June 11, 2010

OSPF Flood Reduction

By default OSPF floods its domain with LSAs every 30 mn, that is half of the Max-Age time which is of 1h to ensure that the LSA database across all routers of the domain are in sync and the network is properly converged.

While this acts as good failsafe in case LSA’s are dropped and LS databases across downstream routers become inconsistent when topology changes occur, it is reasonable on stable networks, to completely disable the LSA scheduled updates and only propagate LSA updates only once the network topology changes.

Doing so, sets the bit set of the LSA’s Age field to “DoNotAge”. That means the LSA will be flushed of the database only where there is a topology change.

Keep in mind that LSA with the DoNotAge bit set are not only generated when setting OSPF flood reduction but by default on “on-demand circuits” and “sham links”.

The IOS interface command will be ip ospf flood-reduction.