We all know that every OSPF nodes floods its LSA 1 and 2 through the area every 30 minutes due to the RefreshTimer. This is because every LSA has a MaxAge timer of 60 minutes, and when it expires, the LSA will be flushed.
But what if we have a real stable network? Do we really need to timeout every LSA and keep flooding every 30 minutes? Keep in mind that the flooding is not global, but per LSA, therefore OSPF process needs to keep track of every LSA in its DB. Luckily, from IOS 11.2 a feature called OPSF Flood-reduction will prevent LSA to age within the area: an additional bit will be set to transform it to a DoNotAge (DNA) LSA. Without DNA we can see normal flooding behavior, imposing age 1 to every refreshed LSAs.
00:37:28: OSPF-1 FLOOD Et0/0: Add Type 1 LSA ID 188.8.131.52 Adv rtr 184.108.40.206 Seq 80000006 to 220.127.116.11 retransmission list 00:37:28: OSPF-1 FLOOD Et0/0: Add Type 1 LSA ID 18.104.22.168 Adv rtr 22.214.171.124 Seq 80000006 to flood list 00:37:28: OSPF-1 FLOOD Et0/0: Flooding update to 126.96.36.199 Area 0 00:37:28: OSPF-1 FLOOD Et0/0: Send Type 1, LSID 188.8.131.52, Adv rtr 184.108.40.206, <strong>age 1,</strong> seq 0x80000006 (0) 00:37:28: OSPF-1 FLOOD Et0/0: Remove Type 1 LSA ID 220.127.116.11 Adv rtr 18.104.22.168 Seq 80000006 flood list 00:37:28: OSPF-1 FLOOD Et0/0: Stop flood timer
But then we enable flood-reduction on an interface basis:
interface Ethernet0/0 ip ospf flood-reduction
Keep in mind that this will just disable aging for only the network which belongs to this interface. So you may want to enable this command on every OSPF enabled interface.
00:44:50: OSPF-1 FLOOD Et0/0: Send Type 1, LSID 22.214.171.124, Adv rtr 126.96.36.199, <strong>age 32773 (DNA</strong>), seq 0x80000001 (0
As you can see the new value is way beyond default MaxAge timer (3600 seconds) which is equal to the DNA flag. And this is how the entry will look like inside the neighboring DB:
R2#sh ip ospf database OSPF Router with ID (188.8.131.52) (Process ID 1) Router Link States (Area 0) Link ID ADV Router Age Seq# Checksum Link count 184.108.40.206 220.127.116.11 5 <strong>(DNA)</strong> 0x80000007 0x009D2E 2
ac41a23 @ 2019-11-20