across multi-vendor platforms. OSPF utilizes Dijkstra’s Shortest Path
First (SPF) algorithm, which allows for faster network convergence. The popularity of OSPF is
continuing to grow with the advent of Multi-Protocol Label Switching (MPLS). Currently, the
only routing protocols MPLS traffic engineering has extensions for are OSPF and IS-IS.
John Moy heads up the working group of OSPF. Two RFCs define OSPF: Version 1 is
defined by RFC 1131, and Version 2 is defined by RFC 2328. Version 2 is the only version to
make it to an operational status. However, many vendors modify OSPF. OSPF is known as a
link-state routing protocol (link-state routing protocols were discussed in Chapter 1, “Routing
Principles”). The Dijkstra algorithm is used to calculate the shortest path through the network.
Within OSPF, links become synonymous with interfaces.
Some of the advantages of OSPF are as follows:

Support of hierarchical network design through the use of areas.

The use of link-state databases reduces the chances of routing loops.

Full support of classless routing behavior.

Decreased size in routing tables through the use of manual route summarization. Automatic
route summarization is not supported by OSPF.

Routing updates are sent only when the information is needed, decreasing the use of network
bandwidth for routing updates.

Utilization of multicast packets decreases the impact on routers not running OSPF and
end stations.

Support of authentication, which allows the user to implement more secure networks.
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OSPF is a robust protocol, and due to its robustness, you must learn many terms in order to
understand the operation of OSPF. The next section covers the terminology necessary to enable
you to understand the many operations and procedures performed by the OSPF process.