Reducing OSPF Traffic in Stable Networks

Reducing OSPF Traffic in Stable Networks

Problem

You have a stable network that doesn't require all of the overhead of LSA flooding.

Solution

You can reduce the overhead due to OSPF LSA flooding by applying the interface-level command ip ospf flood-reduction:

Router9#configure terminal
Enter configuration commands, one per line.  End with CNTL/Z.
Router9(config)#interface Serial0/0
Router9(config-if)#ip address 192.168.10.9 255.255.255.0
Router9(config-if)#ip ospf flood-reduction
Router9(config-if)#exit
Router9(config)#end
Router9#

Discussion

By default, OSPF will flood the area with all known LSAs every 3,600 seconds (1 hour). This is done as a sort of fail-safe to ensure that whatever else might happen to the LSA database, it will be corrected at least once an hour. In most networks this is unnecessary, as OSPF does a good job of forwarding LSA messages whenever there is a change in the network. But it is usually safe and may correct pathological conditions where one or more LSAs have been dropped in the network, which is why the designers of OSPF included the feature. However, if you are concerned about the bandwidth overhead on your network, and if your network is exceptionally stable, you can probably make do without this fail-safe feature. You can see the effect by looking at the show ip ospf interface command:

Router9#show ip ospf interface Serial0/0
Serial0/0 is up, line protocol is up
 Internet Address 192.168.10.9/24, Area 0
 Process ID 1, Router ID 172.19.2.1, Network Type POINT_TO_MULTIPOINT, Cost: 64

 Reduce LSA flooding.
 Transmit Delay is 1 sec, State POINT_TO_MULTIPOINT,
 Timer intervals configured, Hello 30, Dead 120, Wait 120, Retransmit 5
   oob-resync timeout 120
   Hello due in 00:00:17
 Index 1/2, flood queue length 0
 Next 0x0(0)/0x0(0)
 Last flood scan length is 1, maximum is 1
 Last flood scan time is 0 msec, maximum is 0 msec
 Neighbor Count is 1, Adjacent neighbor count is 1
   Adjacent with neighbor 192.168.15.3
 Suppress hello for 0 neighbor(s)
Router9#

This feature actually does two things. In addition to eliminating the standard, once-per-hour LSA flooding, it tells OSPF to send all LSAs with the DoNotAge bit set. This means that they will never be flushed out of the link state databases of downstream routers unless there is a topology change.