Premier IT Outsourcing and Support Services within the UK

User Tools

Site Tools


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 [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
 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
 from R4 to R2 is via R3, so all traffic destined for the backbone and
 backbone-attached areas goes through R3.  Router R3 in turn, having
 only intra-area routes for areas 1 and 2, will drop all traffic not
 destined for the areas directly attached to it.  The same problem can
 occur when a backbone-connected ABR loses all of its adjacencies in
 the backbone---even if there are other normally functioning ABRs in
 the attached areas, all traffic going to the backbone (destined for
 it or for other areas) will be dropped.
 In a standard OSPF implementation this situation can be remedied by
 use of Virtual Links (see section 15 of [Ref1] for more details).  In
 this case, router R3 will have a virtual backbone connection, will
 form an adjacency over it, will receive all LSAs directly from a
 backbone-attached router (R1 or R2, or both in our example) and will
 install intra- or inter-area routes.
 While being an unavoidable technique for repairing a partitioned
 backbone area, the use of virtual links in the described situation
 adds extra configuration headaches and system traffic overhead.
 Another situation where standard ABR behavior does not provide
 acceptable results is when it is necessary to provide redundant
 connectivity to an ASBR via several different OSPF areas.  This would
 allow a provider to aggregate all their customers connecting through
 a single access point into one area while still offering a redundant
 connection through another access point in a different area, as shown
 in Figure 2.

Zinin, et al. Informational [Page 3] RFC 3509 OSPF ABR Behavior April 2003

                          .                .
                           .    Area 0    .
                            +--+      +--+
                          ..|R1|..  ..|R2|..
                         .  +--+  ..  +--+  .
                         .        ..        .
                         .        ..        .
                         . Area1  .. Area2  .
                         .        ..        .
                         .        ..        .
                         .       +--+       .
                              --+-  -+--
                              CN1    CNx
               Customer Networks (CN1--CNx) Advertised
               as AS External or NSSA External Routes
                Figure 2. Dual Homed Customer Router
 This technique is already used in a number of networks including one
 of a major provider.
 The next section describes alternative ABR behaviors, implemented in
 Cisco and IBM routers.  The changes are in the ABR definition and
 inter-area route calculation.  Any other parts of standard OSPF are
 not changed.
 These solutions are targeted to the situation when an ABR has no
 backbone connection.  They imply that a router connected to multiple
 areas without a backbone connection is not an ABR and should function
 as a router internal to every attached area.  This solution emulates
 a situation where separate OSPF processes are run for each area and
 supply routes to the routing table.  It remedies the situation
 described in the examples above by not dropping transit traffic.
 Note that a router following it does not function as a real border
 router---it doesn't originate summary-LSAs.  Nevertheless such a
 behavior may be desirable in certain situations.
 Note that the proposed solutions do not obviate the need of virtual
 link configuration in case an area has no physical backbone
 connection at all.  The methods described here improve the behavior
 of a router connecting two or more backbone-attached areas.

Zinin, et al. Informational [Page 4] RFC 3509 OSPF ABR Behavior April 2003

2 Changes to ABR Behavior

2.1 Definitions

 The following definitions will be used in this document to describe
 the new ABR behaviors:
 Configured area:
    An area is considered configured if the router has at least one
    interface in any state assigned to that area.
 Actively Attached area:
    An area is considered actively attached if the router has at least
    one interface in that area in the state other than Down.
 Active Backbone Connection:
    A router is considered to have an active backbone connection if
    the backbone area is actively attached and there is at least one
    fully adjacent neighbor in it.
 Area Border Router (ABR):
    Cisco Systems Interpretation:
       A router is considered to be an ABR if it has more than one
       area Actively Attached and one of them is the backbone area.
    IBM Interpretation:
       A router is considered to be an ABR if it has more than one
       Actively Attached area and the backbone area Configured.

2.2 Implementation Details

 The following changes are made to the base OSPF, described in [Ref1]:
 1.  The algorithm for Type 1 LSA (router-LSA) origination is changed
     to prevent a multi-area connected router from identifying itself
     as an ABR by the bit B (as described in section 12.4.1 of [Ref1])
     until it considers itself as an ABR according to the definitions
     given in section 2.1.
 2.  The algorithm for the routing table calculation is changed to
     allow the router to consider the summary-LSAs from all attached
     areas if it is not an ABR, but has more than one attached area,
     or it does not have an Active Backbone Connection.  Definitions
     of the terms used in this paragraph are given in section 2.1.

Zinin, et al. Informational [Page 5] RFC 3509 OSPF ABR Behavior April 2003

     So, the paragraph 1 of section 16.2 of [Ref1] should be
     interpreted as follows:
     "The inter-area routes are calculated by examining summary-LSAs.
     If the router is an ABR and has an Active Backbone Connection,
     only backbone summary-LSAs are examined.  Otherwise (either the
     router is not an ABR or it has no Active Backbone Connection),
     the router should consider summary-LSAs from all Actively
     Attached areas..."
 3.  For Cisco ABR approach, the algorithm for the summary-LSAs
     origination is changed to prevent loops of summary-LSAs in
     situations where the router considers itself an ABR but doesn't
     have an Active Backbone Connection (and, consequently, examines
     summaries from all attached areas).  The algorithm is changed to
     allow an ABR to announce only intra-area routes in such a
     So, the paragraph 2 of subsection 12.4.3 of [Ref1] should be
     interpreted as follows:
     "Summary-LSAs are originated by area border routers.  The precise
     summary routes to advertise into an area are determined by
     examining the routing table structure (see Section 11) in
     accordance with the algorithm described below.  Note that while
     only intra-area routes are advertised into the backbone, if the
     router has an Active Backbone Connection, both intra-area and
     inter-area routes are advertised into the other areas; otherwise,
     the router only advertises intra-area routes into non-backbone
     For this policy to be applied we change steps 6 and 7 in the
     summary origination algorithm to be as follows:
     Step 6:
        "Else, if the destination of this route is an AS boundary
        router, a summary-LSA should be originated if and only if the
        routing table entry describes the preferred path to the AS
        boundary router (see Step 3 of Section 16.4).  If so, a Type 4
        summary-LSA is originated for the destination, with Link State
        ID equal to the AS boundary router's Router ID and metric
        equal to the routing table entry's cost.  If the ABR
        performing this algorithm does not have an Active Backbone
        Connection, it can originate Type 4 summary-LSA only if the
        type of the route to the ASBR is intra-area.  Note: Type 4
        summary-LSAs should not be generated if Area A has been
        configured as a stub area."

Zinin, et al. Informational [Page 6] RFC 3509 OSPF ABR Behavior April 2003

     Step 7:
        "Else, the Destination type is network.  If this is an
        inter-area route and the ABR performing this algorithm has an
        Active Backbone Connection, generate a Type 3 summary-LSA for
        the destination, with Link State ID equal to the network's
        address (if necessary, the Link State ID can also have one or
        more of the network's host bits set; see Appendix E for
        details) and metric equal to the routing table cost."
 The changes in the ABR behavior described in this section allow a
 multi-area connected router to successfully route traffic destined
 for the backbone and other areas.  Note that if the router does not
 have a backbone area Configured it does not actively attract
 inter-area traffic, because it does not consider itself an ABR and
 does not originate summary-LSAs.  It still can forward traffic from
 one attached area to another along intra-area routes in case other
 routers in corresponding areas have the best inter-area paths over
 it, as described in section 1.2.
 By processing all summaries when the backbone is not active, we
 prevent the ABR, which has just lost its last backbone adjacency,
 from dropping any packets going through the ABR in question to
 another ABR and destined towards the backbone or other areas not
 connected to the ABR directly.

3 Virtual Link Treatment

 The Cisco ABR approach described in this document requires an ABR to
 have at least one active interface in the backbone area.  This
 requirement may cause problems with virtual links in those rare
 situations where the backbone area is purely virtual, as shown in
 Figure 3, and the state of the VL is determined as in [Ref1].
                   .......    ...........    ......
                          .  .           .  .
                          +--+    VL     +--+
                          +--+           +--+
                   Area 1 .  .  Area 2   .  . Area 3
                   .......    ...........    ......
                      Figure 3. Purely Virtual Backbone
 If R1 and R2 treat virtual links as in [Ref1], their virtual links
 will never go up, because their router-LSAs do not contain the B-bit,
 which is, in turn, because the routers do not have active interfaces
 (virtual links) in the backbone and do not consider themselves ABRs.

Zinin, et al. Informational [Page 7] RFC 3509 OSPF ABR Behavior April 2003

 Note that this problem does not appear if one of the routers has a
 real interface in the backbone, as it usually is in real networks.
 Though the situation described is deemed to be rather rare,
 implementations supporting Cisco ABR behavior may consider changing
 VL-specific code so that a virtual link is reported up (an
 InterfaceUp event is generated) when a router with corresponding
 router-ID is seen via Dijkstra, no matter whether its router-LSA
 indicates that it is an ABR or not.  This means that checking of
 configured virtual links should be done not in step 4 of the
 algorithm in 16.1 of [Ref1] when a router routing entry is added, but
 every time a vertex is added to the SPT in step 3 of the same

4 Compatibility

 The changes of the OSPF ABR operations do not influence any aspects
 of the router-to-router cooperation and do not create routing loops,
 and hence are fully compatible with standard OSPF.  Proof of
 compatibility is outside the scope of this document.

5 Deployment Considerations

 This section discusses the deployments details of the ABR behaviors
 described in this document.  Note that this approach is fully
 compatible with standard ABR behavior, so ABRs acting as described in
 [Ref1] and in this document can coexist in an OSPF domain and will
 function without problems.
 Deployment of ABRs using the alternative methods improves the
 behavior of a router connected to multiple areas without a backbone
 attachment, but can lead to unexpected routing asymmetry, as
 described below.
 Consider an OSPF domain depicted in Figure 4.

Zinin, et al. Informational [Page 8] RFC 3509 OSPF ABR Behavior April 2003

                    .        Backbone         .
                   .                           .
                   .   ---------------------   .
                    .   |1               1|   .
                     ..|R1|.....    ....|R4|..
                    .  +--+     .  .    +--+  .
                    .   1|      .  .     /4   .
                    .    |    8 +--+ 4  /     .
                    .    |    +-|R3|---+      .
                    .   1|   /  +--+\4        .
                    .  +--+ /   .  . \ 4 +--+ .
                    .  |R2|/8   .  .  +--|R5| .
                    .  +--+     .  .     +--+ .
                    .   |       .  .       |  .
                    . --------- .  . -------- .
                    .   net N   .  .  net M   .
                    .           .  .          .
                    .  Area 1   .  .  Area 2  .
                     ...........    ..........
                Figure 4. Inter-area routing asymmetry
 Assume that R3 uses the approach described in this document.  In this
 case R2 will have inter-area routes to network M via ABR R1 only.  R5
 in turn will have its inter-area route to network N via R4, but as
 far as R4 is only reachable via R3, all traffic destined to network N
 will pass through R3.  R3 will have an intra-area route to network N
 via R2 and will, of course, route it directly to it (because
 intra-area routes are always preferred over inter-area ones).
 Traffic going back from network N to network M will pass through R2
 and will be routed to R1, as R2 will not have any inter-area routes
 via R3.  So, traffic from N to M will always go through the backbone
 while traffic from M to N will cross the areas directly via R3 and,
 in this example, will not use a more optimal path through the
 Note that this problem is not caused by the fact that R3 uses the
 alternative approach.  The reason for attracting the attention to it
 is that R3 is not really functioning as an ABR in case this new
 behavior is used, i.e., it does not inject summary-LSAs into the
 attached areas, but inter-area traffic can still go through it.

6 Security Considerations

 The alternative ABR behaviors specified in this document do not raise
 any security issues that are not already covered in [Ref1].

Zinin, et al. Informational [Page 9] RFC 3509 OSPF ABR Behavior April 2003

7 Acknowledgements

 Authors would like to thank Alvaro Retana, Russ White, and Liem
 Nguyen for their review of the document.

8 Disclaimer

 This document describes OSPF ABR implementations of respective
 vendors "as is", only for informational purposes, and without any
 warranties, guarantees or support.  These implementations are subject
 to possible future changes.  For the purposes of easier deployment,
 information about software versions where described behavior was
 integrated is provided below.
 Initial Cisco ABR implementation (slightly different from the one
 described in this memo, requiring non-backbone areas to be
 configured, and not necessarily actively attached in the ABR
 definition) was introduced in Cisco IOS (tm) version 11.1(6).  Cisco
 ABR behavior described in this document was integrated in Cisco IOS
 (tm) in version 12.1(3)T.
 The ABR behavior described as IBM ABR approach was implemented by IBM
 in IBM Nways Multiprotocol Routing Services (MRS) 3.3.
 Note that the authors do not intend to keep this document in sync
 with actual implementations.

10 References

 [Ref1] Moy, J., "OSPF version 2", STD 54, RFC 2328, April 1998.

Zinin, et al. Informational [Page 10] RFC 3509 OSPF ABR Behavior April 2003

11 Authors' Addresses

 Alex Zinin
 Derek M. Yeung
 Procket Networks
 1100 Cadillac Ct
 Milpitas, CA 95035
 Phone: 408-635-7911
 Acee Lindem
 Redback Networks
 102 Carric Bend Court
 Cary, NC 27519 USA
 Phone: 919-387-6971

Zinin, et al. Informational [Page 11] RFC 3509 OSPF ABR Behavior April 2003

12 Full Copyright Statement

 Copyright (C) The Internet Society (2003).  All Rights Reserved.
 This document and translations of it may be copied and furnished to
 others, and derivative works that comment on or otherwise explain it
 or assist in its implementation may be prepared, copied, published
 and distributed, in whole or in part, without restriction of any
 kind, provided that the above copyright notice and this paragraph are
 included on all such copies and derivative works.  However, this
 document itself may not be modified in any way, such as by removing
 the copyright notice or references to the Internet Society or other
 Internet organizations, except as needed for the purpose of
 developing Internet standards in which case the procedures for
 copyrights defined in the Internet Standards process must be
 followed, or as required to translate it into languages other than
 The limited permissions granted above are perpetual and will not be
 revoked by the Internet Society or its successors or assigns.
 This document and the information contained herein is provided on an


 Funding for the RFC Editor function is currently provided by the
 Internet Society.

Zinin, et al. Informational [Page 12]

/data/webs/external/dokuwiki/data/pages/rfc/rfc3509.txt · Last modified: 2003/04/04 18:13 by

Donate Powered by PHP Valid HTML5 Valid CSS Driven by DokuWiki