ZyXEL Communications XS3800-28, User Manual, Page: 501 / 789

Chapter 67 OSPF
XS3800-28 User’s Guide
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HAPTER
67
OSPF
This chapter describes the OSPF (Open Shortest Path First) routing protocol and shows you how to
configure OSPF.
67.1 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.
The Switch uses OSPFv2 for IPv4. OSPFv2 uses plain text or MD5 authentication.
67.1.1 OSPF Autonomous Systems and Areas
An OSPF autonomous system (AS) 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.
The following figure depicts an OSPF network example. The backbone is area 0 with a backbone router.
Table 263 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
Table 264 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 ASs.