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OSPF Flooding Reduction

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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 1.1.1.1 Adv rtr 1.1.1.1 Seq 80000006 to 1.1.1.2 retransmission list
 00:37:28: OSPF-1 FLOOD Et0/0: Add Type 1 LSA ID 1.1.1.1 Adv rtr 1.1.1.1 Seq 80000006 to flood list
 00:37:28: OSPF-1 FLOOD Et0/0: Flooding update to 224.0.0.5 Area 0
 00:37:28: OSPF-1 FLOOD Et0/0: Send Type 1, LSID 1.1.1.1, Adv rtr 1.1.1.1, <strong>age 1,</strong> seq 0x80000006 (0)
 00:37:28: OSPF-1 FLOOD Et0/0: Remove Type 1 LSA ID 1.1.1.1 Adv rtr 1.1.1.1 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 1.1.1.1, Adv rtr 1.1.1.1, <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 (1.1.1.2) (Process ID 1)

		Router Link States (Area 0)

Link ID         ADV Router      Age         Seq#       Checksum Link count
1.1.1.1         1.1.1.1         5     <strong>(DNA)</strong> 0x80000007 0x009D2E 2


© 2018 Matteo Malvica. Illustrations by Sergio Kalisiak.