OSPF Flooding Reduction

Feature Overview

Benefits

Restrictions

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.

Benefits

All OSPF customers with generally stable networks benefit from the reduced traffic overhead provided by the OSPF Flooding Reduction feature.

Restrictions

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.

Supported Platforms

This Cisco IOS OSPF Flooding Reduction feature is supported on the following platforms:

Catalyst 2610
Catalyst 5000 family switches with an installed Route Switch Module
Catalyst 6000
Catalyst 8510
Cisco 800 series
Cisco 1003
Cisco 1004
Cisco 1600 series
Cisco 2500 series
Cisco 2600 series
Cisco 3600 series
Cisco 4000 series
Cisco AS5200
Cisco 5200 series
Cisco AS5300
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

Configuration Tasks

See the following sections for configuration tasks for the OSPF Flooding Reduction feature.

Configuring OSPF Flooding Reduction

Command Purpose

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.

Command	Purpose
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

Command Purpose

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.

Command	Purpose
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.

Configuration Examples

The following example enables OSPF flood-reduction on each interface.

interface serial 0
ip address 192.42.110.201 255.255.255.0
ip ospf flood-reduction
ip ospf authentication-key abcdefgh
ip ospf cost 10
!
interface serial 1
ip address 131.119.251.201 255.255.255.0
ip ospf flood-reduction
ip ospf authentication-key ijklmnop
ip ospf cost 20
ip ospf retransmit-interval 10
ip ospf transmit-delay 2
ip ospf priority 4
!
interface serial 2
ip address 131.119.254.201 255.255.255.0
ip ospf flood-reduction
ip ospf authentication-key abcdefgh
ip ospf cost 10
!
interface serial 3
ip address 36.56.0.201 255.255.0.0
ip ospf flood-reduction
ip ospf authentication-key ijklmnop
ip ospf cost 20
ip ospf dead-interval 80

Command Reference

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

Syntax Description

This command has no arguments or keywords.

Defaults

Disabled

Command Modes

Interface configuration

Command History

Release Modification

12.1(2)T

This command was introduced.

Release	Modification
12.1(2)T	This command was introduced.

Usage Guidelines

All routers supporting the OSPF demand circuit are compatible and can interact with routers supporting flooding reduction.

Examples

The following example reduces the flooding of unnecessary LSAs on serial interface 0:

interface serial 0
ip ospf flood-reduction

Related Commands

Command Description

show ip ospf interface

Display OSPF-related interface information.

show ip ospf neighbor

Display OSPF-neighbor information on a per-interface basis.

Command	Description
show ip ospf interface	Display OSPF-related interface information.
show ip ospf neighbor	Display OSPF-neighbor information on a per-interface basis.

Glossary

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.

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