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
draft-pillay-esnault-ospf-flooding-07.txt
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 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
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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.
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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.
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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.
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9. Authors' Addresses
Padma Pillay-Esnault
Juniper Networks
1194 N, Mathilda Avenue
Sunnyvale, CA 94089-1206
Email: padma@juniper.net
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