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OSPF Mechanism to Exclude Connected IP Prefixes from LSA Advertisements

Contents

Prerequisites for the OSPF Mechanism to Exclude Connected IP Prefixes from LSA Advertisements

Information About the OSPF Mechanism to Exclude Connected IP Prefixes from LSA Advertisements

Previous Methods to Limit the Number of IP Prefixes Carried in LSAs

Feature Overview of the OSPF Mechanism to Exclude Connected IP Prefixes from LSA Advertisements

How to Exclude Connected IP Prefixes from OSPF LSAs

Excluding IP Prefixes per OSPF Process

Examples

Excluding IP Prefixes on a Per-Interface Basis

Examples

Troubleshooting IP Prefix Suppression

Examples

Configuration Examples for the OSPF Mechanism to Exclude Connected IP Prefixes from LSA Advertisements

Excluding IP Prefixes from LSA Advertisements for an OSPF Process: Example

Excluding IP Prefixes from LSA Advertisements for a Specified Interface: Example

Additional References

Related Documents

Standards

MIBs

RFCs

Technical Assistance

Command Reference

ip ospf prefix-suppression

prefix-suppression

Feature Information for the OSPF Mechanism to Exclude Connected IP Prefixes from LSA Advertisements

Glossary

OSPF Mechanism to Exclude Connected IP Prefixes from LSA Advertisements

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First Published: June 28, 2007

Last Updated: June 28, 2007

This document describes the Open Shortest Path First (OSPF) mechanism to exclude IP prefixes of connected networks from link-state advertisements (LSAs). When OSPF is deployed in large networks, limiting the number of IP prefixes that are carried in the OSPF LSAs can speed up OSPF convergence.

This feature can also be utilized to enhance the security of an OSPF network by allowing the network administrator to prevent IP routing toward internal nodes.

Finding Feature Information in This Module

Your Cisco IOS software release may not support all of the features documented in this module. To reach links to specific feature documentation in this module and to see a list of the releases in which each feature is supported, use the "Feature Information for the OSPF Mechanism to Exclude Connected IP Prefixes from LSA Advertisements" section.

Finding Support Information for Platforms and Cisco IOS and Catalyst OS Software Images

Use Cisco Feature Navigator to find information about platform support and Cisco IOS and Catalyst OS software image support. To access Cisco Feature Navigator, go to http://www.cisco.com/go/cfn. An account on Cisco.com is not required.

Contents

•Prerequisites for the OSPF Mechanism to Exclude Connected IP Prefixes from LSA Advertisements

•Information About the OSPF Mechanism to Exclude Connected IP Prefixes from LSA Advertisements

•How to Exclude Connected IP Prefixes from OSPF LSAs

•Configuration Examples for the OSPF Mechanism to Exclude Connected IP Prefixes from LSA Advertisements

•Additional References

•Command Reference

•Feature Information for the OSPF Mechanism to Exclude Connected IP Prefixes from LSA Advertisements

•Glossary

Prerequisites for the OSPF Mechanism to Exclude Connected IP Prefixes from LSA Advertisements

Before you can use the mechanism to exclude IP prefixes from LSAs, the OSPF routing protocol must be configured.

Information About the OSPF Mechanism to Exclude Connected IP Prefixes from LSA Advertisements

One way to improve OSPF network convergence is to limit the number of IP prefixes carried in LSAs.

•Previous Methods to Limit the Number of IP Prefixes Carried in LSAs

•Feature Overview of the OSPF Mechanism to Exclude Connected IP Prefixes from LSA Advertisements

Previous Methods to Limit the Number of IP Prefixes Carried in LSAs

Configuring interfaces as unnumbered limits IP prefixes. However, for network management and the ease of identifying and troubleshooting numbered interfaces, you might want to have numbered interfaces and also want to limit the number of IP advertisements.

Feature Overview of the OSPF Mechanism to Exclude Connected IP Prefixes from LSA Advertisements

The OSPF mechanism to exclude connected IP prefixes from LSAs allows network administrators to control what IP prefixes are installed into LSAs. This functionality is implemented for router and network LSAs in the following manner:

•For the router LSA, to exclude prefixes, the feature excludes link type 3 (stub link).

•For the network LSA, the OSPF Designated Router (DR) generates LSAs with a special /32 network mask (0xFFFFFFFF).

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Note Previous versions of Cisco IOS software that do not have this feature will install the /32 prefix into the routing table.

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Globally Suppressing IP Prefix Advertisements per OSPF Process

You can reduce OSPF convergence time by configuring the OSPF process on a router to prevent the advertisement of all IP prefixes by using the prefix-suppression command in router configuration mode.

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Note Prefixes that are associated with loopbacks, secondary IP addresses, and passive interfaces are excluded because typical network designs require those to remain reachable.

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Suppressing IP Prefix Advertisements on a Per-Interface Basis

You can explicitly configure an OSPF interface not to advertise its IP network to its neighbors by using the ip ospf prefix-suppression command in interface configuration mode.

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Note If you have globally suppressed IP prefixes from connected IP networks by configuring the prefix-suppression router configuration command, the interface configuration command takes precedence over the router configuration mode command.

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How to Exclude Connected IP Prefixes from OSPF LSAs

This section describes how to configure two alternative methods to suppress IP prefix advertisements. You can suppress IP prefix advertisements per OSPF process or per interface. This section also explains how you can troubleshoot IP prefix suppression.

•Excluding IP Prefixes per OSPF Process

•Excluding IP Prefixes on a Per-Interface Basis

•Troubleshooting IP Prefix Suppression

Excluding IP Prefixes per OSPF Process

This section provides the necessary steps to exclude IP prefixes from connected networks for all interface types except for loopbacks, secondary IP addresses, and passive interfaces for an OSPF process.

SUMMARY STEPS

1. enable

2. configure terminal

3. router ospf [process-id] [vrf vpn-name]

4. prefix-suppression

5. end

6. show ip ospf

DETAILED STEPS

	Command or Action	Purpose
Step 1	enable Example: Router> enable	Enables privileged EXEC mode. •Enter your password if prompted.
Step 2	configure terminal Example: Router# configure terminal	Enters global configuration mode.
Step 3	router ospf process-id [vrf vpn-name] Example: Router(config)# router ospf 23	Configures an OSPFv2 routing process and enters router configuration mode.
Step 4	prefix-suppression Example: Router(config-router)# prefix-suppression	Prevents OSPF from advertising all IP prefixes except prefixes that are associated with loopbacks, secondary IP addresses, and passive interfaces.
Step 5	end Example: Router(config-router)# end	Returns to privileged EXEC mode.
Step 6	show ip ospf Example: Router# show ip ospf	Displays general information about OSPF routing processes. Note Use this command to verify that IP prefix suppression has been enabled.

Examples

In the following example, output from the show ip ospf command shows that IP prefix advertisement has been suppressed for OSPF process 1.

Router# show ip ospf 

Routing Process "ospf 1" with ID 10.0.0.6

Start time: 00:00:04.912, Time elapsed: 00:02:35.184

Supports only single TOS(TOS0) routes

Supports opaque LSA

Supports Link-local Signaling (LLS)

Supports area transit capability

It is an area border router

Router is not originating router-LSAs with maximum metric

Initial SPF schedule delay 5000 msecs

Minimum hold time between two consecutive SPFs 10000 msecs

Maximum wait time between two consecutive SPFs 10000 msecs

Incremental-SPF disabled

Minimum LSA interval 5 secs

Minimum LSA arrival 1000 msecs

LSA group pacing timer 240 secs

Interface flood pacing timer 33 msecs

Retransmission pacing timer 66 msecs

Number of external LSA 2. Checksum Sum 0x0132C8

Number of opaque AS LSA 0. Checksum Sum 0x000000

Number of DCbitless external and opaque AS LSA 0

Number of DoNotAge external and opaque AS LSA 0

Number of areas in this router is 3. 3 normal 0 stub 0 nssa

Number of areas transit capable is 1

External flood list length 0

IETF NSF helper support enabled

Cisco NSF helper support enabled

Prefix-suppression is enabled

.

.

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Excluding IP Prefixes on a Per-Interface Basis

This section provides the steps necessary to prevent OSPF routers from advertising all IP prefixes that are associated with a specific interface. When you enter the ip ospf prefix suppression command in interface configuration mode, it takes precedence over the prefix-suppression command that is entered in router configuration mode.

SUMMARY STEPS

1. enable

2. configure terminal

3. interface type number

4. ip ospf prefix-suppression [disable]

5. end

6. show ip ospf interface

DETAILED STEPS

	Command or Action	Purpose
Step 1	enable Example: Router> enable	Enables privileged EXEC mode. •Enter your password if prompted.
Step 2	configure terminal Example: Router# configure terminal	Enters global configuration mode.
Step 3	interface type number Example: Router(config)# interface serial 0/0	Configures an interface type and enters interface configuration mode.
Step 4	ip ospf prefix-suppression [disable] Example: Router(config-if)# ip ospf prefix-suppression	Prevents OSPF from advertising IP prefixes that belong to a specific interface, except those that are associated with secondary IP addresses.
Step 5	end Example: Router(config-if)# end	Returns to privileged EXEC mode.
Step 6	show ip ospf interface Example: Router# show ip ospf interface	Displays OSPF-related interface information. Note Use this command to verify that IP prefix suppression has been enabled for a specific interface.

Examples

In the following example, the output from the show ip ospf interface command verifies that prefix suppression has been enabled for Ethernet interface 0/0.

Router# show ip ospf interface 

Ethernet0/0 is up, line protocol is up 

  Internet Address 192.168.130.2/24, Area 2 

  Process ID 1, Router ID 10.0.0.6, Network Type BROADCAST, Cost: 10

  Prefix-suppression is enabled

.

.

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Troubleshooting IP Prefix Suppression

This section describes how to troubleshoot IP prefix suppression by using the debug ip ospf lsa-generation command.

SUMMARY STEPS

1. enable

2. debug ip ospf lsa-generation

3. debug condition interface interface-type interface-number [dlci dlci] [vc {vci | vpi | vci}]

4.

5. show debugging

6. show logging

DETAILED STEPS

	Command or Action	Purpose
Step 1	enable Example: Router> enable	Enables privileged EXEC mode. •Enter your password if prompted.
Step 2	debug ip ospf lsa-generation Example: Router# debug ip ospf lsa-generation	Displays informations about each OSPF LSA generated.
Step 3	debug condition interface interface-type interface-number [dlci dlci] [vc {vci | vpi | vci}] Example: Router# debug interface serial 0/0	Limits output for some debug commands on the basis of the interface or virtual circuit.
Step 4	show debugging Example: Router# show debugging	Displays information about the types of debugging that are enabled for your router.
Step 5	show logging [slot slot-number | summary] Example: Router# show logging	Displays the state of syslog and the contents of the standard system logging buffer.

Examples

The following sample output from the debug ip ospf lsa-generation command verifies that for the Ethernet interface 0/0, IP prefixes from the connected network 192.168.131.0 are excluded.

Router# debug ip ospf lsa-generation 

OSPF summary lsa generation debugging is on

Router# debug condition interface e0/0

Condition 1 set

Router# show debugging 

IP routing:

   OSPF summary lsa generation debugging is on

Condition 1: interface Et0/0 (1 flags triggered)

        Flags: Et0/0

Router# show logging

*Jun  5 21:54:47.295: OSPF: Suppressing 192.168.131.0/24 on Ethernet1/0 from router LSA

*Jun  5 21:54:52.355: OSPF: Suppressing 192.168.131.0/24 on Ethernet1/0 from router LSA

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.

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Configuration Examples for the OSPF Mechanism to Exclude Connected IP Prefixes from LSA Advertisements

This section contains the following examples:

•Excluding IP Prefixes from LSA Advertisements for an OSPF Process: Example

•Excluding IP Prefixes from LSA Advertisements for a Specified Interface: Example

Excluding IP Prefixes from LSA Advertisements for an OSPF Process: Example

The following example configures IP prefix suppression for OSPF routing process 23.

router ospf 23

 prefix-suppression

 end

When the show ip ospf command is entered, the displayed output verifies that IP prefix suppression has been enabled for OSPF process 23.

Router# show ip ospf

outing Process "ospf 23" with ID 10.0.0.6

Start time: 00:00:04.912, Time elapsed: 00:02:35.184

Supports only single TOS(TOS0) routes

Supports opaque LSA

Supports Link-local Signaling (LLS)

Supports area transit capability

It is an area border router

Router is not originating router-LSAs with maximum metric

Initial SPF schedule delay 5000 msecs

Minimum hold time between two consecutive SPFs 10000 msecs

Maximum wait time between two consecutive SPFs 10000 msecs

Incremental-SPF disabled

Minimum LSA interval 5 secs

Minimum LSA arrival 1000 msecs

LSA group pacing timer 240 secs

Interface flood pacing timer 33 msecs

Retransmission pacing timer 66 msecs

Number of external LSA 2. Checksum Sum 0x0132C8

Number of opaque AS LSA 0. Checksum Sum 0x000000

Number of DCbitless external and opaque AS LSA 0

Number of DoNotAge external and opaque AS LSA 0

Number of areas in this router is 3. 3 normal 0 stub 0 nssa

Number of areas transit capable is 1

External flood list length 0

IETF NSF helper support enabled

Cisco NSF helper support enabled

Prefix-suppression is enabled

.

.

.

Excluding IP Prefixes from LSA Advertisements for a Specified Interface: Example

The following example configures the suppression of all IP prefixes that are associated with Ethernet interface 0/0:

interface Ethernet 0/0

 ip ospf prefix-suppression

 end

When the show ip ospf interface command is entered, the displayed output verifies that IP prefix suppression is enabled for Ethernet interface 0/0.

Router# show ip ospf interface 

Ethernet0/0 is up, line protocol is up 

  Internet Address 192.168.130.2/24, Area 2 

  Process ID 1, Router ID 10.0.0.6, Network Type BROADCAST, Cost: 10

  Prefix-suppression is enabled

.

.

.

Additional References

The following sections provide references related to the OSPF Mechanism to Exclude Connected IP Prefixes from LSA Advertisements feature.

Related Documents

Related Topic	Document Title
OSPF commands	"OSPF Commands" chapter of the Cisco IOS IP Routing Protocols Command Reference, Release 12.4T.
OSPF configuration tasks	"Configuring OSPF" chapter of the Cisco IOS Routing Protocols Configuration Guide, Release 12.4T.

Standards

Standard	Title
None	—

MIBs

MIB	MIBs Link
There are no new MIBs that are associated with this feature.	To locate and download MIBs for selected platforms, Cisco IOS releases, and feature sets, use Cisco MIB Locator found at the following URL: http://www.cisco.com/go/mibs

RFCs

RFC	Title
None	—

Technical Assistance

Description

Link

The Cisco Support website provides extensive online resources, including documentation and tools for troubleshooting and resolving technical issues with Cisco products and technologies. To receive security and technical information about your products, you can subscribe to various services, such as the Product Alert Tool (accessed from Field Notices), the Cisco Technical Services Newsletter, and Really Simple Syndication (RSS) Feeds. Access to most tools on the Cisco Support website requires a Cisco.com user ID and password.

http://www.cisco.com/techsupport

Command Reference

This section documents only commands that are new or modified.

•ip ospf prefix-suppression

•prefix-suppression

ip ospf prefix-suppression

To prevent OSPF from advertising all IP prefixes that belong to a specific interface, except for prefixes that are associated with secondary IP addresses, use the ip ospf prefix-suppression command in interface configuration mode. To remove the per-interface configuration from the interface and allow the interface to inherit the IP prefix suppression setting from the router configuration, use the no form of this command.

ip ospf prefix-suppression [disable]

no ip ospf prefix-suppression

Syntax Description

disable

(Optional) Specifies that OSPF will advertise the interface IP prefix, regardless of the router mode configuration for IP prefix suppression.

Command Default

All IP prefixes that are associated with the interface are advertised.

Command Modes

Interface configuration (config-if)

Command History

Release	Modification
12.4(15)T	This command was introduced.

Usage Guidelines

You can suppress IP prefixes on a per-interface basis for all interface types by entering the ip ospf prefix-suppression command in interface configuration mode.

When the ip ospf prefix-suppression command is configured, it takes precedence over the prefix-suppression router configuration command. If the prefix-suppression router configuration command has been entered, the interfaces for the specified OSPF process will inherit the prefix suppression setting from the router configuration command.

When you enter the ip ospf prefix-suppression command, prefix generation for any interface type, including loopbacks and passive interfaces, are suppressed. Only prefixes associated with secondary IP addressees remain unaffected.

Examples

The following example suppresses all IP prefixes, except for secondary IP addresses, for Ethernet interface 1/1:

interface ethernet 1/1

 ip ospf prefix-suppression

Related Commands

Command	Description
prefix-suppression	Prevents OSPF from advertising all IP prefixes except prefixes associated with loopbacks, secondary IP addresses, and passive interfaces for a specific OSPF process.

prefix-suppression

To prevent OSPF from advertising all IP prefixes except prefixes that are associated with loopbacks, secondary IP addresses, and passive interfaces for a specific OSPF process, use the prefix-suppression command in router configuration mode. To advertise all IP prefixes, use the no form of this command.

prefix-suppression

no prefix-suppression

Syntax Description

This command has no arguments or keywords.

Command Default

All IP prefixes are advertised.

Command Modes

Router configuration (config-router)

Command History

Release	Modification
12.4(15)T	This command was introduced.

Usage Guidelines

You can globally suppress all IP prefixes (except prefixes that are associated with loopbacks, secondary IP addresses, and passive interfaces) for an entire OSPF process by using the prefix-suppression command in router configuration mode. You can also suppress IP prefixes on a per-interface basis by using the ip ospf prefix-suppression command in interface configuration mode. When the ip ospf prefix-suppression command is configured, it takes precedence over the prefix-suppression router configuration command.

Examples

The following example globally suppresses all IP prefixes except prefixes that are associated with loopbacks, secondary IP addresses, and passive interfaces for OSPF process 4:

router ospf 4

 prefix-suppression

Related Commands

Command	Description
ip ospf prefix-suppression	Prevents OSPF from advertising all IP prefixes that belong to a specific interface, except for IP prefixes that are associated with secondary IP addresses.

Feature Information for the OSPF Mechanism to Exclude Connected IP Prefixes from LSA Advertisements

Table 1 lists the release history for this feature.

Not all commands may be available in your Cisco IOS software release. For release information about a specific command, see the command reference documentation.

Use Cisco Feature Navigator to find information about platform support and software image support. Cisco Feature Navigator enables you to determine which Cisco IOS and Catalyst OS software images support a specific software release, feature set, or platform. To access Cisco Feature Navigator, go to http://www.cisco.com/go/cfn. An account on Cisco.com is not required.

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Note Table 1 lists only the Cisco IOS software release that introduced support for a given feature in a given Cisco IOS software release train. Unless noted otherwise, subsequent releases of that Cisco IOS software release train also support that feature.

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Table 1 Feature Information for the OSPF Mechanism to Exclude Connected IP Prefixes from LSA Advertisements

Feature Name	Releases	Feature Information
OSPF Mechanism to Exclude Connected IP Prefixes from LSA Advertisements	12.4(15)T	This document describes the Open Shortest Path First (OSPF) mechanism to exclude IP prefixes of connected networks from link-state advertisements (LSAs). When OSPF is deployed in large networks, limiting the number of IP prefixes that are carried in the OSPF LSAs can speed up OSPF convergence. This feature can also be utilized to enhance the security of an OSPF network by allowing the network administrator to prevent IP routing toward internal nodes.

Glossary

network LSA—The link-state advertisement created by the designated router (DR) or pseudonode that represents a group of routers on the same interface. The network LSA advertises summary information to represent the group of routers on the network.

router LSA—The link-state advertisement that is generated by a router. The router LSA advertises routing information (connected routes) for the router.

Any Internet Protocol (IP) addresses used in this document are not intended to be actual addresses. Any examples, command display output, and figures included in the document are shown for illustrative purposes only. Any use of actual IP addresses in illustrative content is unintentional and coincidental.

© 2007 Cisco Systems, Inc. All rights reserved.