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218 Chapter 7: OSPF, EIGRP, and IGRP

Foundation Topics

The Routing Information Protocol (RIP), which is defined in RFC 1058, was the first routing
protocol for the Internet. In today's complex internetworking environments, RIP's limitations
have made it virtually nonexistent. Today RIP exists, for the most part, in small networks where
servers perform as low-end routers. Simple Windows NT networks can perform server-based
routing scenarios. RIP's limited range, slow convergence, and susceptibility to routing loops
have rendered it unacceptable in today's Internet environment. Open Shortest Path First
(OSPF), defined in RFC 1583, was created to overcome the limitations of RIP. OSPF is now
considered to be the de facto standards-based routing protocol for IP. OSPF is built on a ladder
of network components. The highest level in the pecking order is the autonomous system (AS).
An autonomous system is a group of networks that share the same routing and administration
characteristics under the same domain.

An area represents a collection of adjacent routers that maintain the same link-state database.
Areas limit the area to which routing advertisements are broadcast.

NOTE

OSPF is named after the Dijkstra algorithm, which is also called the Shortest-Path Algorithm.

OSPF addresses the following limitations of RIP:

RIP has a hop-count limitation of 15. RIP always uses the lowest hop-count as the metric,
regardless of a link's speed or reliability. OSPF is a link-state protocol, which means that
it can consider metrics based on link speed when determining the best path to a destination
network. In addition, OSPF has no hop-count limit.

OSPF uses variable-length subnet masking, which provides for more efficient allocation.
RIP uses fixed-length subnet masking (FLSM), which uses fixed network boundaries. RIP
requires that all networks in the internetwork have the same subnet mask.

OSPF has better network convergence. OSPF routes are propagated only during link-state
changes. OSPF has reachability information for the whole area. RIP propagates every 30
seconds and must rely on its neighboring routers for reachability information.

OSPF Network Types

There are four types of OSPF router network connection types, three of which are shown in
Figure 7-2:

Point-to-point network--Joins a single pair of OSPF routers. ISDN and T1 connections
are examples of point-to-point networks.

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