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Enterasys Xpedition User Reference Manual 131

Chapter 13

OSPF Configuration

Guide

OSPF Overview

Open Shortest Path First Routing (OSPF) is a shortest path first or link-state protocol. The

XP supports OSPF Version 2.0, as defined in RFC 1583. OSPF is an interior gateway

protocol that distributes routing information between routers in a single autonomous

system. OSPF chooses the least-cost path as the best path. OSPF is suitable for complex

networks with a large number of routers because it provides equal-cost multi-path routing

where packets to a single destination can be sent via more than one interface

simultaneously.

In a link-state protocol, each router maintains a database that describes the entire AS

topology, which it builds out of the collected link state advertisements of all routers. Each

participating router distributes its local state (i.e., the router’s usable interfaces and

reachable neighbors) throughout the AS by flooding. Each multi-access network that has

at least two attached routers has a designated router and a backup designated router. The

designated router floods a link state advertisement for the multi-access network and has

other special responsibilities. The designated router concept reduces the number of

adjacencies required on a multi-access network.

OSPF allows networks to be grouped into areas. Routing information passed between

areas is abstracted, potentially allowing a significant reduction in routing traffic. OSPF

uses four different types of routes, listed in order of preference:

• Intra-area

• Inter-area

• Type 1 ASE

• Type 2 ASE

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Cabletron Systems SSIM-R8-02 Spécifications Page 151