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Network Working Group                               Padma Pillay-Esnault
Internet Draft                                          Juniper Networks
                                                            January 2003
Category: Standards Track
Expires: June 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.

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Copyright Notice

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

1. Abstract

   This document describes extension to the OSPF protocol to eliminate
   or reduce periodic flooding of Link State Advertisements in stable

   The current behavior of OSPF requires that all LSAs be refreshed
   every 30 minutes regardless of the stability of the network except
   for DoNotAge LSAs. This document proposes to generalize the use of
   DoNotAge LSAs so as to reduce protocol traffic in stable topologies.

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2. Motivation

   The explosive growth of IP based networks has placed the focus on the
   scalability of the Interior Gateway Protocols such as OSPF. Networks
   using OSPF are larger everyday and will continue to expand to
   accommodate the demand to connect to the Internet or intranets.

   Internet Service Providers and users having large networks have
   noticed non-negligible protocol traffic even when their network
   topology was 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 heavily on the OSPF Demand Circuit extension.
   The details of the implementation of the DC-bit, DoNotAge bit and
   the Indication-LSA are specified in "Extending OSPF to Support
   Demand Circuits" [2].

   The Flooding Reduction capable routers will continue to send hellos
   to their neighbors but will flood their Link State Advertisements
   (LSAs) with the DoNotAge bit set. This will reduce the protocol
   traffic overhead while allowing changes to be flooded immediately.

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4. Deployment

4.1 Routers supporting the OSPF Demand Circuit capability.

   All routers supporting OSPF Demand Circuit will be able to
   interoperate with the routers supporting the flooding reduction.

   For routers supporting OSPF Demand Circuits but do NOT support the
   new Flooding Reduction capability but have to interoperate with
   routers having the  Flooding Reduction capability there are two

   (1) Demand Circuit is not configured
   In this case, the router older implementation will send its LSAs
   without the DoNotAge bit set and it will need to refresh its LSAs
   periodically. It will however receive DoNotAge LSAs from the
   flooding reduction capable routers and will keep them as such in
   its own database.

   (2) Demand Circuit is configured
   All DC routers will set the DoNotAge bit on their own LSAs and will
   suppress hellos. The flooding reduction capable routers will run as
   DC as well.

4.2 Router not supporting the OSPF Demand Circuit capability.

   For routers that do not support OSPF Demand Circuit Feature have no
   knowledge how to handle DoNotAge LSAs and the LSAs with the DoNotAge
   bit set will appear as expired LSAs in their own database.

   The DCbitless LSAs must be used here to detect the presence of those
   routers not supporting the OSPF Demand Circuit and indication LSAs
   will be use as described in [2] to inform other routers of the
   presence of routers incapable to handling DoNotAge LSAs. In the
   presence of routers not supporting DC-bit, the Flooding Reduction
   capable routers must flush all the DoNotAge LSAs and revert to
   sending normal aging LSAs.

5. Configuration of the Flooding Reduction capable routers

   The implementations of Flooding Reduction capability must provide a
   knob to activate/deactivate the feature and by default it should be
   disabled. It should be also possible to specify a forced periodic
   refresh interval of Link State Advertisements.

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6. 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].

7. Acknowledgments

   The author would like to thank Jean-Michel Esnault, Barry Friedman,
   Thomas Kramer, Peter Psenak and Henk Smit 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.

9. Authors' Addresses

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

   Email: padma@juniper.net

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