By design, OSPF requires link-state advertisements (LSAs) to be refreshed as they expire after 3600 sec.
When OSPF network is stable. The OSPF Flooding Reduction feature is used to reducing unnecessary refreshing and flooding of already known and unchanged information. To achieve this reduction, the LSAs are now flooded with the higher bit set called DNA bit, thus making them DoNotAge (DNA) LSAs.
To suppress the unnecessary flooding of link-state advertisements (LSAs) in a stable topology we can use the following commands:
-With OSPFv2 use the ip ospf flood-reduction command.
-With OSPFv3, use the ipv6 flood-reduction command.
-With OSPFv3 AF, use the ospfv3 flood-reduction command.
RFC 2328 says on page 216 that OSPF LSAs age out after 60 minutes :
B. Architectural Constants
LSRefreshTime
The maximum time between distinct originations of any particular
LSA. If the LS age field of one of the router's self-originated
LSAs reaches the value LSRefreshTime, a new instance of the LSA
is originated, even though the contents of the LSA (apart from
the LSA header) will be the same. The value of LSRefreshTime is
set to 30 minutes.
MaxAge
The maximum age that an LSA can attain. When an LSA's LS age
field reaches MaxAge, it is reflooded in an attempt to flush the
LSA from the routing domain (See Section 14). LSAs of age MaxAge
are not used in the routing table calculation. The value of
MaxAge is set to 1 hour.
Therefore routers must refresh (flood) their LSAs every 30 minutes. This can create a fair amount of unnecessary traffic if the LSDB (Link State Database) has a large number of LSAs and is relatively stable. To mitigate the this behavior use the flood reduction feature.
The RFC 4136 explains the flood reduction feature, it allows the routers to flood their self-originated LSAs with the DoNotAge (DNA) bit set:
2. Changes in the Existing Implementation
This enhancement relies on the implementation of the DoNotAge bit and
the Indication-LSA. The details of the implementation of the
DoNotAge bit and the Indication-LSA are specified in "Extending OSPF
to Support Demand Circuits" [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.
Another way to reduce flooding of unnecessary LSAs is the OSPF demand circuit options.
The OSPF flooding reduction and demand circuit features us ethe same mechanism to eliminate the need for the periodic LSA refresh in other words they set the DNA bit.
The main difference is that the OSPF demand circuit suppresses hellos while the OSPF flood reduction does not.
