Technical Documentation

Configuring OSPF Refresh and Flooding Reduction in Stable Topologies

The OSPF standard requires that every link-state advertisement (LSA) be refreshed every 30 minutes. The Juniper implementation refreshes LSAs every 50 minutes. By default, any LSA that is not refreshed expires after 60 minutes. This requirement can result in traffic overhead that makes it difficult to scale OSPF networks. You can override the default behavior by specifying that the DoNotAge bit be set in self-originated LSAs when they are initially sent by the router. Any LSA with the DoNotAge bit set is reflooded only when a change occurs in the LSA. This feature thus reduces protocol traffic overhead while permitting any changed LSAs to be flooded immediately. Routers enabled for flood reduction continue to send hello packets to their neighbors and to age self-originated LSAs in their databases.

The Juniper implementation of OSPF refresh and flooding reduction is based on RFC 4136, OSPF Refresh and Flooding Reduction in Stable Topologies. However, the Juniper implementation does not include the forced-flooding interval defined in the RFC. Not implementing the forced-flooding interval ensures that LSAs with he DoNotAge big set are reflooded only when a change occurs.

This feature is supported for the following:

  • OSPFv2 and OSPFv3 interfaces
  • OSPFv3 realms
  • OSPFv2 and OSPFv3 virtual links
  • OSPFv2 sham links
  • OSPFv2 peer interfaces
  • All routing instances supported by OSPF
  • Logical systems

To configure flooding reduction for an OSPF interface, include the flood-reduction statement at the [edit protocols (ospf | ospf3) area area-id interface interface-id hierarchy level.

Note: If you configure flooding reduction for an interface configured as a demand circuit, the LSAs are not initially flooded, and LSAs are sent only when their content has changed. Hello packets and LSAs are sent and received on a demand-circuit interface only when a change occurs in the network topology.

In the following example, the OSPF interface so-0/0/1.0 is configured for flooding reduction. As a result, all the LSAs generated by the routes that traverse the specified interface have the DoNotAge bit set when they are initially flooded, and LSAs are refreshed only when a change occurs.

[edit]protocols ospf {area 0.0.0.0 {interface so-0/0/1.0 {flood-reduction;}interface lo0.0interface so-0/0/0.0;}}

Published: 2010-04-14