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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.






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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.








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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.

































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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 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 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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Full Copyright Statement

Copyright (C) The Internet Society (2005).

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Acknowledgement

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Internet Society.







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