Alternative Implementations of OSPF Area Border Routers
RFC 3509

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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
   inter-area routing loops by implementing a split-horizon mechanism,
   allowing ABRs to inject into the backbone only Summary-LSAs derived
   from the
   intra-area routes, and limiting ABRs' SPF calculation to consider
   only Summary-LSAs in the backbone area's link-state database.

   The last restriction leads to a problem when an ABR has no backbone
   connection (in OSPF, an ABR does not need to be attached to the
   backbone).  Consider a sample OSPF domain depicted in the Figure 1.

                          .                .
                           .    Area 0    .
                            +--+      +--+
                          ..|R1|..  ..|R2|..
                         .  +--+  ..  +--+  .
                         .        ..        .
                         .       +--+       .
                         . Area1 |R3| Area2 .
                         .       +--+  +--+ .
                         .        ..   |R4| .
                         .       .  .  +--+ .
                          .......    .......

                  Figure 1. ABR dropping transit traffic

   In this example R1, R2, and R3 are ABRs.  R1 and R2 have backbone
   connections, while R3 doesn't.

   Following the section 12.4.1 of [Ref1], R3 will identify itself as an
   ABR by setting the bit B in its router-LSA.  Being an ABR, R3 can
   only consider summary-LSAs from the backbone when building the
   routing table (according to section 16.2 of [Ref1]), so it will not
   have any inter-area routes in its routing table, but only intra-area
   routes from both Area 1 and Area 2.  Consequently, according to
   section 12.4.3 of [Ref1], R3 will originate into Areas 1 and 2 only
   summary-LSAs covering destinations in the directly attached areas,
   i.e., in Area 2---the summary-LSAs for Area 1, and in Area 1---the
   summary-LSAs for Area 2.

Zinin, et al.                Informational                      [Page 2]
RFC 3509                   OSPF ABR Behavior                  April 2003

   At the same time, router R2, as an ABR connected to the backbone,
   will inject into Area 2 summary-LSAs describing the destinations in
   Area 0 (the backbone), Area 1 and other areas reachable through the
   backbone.

   This results in a situation where internal router R4 calculates its
   routes to destinations in the backbone and areas other than Area 1
   via R2.  The topology of Area 2 itself can be such that the best path