Table Of Contents
OSPF Flooding Reduction
This document describes the Open Shortest Path First (OSPF) Flooding Reduction feature in the following sections:
The explosive growth of the Internet has placed the focus on the scalability of Interior Gateway Protocols such as OSPF. The networks using OSPF are becoming larger every day and will continue to expand to accommodate the demand to connect to the Internet.
Internet Service Providers and customers with large networks have regularly complained that OSPF has a traffic overhead, even when the network topology is stable.
By design, OSPF requires link-state advertisements (LSAs) to be refreshed as they expire after 3600 sec. Some implementations have tried to improve the flooding by reducing the frequency to refresh from 30 min to around 50 min or so. This solution reduces the amount of refresh traffic but requires at least one refresh before the LSA expires.
The OSPF Flooding Reduction feature 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, thus making them DoNotAge (DNA) LSAs.
All OSPF customers with generally stable networks benefit from the reduced traffic overhead provided by the OSPF Flooding Reduction feature.
All routers and access servers in the area must be running Cisco IOS Release 11.2 or higher or the feature will not be active.
•IP and IP Routing Configuration Guide, Cisco IOS Release 12.1
•IP and IP Routing Command Reference, Cisco IOS Release 12.1
This Cisco IOS OSPF Flooding Reduction feature is supported on the following platforms:
•Catalyst 5000 family switches with an installed Route Switch Module
•Cisco 1600 series
•Cisco 2500 series
•Cisco 2600 series
•Cisco 3600 series
•Cisco 4500 family routers
•Cisco 4700 family routers
•Cisco 5200 series
•Cisco 6400 series (NRP)
•Cisco 7000 series
•Cisco 7100 series
•Cisco 7200 series
•Cisco 7500 series with a Route Switch Module
•Cisco 12000 series
•Cisco uBR7200 series
See the following sections for configuration tasks for the OSPF Flooding Reduction feature.
Configuring OSPF Flooding Reduction
Router(config-if)#ip ospf flood-reduction
Reduces unnecessary flooding and refreshing of LSAs in stable networks. You must configure this feature on a per-interface basis.
Verifying OSPF Flooding Reduction
Step 1 Enter the show running-config command.
Step 2 Verify the configuration in NVRAM.
Monitoring and Maintaining OSPF Flooding Reduction
Router# show ip ospf database
Display lists of information related to the OSPF database. Should display low sequence numbers on LSAs that are not originated in the local environment.
The following example enables OSPF flood-reduction on each interface.interface serial 0ip address 126.96.36.199 255.255.255.0ip ospf flood-reductionip ospf authentication-key abcdefghip ospf cost 10!interface serial 1ip address 188.8.131.52 255.255.255.0ip ospf flood-reductionip ospf authentication-key ijklmnopip ospf cost 20ip ospf retransmit-interval 10ip ospf transmit-delay 2ip ospf priority 4!interface serial 2ip address 184.108.40.206 255.255.255.0ip ospf flood-reductionip ospf authentication-key abcdefghip ospf cost 10!interface serial 3ip address 220.127.116.11 255.255.0.0ip ospf flood-reductionip ospf authentication-key ijklmnopip ospf cost 20ip ospf dead-interval 80
This section documents the new ip ospf flood-reduction command. All other commands used with this feature are documented in the Cisco IOS Release 12.1 command reference publications.
ip ospf flood-reduction
To suppress the unnecessary flooding of link-state advertisements (LSAs) in stable topologies, use the ip ospf flood-reduction interface configuration command. To disable this feature, use the no form of this command.
ip ospf flood-reduction
no ip ospf flood-reduction
This command has no arguments or keywords.
All routers supporting the OSPF demand circuit are compatible and can interact with routers supporting flooding reduction.
The following example reduces the flooding of unnecessary LSAs on serial interface 0:interface serial 0ip ospf flood-reduction
show ip ospf interface
Display OSPF-related interface information.
show ip ospf neighbor
Display OSPF-neighbor information on a per-interface basis.
DoNotAge (DNA)—The DoNotAge bit is the most significant bit of the LS Age field. LSAs having the DoNotAge bit set are not aged as they are in the OSPF router's link-state database, which means that these LSAs do not have to be refreshed every 30 minutes.
link-state advertisement (LSA)—Broadcast packet used by link-state protocols that contains information about neighbors and path costs. LSAs are used by the receiving routers to maintain their routing tables.