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Configuring OSPF
Reducing LSA Flooding
IPC-238
Cisco IOS IP Configuration Guide
On point-to-multipoint networks, to prevent flooding of OSPF LSAs, use the following command in
router configuration mode:
For an example of blocking LSA flooding, see the section “Block LSA Flooding Example” at the end of
this chapter.
Reducing LSA Flooding
The explosive growth of the Internet has placed the focus on the scalability of IGPs such as OSPF. By
design, OSPF requires LSAs to be refreshed as they expire after 3600 seconds. Some implementations
have tried to improve the flooding by reducing the frequency to refresh from 30 minutes to about 50
minutes. This solution reduces the amount of refresh traffic but requires at least one refresh before the
LSA expires. The OSPF flooding reduction solution works by 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. The LSAs are now set as “do not age.”
To reduce unnecessary refreshing and flooding of LSAs on your network, use the following command
in interface configuration mode:
Ignoring MOSPF LSA Packets
Cisco routers do not support LSA type 6 Multicast OSPF (MOSPF), and they generate syslog messages
if they receive such packets. If the router is receiving many MOSPF packets, you might want to configure
the router to ignore the packets and thus prevent a large number of syslog messages. To do so, use the
following command in router configuration mode:
For an example of suppressing MOSPF LSA packets, see the section “Ignore MOSPF LSA Packets
Example” at the end of this chapter.
Command Purpose
Router(config-if)# ospf database-filter all
out
Blocks the flooding of OSPF LSA packets to the interface.
Command Purpose
Router(config-router)# neighbor ip-address
database-filter all out
Blocks the flooding of OSPF LSA packets to the specified neighbor.
Command Purpose
Router(config-if)# ip ospf flood-reduction
Suppresses the unnecessary flooding of LSAs in stable topologies.
Command Purpose
Router(config-router)# ignore lsa mospf
Prevents the router from generating syslog messages when it receives
MOSPF LSA packets.