Network Working Group Padma Pillay-Esnault Internet Draft Cisco Systems Expiration Date:Nov 2000 November 1999 OSPF Refresh and flooding reduction in stable topologies draft-pillay-esnault-ospf-flooding-01.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 documents of the Internet Engineering Task Force (IETF), its areas, and its working groups. Note that other groups may also distribute working documents as Internet-Drafts. Internet-Drafts are draft documents valid for a maximum of six months and may be updated, replaced, or obsoleted by other documents at any time. It is inappropriate to use Internet- Drafts as reference material or to cite them other than as "work in progress." The list of current Internet-Drafts can be accessed at http://www.ietf.org/ietf/1id-abstracts.txt The list of Internet-Draft Shadow Directories can be accessed at http://www.ietf.org/shadow.html. 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. Pillay-Esnault [Page 1] Internet Draft draft-pillay-esnault-ospf-flooding-00.txt November 1999 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. Pillay-Esnault [Page 2] Internet Draft draft-pillay-esnault-ospf-flooding-00.txt November 1999 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. Pillay-Esnault [Page 3] Internet Draft draft-pillay-esnault-ospf-flooding-00.txt November 1999 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 Voice: +1 408 526 6640 Pillay-Esnault [Page 4]