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Dimension ES-4024 Ethernet Switch
OSPF 26-1
This chapter describes the OSPF (Open Shortest Path First) routing protocol and shows you how to
configure OSPF on the ES.
26.1 OSPF Overview
OSPF (Open Shortest Path First) is a link-state protocol designed to distribute routing information within an
autonomous system (AS). An autonomous system is a collection of networks using a common routing protocol to
exchange routing information.
OSPF offers some advantages over traditional vector-space routing protocols (such as RIP). The following table
summarizes some of the major differences between OSPF and RIP.
Figure 26-1 OSPF vs. RIP
OSPF RIP
Network Size Large Small (with up to 15 routers)
Metrics Bandwidth, hop count, throughput, round trip
time and reliability.
Hop count
Convergence Fast Slow
26.1.1 OSPF Autonomous Systems and Areas
An OSPF autonomous system can be divided into logical areas. Each area represents a group of adjacent networks.
All areas are connected to a backbone (also known as area 0). The backbone is the transit area to route packets
between two areas. A stub area, at the edge of an AS, is not a transit area since there is only one connection to the
stub area.
The following table describes the four classes of OSPF routers.
Table 26-1 OSPF Router Types
TYPE DESCRIPTION
Internal Router (IR) An Internal or intra-area router is a router in an area.
Area Border Router (ABR) An Area Border Router connects two or more areas.
Backbone Router (BR) A backbone router has an interface to the backbone.
AS Boundary Router An AS boundary router exchanges routing information with routers in other ASes.
The following figure depicts an OSPF network example. The backbone is area 0 with a backbone router. The
internal routers are in area 1 and 2. The area border routers connect area 1 and 2 to the backbone.
Chapter 26
OSPF