Movatterモバイル変換

Internet Engineering Task Force (IETF)                         A. RetanaRequest for Comments: 6987                                     L. NguyenObsoletes:3137                                      Cisco Systems, Inc.Category: Informational                                         A. ZininISSN: 2070-1721                                          Cinarra Systems                                                                R. White                                                            D. McPherson                                                          Verisign, Inc.                                                          September 2013OSPF Stub Router AdvertisementAbstract   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 obsoletesRFC 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; seeSection 2 of RFC 5741.   Information about the current status of this document, any errata,   and how to provide feedback on it may be obtained athttp://www.rfc-editor.org/info/rfc6987.Retana, et al.                Informational                     [Page 1]

RFC 6987             OSPF Stub Router Advertisement       September 2013Copyright Notice   Copyright (c) 2013 IETF Trust and the persons identified as the   document authors.  All rights reserved.   This document is subject toBCP 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 Contents1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .22.  Solutions . . . . . . . . . . . . . . . . . . . . . . . . . .32.1.  OSPFv3-Only Solution  . . . . . . . . . . . . . . . . . .33.  Maximum Link Metric . . . . . . . . . . . . . . . . . . . . .44.  Deployment Considerations . . . . . . . . . . . . . . . . . .45.  Security Considerations . . . . . . . . . . . . . . . . . . .46.  Acknowledgements  . . . . . . . . . . . . . . . . . . . . . .57.  References  . . . . . . . . . . . . . . . . . . . . . . . . .57.1.  Normative References  . . . . . . . . . . . . . . . . . .57.2.  Informative References  . . . . . . . . . . . . . . . . .5Appendix A.  Changes fromRFC 3137  . . . . . . . . . . . . . . .61.  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 inSection 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.  SeeSection 4 for more details.Retana, et al.                Informational                     [Page 3]

RFC 6987             OSPF Stub Router Advertisement       September 20133.  Maximum Link MetricSection 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 (seeSection 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 20136.  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.  References7.1.  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 2013Appendix A.  Changes fromRFC 3137   This document obsoletes [RFC3137].   In addition to editorial updates, this document defines a new   architectural constant (MaxLinkMetric inSection 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 2013Authors' 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.comRetana, et al.                Informational                     [Page 7]