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Versions: 00 01 02 03 04 05 06 07 rfc4136                  Informational
                                                        IPR declarations
Network Working Group                               Padma Pillay-Esnault
Internet Draft                                          Juniper Networks
                                                               June 2003
Category: Informational
Expires: December 2003

         OSPF Refresh and Flooding Reduction in Stable Topologies


Status of this Memo

   This document is an Internet-Draft and is in full conformance with
   all provisions of Section 10 of RFC2026.

   Internet-Drafts are working documents of the Internet Engineering
   Task Force (IETF), its areas, and its working groups.  Note that
   other groups may also distribute working documents as Internet-

   Internet-Drafts are draft documents valid for a maximum of six months
   and may be updated, replaced, or obsoleted by other documents at any
   time.  It is inappropriate to use Internet-Drafts as reference
   material or to cite them other than as "work in progress."

   The list of current Internet-Drafts can be accessed at

The list of Internet-Draft Shadow Directories can be accessed at

Copyright Notice

   Copyright (C) The Internet Society (2002).  All Rights Reserved.

1. Abstract

   This document describes an extension to the OSPF protocol to
   reduce periodic flooding of Link State Advertisements in
   stable topologies.

   The OSPF current behavior requires that all LSAs other than DoNotAge
   LSAs to be refreshed every 30 minutes. This document proposes to
   generalize the use of DoNotAge LSAs to reduce protocol traffic in
   stable topologies

Pillay-Esnault                                                  [Page 1]

Internet Draft       OSPF Refresh and Flooding Reduction      March 2003

2. Motivation

   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 having 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.

3. 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, they will flood their self-originated LSAs
   with the DoNotAge bit set. Hence, self-originated LSAs do not
   have to be reflooded every 30 minutes and the reflooding 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 both as 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.

Pillay-Esnault                                                  [Page 2]

Internet Draft       OSPF Refresh and Flooding Reduction      March 2003

4. 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 which 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 capability will be
   subject to the demand circuit interoperability constraints
   articulated in section 2.5 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.

5. 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].

6. Intellectual Property Considerations

   The IETF has been notified by Cisco Systems of intellectual property
   rights claimed in regard to some or all of the specifications
   contained in this document. For more information please refer to the
   IETF web page http://www.ietf.org/ietf/IPR/CISCO-OSPF-REFRESH.txt

7. 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.

8. Normative References

   [1] RFC 2328 OSPF Version 2. J. Moy. April 1998.
   [2] RFC 1793 Extending OSPF to Support Demand Circuits. J. Moy.
   April 1995.

Pillay-Esnault                                                  [Page 3]

Internet Draft       OSPF Refresh and Flooding Reduction      March 2003

A. 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 flooding reduction feature
       enabled. By default, it 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 reflooding of self-originated
       LSAs that have not changed.

Pillay-Esnault                                                  [Page 4]

Internet Draft       OSPF Refresh and Flooding Reduction      March 2003

9. Authors' Addresses

   Padma Pillay-Esnault
   Juniper Networks
   1194 N, Mathilda Avenue
   Sunnyvale, CA 94089-1206

   Email: padma@juniper.net

Pillay-Esnault                                                  [Page 5]