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).
AbstractThis 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]. 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.
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. 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  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" . 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.
1]. The minimum value is also 30 minutes. A value of infinity will prevent re-flooding of self- originated LSAs that have not changed.
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