RFC 3509 (rfc3509) - Page 1 of 12

Alternative Implementations of OSPF Area Border 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.


   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

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