Network Working Group                                  P. Pillay-Esnault
Request for Comments: 4136                                 Cisco Systems
Category: Informational                                        July 2005


        OSPF Refresh and Flooding Reduction in Stable Topologies

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 (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.

2.  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 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.

4.  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.

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 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.com

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