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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.
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