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Network Working Group                                  P. Pillay-EsnaultRequest for Comments: 4136                                 Cisco SystemsCategory: Informational                                        July 2005OSPF Refresh and Flooding Reduction in Stable TopologiesStatus 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 (2005).Abstract   This document describes an extension to the OSPF protocol to reduce   periodic flooding of Link State Advertisements (LSAs) in stable   topologies.   Current OSPF behavior requires that all LSAs, except DoNotAge LSAs,   to be refreshed every 30 minutes.  This document proposes to   generalize the use of DoNotAge LSAs in order to reduce protocol   traffic in stable topologies.1.  Introduction   The explosive growth of IP-based networks has placed focus on the   scalability of Interior Gateway Protocols such as OSPF.  Networks   using OSPF are growing every day and will continue to expand to   accommodate the demand for connections to the Internet or intranets.   Internet Service Providers and users that have large networks have   noticed non-negligible protocol traffic, even when their network   topologies were stable.   OSPF requires every LSA to be refreshed every 1800 seconds or else   they will expire when they reach 3600 seconds [1].   This document proposes to overcome the LSA expiration by generalizing   the use of DoNotAge LSAs.  This technique will facilitate OSPF   scaling by reducing OSPF traffic overhead in stable topologies.Pillay-Esnault               Informational                      [Page 1]

RFC 4136          OSPF Refresh and Flooding Reduction          July 20052.  Changes in the Existing Implementation   This enhancement relies on the implementation of the DoNotAge bit and   the Indication-LSA.  The details of the implementation of the   DoNotAge bit and the Indication-LSA are specified in "Extending OSPF   to Support Demand Circuits" [2].   Flooding-reduction-capable routers will continue to send hellos to   their neighbors and keep aging their self-originated LSAs in their   database.  However, these routers will flood their self-originated   LSAs with the DoNotAge bit set.  Thus, self-originated LSAs do not   have to be re-flooded every 30 minutes and the re-flooding interval   can be extended to the configured forced-flooding interval.  As in   normal OSPF operation, any change in the contents of the LSA will   cause a reoriginated LSA to be flooded with the DoNotAge bit set.   This will reduce protocol traffic overhead while allowing changes to   be flooded immediately.   Flooding-reduction-capable routers will flood received non-self-   originated LSAs with the DoNotAge bit set on all normal or flooding-   reduction-only interfaces within the LSA's flooding scope.  If an   interface is configured as both flooding-reduction-capable and   Demand-Circuit, then the flooding is done if and only if the contents   of the LSA have changed.  This allows LSA flooding for unchanged LSAs   to be periodically forced by the originating router.3.  Backward Compatibility   Routers supporting the demand circuit extensions [2] will be able to   correctly process DoNotAge LSAs flooded by routers supporting the   flooding reduction capability described herein.  These routers will   also suppress flooding DoNotAge LSAs on interfaces configured as   demand circuits.  However, they will also flood DoNotAge LSAs on   interfaces that are not configured as demand circuits.   When there are routers in the OSPF routing domain, stub area, or NSSA   area, that do not support the demand circuit extensions [2] then the   use of these flooding reduction capabilities will be subject to the   demand circuit interoperability constraints articulated insection2.5 of "Extending OSPF to Support Demand Circuits" [2].  This implies   that detection of an LSA, with the DC bit clear, will result in the   re-origination of self-originated DoNotAge LSAs with the DoNotAge   clear and purging of non-self-originated DoNotAge LSAs.Pillay-Esnault               Informational                      [Page 2]

RFC 4136          OSPF Refresh and Flooding Reduction          July 20054.  Security Considerations   This memo does not create any new security issues for the OSPF   protocol.  Security considerations for the base OSPF protocol are   covered in [1].5.  Acknowledgments   The author would like to thank Jean-Michel Esnault, Barry Friedman,   Thomas Kramer, Acee Lindem, Peter Psenak, Henk Smit, and Alex Zinin   for their helpful comments on this work.6.  Normative References   [1] Moy, J., "OSPF Version 2", STD 54,RFC 2328, April 1998.   [2] Moy, J., "Extending OSPF to Support Demand Circuits",RFC 1793,       April 1995.Pillay-Esnault               Informational                      [Page 3]

RFC 4136          OSPF Refresh and Flooding Reduction          July 2005A.  Configurable Parameters   This memo defines new configuration parameters for the flooding   reduction feature.  The feature must be enabled by configuration on a   router and is, by default, off.    flooding-reduction <all | list of interfaces> Indicates that the       router has the flooding reduction feature enabled.  By default,       this parameter applies to all interfaces running under the OSPF       instance to which it applies.  The feature can be enabled on a       subset of explicitly specified interfaces.    flooding-interval <n minutes> Indicates the interval in minutes for       the periodic flooding of self-originated LSAs.  By default, this       value is 30 minutes as per [1].  The minimum value is also 30       minutes.  A value of infinity will prevent re-flooding of self-       originated LSAs that have not changed.Author's Address   Padma Pillay-Esnault   Cisco Systems   170 W. Tasman Drive   San Jose, CA  95134   EMail: ppe@cisco.comPillay-Esnault               Informational                      [Page 4]

RFC 4136          OSPF Refresh and Flooding Reduction          July 2005Full Copyright Statement   Copyright (C) The Internet Society (2005).   This document is subject to the rights, licenses and restrictions   contained inBCP 78, and except as set forth therein, the authors   retain all their rights.   This document and the information contained herein are provided on an   "AS IS" basis and THE CONTRIBUTOR, THE ORGANIZATION HE/SHE REPRESENTS   OR IS SPONSORED BY (IF ANY), THE INTERNET SOCIETY AND THE INTERNET   ENGINEERING TASK FORCE DISCLAIM ALL WARRANTIES, EXPRESS OR IMPLIED,   INCLUDING BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF THE   INFORMATION HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED   WARRANTIES OF MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE.Intellectual Property   The IETF takes no position regarding the validity or scope of any   Intellectual Property Rights or other rights that might be claimed to   pertain to the implementation or use of the technology described in   this document or the extent to which any license under such rights   might or might not be available; nor does it represent that it has   made any independent effort to identify any such rights.  Information   on the procedures with respect to rights in RFC documents can be   found inBCP 78 andBCP 79.   Copies of IPR disclosures made to the IETF Secretariat and any   assurances of licenses to be made available, or the result of an   attempt made to obtain a general license or permission for the use of   such proprietary rights by implementers or users of this   specification can be obtained from the IETF on-line IPR repository athttp://www.ietf.org/ipr.   The IETF invites any interested party to bring to its attention any   copyrights, patents or patent applications, or other proprietary   rights that may cover technology that may be required to implement   this standard.  Please address the information to the IETF at ietf-   ipr@ietf.org.Acknowledgement   Funding for the RFC Editor function is currently provided by the   Internet Society.Pillay-Esnault               Informational                      [Page 5]