Cabletron Systems 9032578-05 manual Ospf Overview

Chapter 11

OSPF

Configuration

Guide

OSPF Overview

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

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