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Alternative Implementations of OSPF Area Border Routers (RFC3509)

IP.com Disclosure Number: IPCOM000012090D
Original Publication Date: 2003-Apr-01
Included in the Prior Art Database: 2003-Apr-08
Document File: 13 page(s) / 24K

Publishing Venue

Internet Society Requests For Comment (RFCs)

Related People

A. Zinin: AUTHOR [+3]

Abstract

Open Shortest Path First (OSPF) is a link-state intra-domain routing protocol used for routing in IP networks. Though the definition of the Area Border Router (ABR) in the OSPF specification does not require a router with multiple attached areas to have a backbone connection, it is actually necessary to provide successful routing to the inter-area and external destinations. If this requirement is not met, all traffic destined for the areas not connected to such an ABR or out of the OSPF domain, is dropped. This document describes alternative ABR behaviors implemented in Cisco and IBM routers.

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Network Working Group                                           A. Zinin

Request for Comments: 3509                                       Alcatel

Category: Informational                                        A. Lindem

                                                        Redback Networks

                                                                D. Yeung

                                                        Procket Networks

                                                              April 2003

        Alternative Implementations of OSPF Area Border Routers

Status of this Memo

   This memo provides information for the Internet community.  It does

   not specify an Internet standard of any kind.  Distribution of this

   memo is unlimited.

Copyright Notice

   Copyright (C) The Internet Society (2003).  All Rights Reserved.

Abstract

   Open Shortest Path First (OSPF) is a link-state intra-domain routing

   protocol used for routing in IP networks.  Though the definition of

   the Area Border Router (ABR) in the OSPF specification does not

   require a router with multiple attached areas to have a backbone

   connection, it is actually necessary to provide successful routing to

   the inter-area and external destinations.  If this requirement is not

   met, all traffic destined for the areas not connected to such an ABR

   or out of the OSPF domain, is dropped.  This document describes

   alternative ABR behaviors implemented in Cisco and IBM routers.

1 Overview

1.1 Introduction

   An OSPF routing domain can be split into several subdomains, called

   areas, which limit the scope of LSA flooding.  According to [Ref1] a

   router having attachments to multiple areas is called an "area border

   router" (ABR).  The primary function of an ABR is to provide its

   attached areas with Type-3 and Type-4 LSAs, which are used for

   describing routes and AS boundary routers (ASBRs) in other areas, as

   well as to perform actual inter-area routing.

Zinin, et al.                Informational                      [Page 1]

RFC 3509                   OSPF ABR Behavior                  April 2003

1.2 Motivation

   In OSPF domains the area topology is restricted so that there must be

   a backbone area (area 0) and all other areas must have either

   physical or virtual connections to the backbone.  The reason for this

   star-like topology is that OSPF inter-area routing uses the

   distance-vector approach and a strict area hierarchy permits

   avoidance of the "counting to infinity" problem.  OSPF prevents

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