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rfc:rfc6987

Internet Engineering Task Force (IETF) A. Retana Request for Comments: 6987 L. Nguyen Obsoletes: 3137 Cisco Systems, Inc. Category: Informational A. Zinin ISSN: 2070-1721 Cinarra Systems

                                                              R. White
                                                          D. McPherson
                                                        Verisign, Inc.
                                                        September 2013
                   OSPF Stub Router Advertisement

Abstract

 This document describes a backward-compatible technique that may be
 used by OSPF (Open Shortest Path First) implementations to advertise
 a router's unavailability to forward transit traffic or to lower the
 preference level for the paths through such a router.
 This document obsoletes RFC 3137.

Status of This Memo

 This document is not an Internet Standards Track specification; it is
 published for informational purposes.
 This document is a product of the Internet Engineering Task Force
 (IETF).  It represents the consensus of the IETF community.  It has
 received public review and has been approved for publication by the
 Internet Engineering Steering Group (IESG).  Not all documents
 approved by the IESG are a candidate for any level of Internet
 Standard; see Section 2 of RFC 5741.
 Information about the current status of this document, any errata,
 and how to provide feedback on it may be obtained at
 http://www.rfc-editor.org/info/rfc6987.

Retana, et al. Informational [Page 1] RFC 6987 OSPF Stub Router Advertisement September 2013

Copyright Notice

 Copyright (c) 2013 IETF Trust and the persons identified as the
 document authors.  All rights reserved.
 This document is subject to BCP 78 and the IETF Trust's Legal
 Provisions Relating to IETF Documents
 (http://trustee.ietf.org/license-info) in effect on the date of
 publication of this document.  Please review these documents
 carefully, as they describe your rights and restrictions with respect
 to this document.  Code Components extracted from this document must
 include Simplified BSD License text as described in Section 4.e of
 the Trust Legal Provisions and are provided without warranty as
 described in the Simplified BSD License.

Table of Contents

 1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   2
 2.  Solutions . . . . . . . . . . . . . . . . . . . . . . . . . .   3
   2.1.  OSPFv3-Only Solution  . . . . . . . . . . . . . . . . . .   3
 3.  Maximum Link Metric . . . . . . . . . . . . . . . . . . . . .   4
 4.  Deployment Considerations . . . . . . . . . . . . . . . . . .   4
 5.  Security Considerations . . . . . . . . . . . . . . . . . . .   4
 6.  Acknowledgements  . . . . . . . . . . . . . . . . . . . . . .   5
 7.  References  . . . . . . . . . . . . . . . . . . . . . . . . .   5
   7.1.  Normative References  . . . . . . . . . . . . . . . . . .   5
   7.2.  Informative References  . . . . . . . . . . . . . . . . .   5
 Appendix A.  Changes from RFC 3137  . . . . . . . . . . . . . . .   6

1. Introduction

 In some situations, it may be advantageous to inform routers in a
 network not to use a specific router as a transit point but to still
 route to it.  Possible situations include the following:
 o  The router is in a critical condition (for example, has a very
    high CPU load or does not have enough memory to store all Link
    State Advertisements (LSAs) or build the routing table).
 o  Graceful introduction and removal of the router to/from the
    network.
 o  Other (administrative or traffic engineering) reasons.

Retana, et al. Informational [Page 2] RFC 6987 OSPF Stub Router Advertisement September 2013

 Note that the solution introduced in this document does not remove
 the router from the topology view of the network (as could be done by
 just flushing that router's router-LSA) but discourages other routers
 from using it for transit routing, while still routing packets to the
 router's own IP addresses, i.e., the router is announced as a stub.
 It must be emphasized that the solution provides real benefits in
 networks designed with at least some level of redundancy, so that
 traffic can be routed around the stub router.  Otherwise, traffic
 destined for the networks and reachable through such a stub router
 may still be routed through it.

2. Solutions

 The solution introduced in this document solves two challenges
 associated with the outlined problem.  In the description below,
 router X is the router announcing itself as a stub.  The challenges
 are
 1) Making other routers prefer routes around router X while
    performing the Dijkstra calculation.
 2) Allowing other routers to reach IP prefixes directly connected to
    router X.
 Note that it would be easy to address issue 1) alone by just flushing
 router X's router-LSA from the domain.  However, it does not solve
 problem 2), since other routers will not be able to use links to
 router X in Dijkstra (no back link), and because router X will not
 have links to its neighbors.
 To address both problems, router X announces its router-LSA to the
 neighbors with the cost of all non-stub links (links of the types
 other than 3) being set to MaxLinkMetric (defined in Section 3).
 The solution above applies to both OSPFv2 [RFC2328] and OSPFv3
 [RFC5340].

2.1. OSPFv3-Only Solution

 OSPFv3 [RFC5340] introduces additional options to provide similar
 control of the forwarding topology; the R-bit provides an indication
 of whether a router is active and should be used for transit traffic.
 It is left to network operators to decide which technique to use in
 their network.  See Section 4 for more details.

Retana, et al. Informational [Page 3] RFC 6987 OSPF Stub Router Advertisement September 2013

3. Maximum Link Metric

 Section 2 refers to the cost of all non-stub links as MaxLinkMetric,
 which is a new fixed architectural value introduced in this document.
 MaxLinkMetric
    The metric value indicating that a router-LSA link (see Section 2)
    should not be used for transit traffic.  It is defined to be the
    16-bit binary value of all ones: 0xffff.

4. Deployment Considerations

 When using MaxLinkMetric, some inconsistency may be seen if the
 network is constructed of routers that perform an intra-area Dijkstra
 calculation as specified in [RFC1247] (discarding link records in
 router-LSAs that have a MaxLinkMetric cost value) and routers that
 perform it as specified in [RFC1583] and higher (do not treat links
 with MaxLinkMetric cost as unreachable).  Note that this
 inconsistency will not lead to routing loops, because if there are
 some alternate paths in the network, both types of routers will agree
 on using them rather than the path through the stub router.  If the
 path through the stub router is the only one, the routers of the
 first type will not use the stub router for transit (which is the
 desired behavior), while the routers of the second type will still
 use this path.
 On the other hand, clearing the R-bit will consistently result in the
 router not being used for transit.
 The use of MaxLinkMetric or the R-bit in a network depends on the
 objectives of the operator.  One of the possible considerations for
 selecting one or the other is in the desired behavior if the path
 through the stub router is the only one available.  Using
 MaxLinkMetric allows for that path to be used while the R-bit
 doesn't.

5. Security Considerations

 The technique described in this document does not introduce any new
 security issues into the OSPF protocol.

Retana, et al. Informational [Page 4] RFC 6987 OSPF Stub Router Advertisement September 2013

6. Acknowledgements

 The authors of this document do not make any claims on the
 originality of the ideas described.  Among other people, we would
 like to acknowledge Henk Smit for being part of one of the initial
 discussions around this topic.
 We would like to thank Shishio Tsuchiya, Gunter Van de Velde,
 Tomohiro Yamagata, Faraz Shamim, and Acee Lindem who provided
 significant input for the latest draft version of this document.
 Dave Cridland and Tom Yu also provided valuable comments.

7. References

7.1. Normative References

 [RFC2328]  Moy, J., "OSPF Version 2", STD 54, RFC 2328, April 1998.
 [RFC5340]  Coltun, R., Ferguson, D., Moy, J., and A. Lindem, "OSPF
            for IPv6", RFC 5340, July 2008.

7.2. Informative References

 [RFC1247]  Moy, J., "OSPF Version 2", RFC 1247, July 1991.
 [RFC1583]  Moy, J., "OSPF Version 2", RFC 1583, March 1994.
 [RFC3137]  Retana, A., Nguyen, L., White, R., Zinin, A., and D.
            McPherson, "OSPF Stub Router Advertisement", RFC 3137,
            June 2001.

Retana, et al. Informational [Page 5] RFC 6987 OSPF Stub Router Advertisement September 2013

Appendix A. Changes from RFC 3137

 This document obsoletes [RFC3137].
 In addition to editorial updates, this document defines a new
 architectural constant (MaxLinkMetric in Section 3) to eliminate any
 confusion about the interpretation of LSInfinity.  It also
 incorporates and explains the use of the R-bit [RFC5340] as a
 solution to the problem addressed in the text.

Retana, et al. Informational [Page 6] RFC 6987 OSPF Stub Router Advertisement September 2013

Authors' Addresses

 Alvaro Retana
 Cisco Systems, Inc.
 7025 Kit Creek Rd.
 Research Triangle Park, NC  27709
 USA
 EMail: aretana@cisco.com
 Liem Nguyen
 Cisco Systems, Inc.
 3750 Cisco Way
 San Jose, CA  95134
 USA
 EMail: lhnguyen@cisco.com
 Alex Zinin
 Cinarra Systems
 Menlo Park, CA
 USA
 EMail: alex.zinin@gmail.com
 Russ White
 1500 N. Greenville Avenue
 Suite 1100
 Richardson, TX  75081
 USA
 EMail: Russ.White@vce.com
 Danny McPherson
 Verisign, Inc.
 12061 Bluemont Way
 Reston, VA  20190
 USA
 EMail: dmcpherson@verisign.com

Retana, et al. Informational [Page 7]

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