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
draft-pillay-esnault-ospf-flooding-04.txt
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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
topologies.
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
possibilities:
(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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