Illustrated TCP/IP by Matthew G. Naugle Wiley Computer Publishing, John Wiley & Sons, Inc. ISBN: 0471196568   Pub Date: 11/01/98

Chapter 23
Introduction to Routing Protocols (OSPF)

OSPF is also routing protocol, but it does not compare to RIP with the exception that it, too, is an IGP. Of course, let’s be fair. In the beginning, when the Internet was created, the processors that we had were nowhere near the power of what we have today. In fact, a Honeywell 516 minicomputer was used as the first router (then called an Internet Message Processor, or IMP). The only micro–CPU in those days was the Z80 from Zilog. RIP worked great on the routers that we had at that time. It had very low overhead (computationally speaking). OSPF is a great protocol, but at the time of RIP, there was no machine that could run it economically.

Today, with the faster processors and plentiful memory, OSPF is the routing protocol of choice (for open routing protocols, that is). It is very efficient when it comes to the network, although it is a complicated protocol and is very CPU intensive when it builds its routing table.

OSPF is an IGP protocol. It exchanges routing information within a single autonomous system (described as those networks and routers grouped into a single domain under one authority). It can be used in small, medium, or large internetworks, but the most dramatic effects will be readily noticed on large IP networks. As opposed to RIP (a distance vector protocol), OSPF is a link–state protocol. It maintains the state of every link in the domain, and information is flooded to all routers in the domain. Flooding is the process of receiving the information on one port and transmitting it to all other active ports on the router. In this way, all routers receive the same information. This information is stored in a database called the link–state database, which is identical on very router in the AS (or every area if the domain is split into multiple areas). Based on information in the link–state database, an algorithm known as the Dykstra algorithm runs and produces a shortest–path tree based on the metrics, using itself as the root of the tree. The information this produces is used to build the routing table.