Network Working Group Padma Pillay-Esnault
Internet Draft Cisco Systems
Expiration Date:Jun 2001 November 1999
OSPF Refresh and flooding reduction in stable topologies
draft-pillay-esnault-ospf-flooding-02.txt
Status
This document is an Internet-Draft and is in full conformance with
all provisions of Section 10 of RFC2026. Internet-Drafts are working
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1. Abstract
This document describes extension to the OSPF protocol [1] to
optimize flooding of Link State Advertisements (LSA) in stable
topologies.
The current behaviour of OSPF requires that all LSA be refreshed
every 30 minutes regardless of the stability of the network except
for Do Not Age (DNA) LSA [2]. This document proposes to generalize
the use of DNA LSA so as to reduce protocol traffic in stable
networks.
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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. The
networks using OSPF are larger everyday and will continue to expand
to accomodate the demand to connect to the Internet or intranets.
Internet Service Providers and users having large networks have
noticed of a non negligible protocol traffic even when their network
topology was stable.
By design OSPF requires LSA to be refreshed as they expire after
3600 seconds. Some implementations have tried to improve the flooding
by reducing its frequency to refresh from 30 mins to around 50 mins
or so. This solution presents the advantage of cutting down the amount
of refresh traffic but will require at least one refresh before the LSA
expires.
This document proposes to overcome the LSA expiration by implementing
the generalization of DO NOT AGE LSA use. By reducing considerably the
traffic overhead in stable topologies OSPF will scale better.
3. Changes in the existing implementation.
The existing OSPF Demand Circuit feature [2] provides the premise of the
Do Not Age LSA implementation.
The goal here is to reduce refreshing and flooding of already known
and unchanged information. To achieve this, the LSA will now be flooded
with the higher bit set thus making them DO NOT AGE LSA.
Unlike in the implementation of DC, there is no suppression of hellos
between adjacent neighbors. The objective being reduced overhead but fast
convergence and recovery is primordial in case there is a change in the
topology.
The suppression of hellos will delay the knowledge that the neighbor is
down. By keeping the hellos, the routers are fully aware of their neighbor
states after the DEAD timer interval at the most.
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4. Deployment
All routers supporting OSPF Demand Circuit will be able to have no problem
to interact with the routers supporting the flooding reduction.
There are two possibilities :
(1) The routers supporting DC but do not have the Flooding Reduction
enhancement are NOT configured to run as DC on their links with
the routers supporting the Flooding Reduction Enhancement (FRE). In this
case, the older implementation will send its LSA without the DNA bit
set and will need to refresh its LSA periodically. It will however
receive DNA LSA from the FRE routers and will keep them as such in its
own database.
(2) The routers supporting DC but do not have the Flooding Reduction
enhancement are configured to run as DC on their links with the routers
supporting the Flooding Reduction Enhancement (FRE). All peers will
set the DNA age on their own LSA and will suppression hellos. The FRE
routers will run as DC as well.
All routers that do not support OSPF Demand Circuit Feature have no
knowledge how to handle DNA LSA and these will appear as expired LSA
in their own database. The DCbitless LSA will be used here to detect
the presence of routers not supporting the DC and indication LSA will be
us in a similar manner as in [2] to inform other routers of the presence
of routers incapable to handle DNA LSA. All the DNA LSA will be flushed
and only aging LSA will then be sent.
The interoperability with routers not supporting DNA LSA implies that
they are in a stub area for the FRE routers to perform the flooding
Reduction. The flooding scope of the type 5 LSA introduces this
constraint. .
5. Configuration of the FRE routers
The FRE routers will have the possibility either to act globally on all
its OSPF interfaces or to implement flooding reduction only on some of its
interfaces. This will give a greater ease in its deployment and a greater
liberty as to how the user want to shape the refreshing its their topology.
6. Lost Functionality
The enhancement rely heavily on the Demand Circuit mechanism and come at
the same costs. The reduction of OSPF traffic will have an impact on the
robustness and database checksum as described in [2] section 6.
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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. References
[1] RFC 2328 OSPF Version 2. J. Moy. April 1998. (Format: TXT=447367
bytes) (Obsoletes RFC2178) (Also STD0054) (Status: STANDARD)
[2] RFC 1793 Extending OSPF to Support Demand Circuits. J. Moy.
April 1995. (Format: TXT=78728 bytes) (Status: PROPOSED STANDARD)
9. Authors' Addresses
Padma Pillay-Esnault
Cisco Systems, Inc.
170 West Tasman Drive
San Jose, CA 95134
Email: ppe@cisco.com
ppesnault@home.com
Voice: +1 408 526 6640
Pillay-Esnault [Page 4]
