OSPF Mechanism to Exclude Connected IP Prefixes from LSA Advertisements

First Published: June 28, 2007
Last Updated: February 15, 2010

The Open Shortest Path First (OSPF) mechanism to exclude (suppress) IP prefixes of connected networks from link-state advertisements (LSAs) is used when OSPF is deployed in large networks. It allows the network administrator to limit the number of IP prefixes that are carried in the OSPF LSAs, which can speed up OSPF convergence.It also allows the network administrator to prevent IP routing toward internal nodes, which enhances the security of the OSPF network.

Finding Feature Information

For the latest feature information and caveats, see the release notes for your platform and software release. To find information about the features documented in this module, and to see a list of the releases in which each feature is supported, see the "Feature Information for the OSPF Mechanism to Exclude Connected IP Prefixes from LSA Advertisements" section.

Use Cisco Feature Navigator to find information about platform support and Cisco IOS XE 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

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

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

Before choosing to suppress IP Prefixes in LSA advertisements, you should understand the following concepts:

•Unnumbered Interfaces

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

Unnumbered Interfaces

Configuring interfaces as unnumbered interfaces limits the number of IP prefixes carried in LSAs and improves OSPF network convergence.

However, for network management reasons (such as identifying and troubleshooting interfaces), you might want to use numbered interfaces and also limit the number of IP advertisements. In deployments where you need to use numbered interfaces, you should use the OSPF mechanism to exclude connected IP prefixes from LSA advertisements.

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).

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

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.

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

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.

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.

How to Exclude Connected IP Prefixes from OSPF LSAs

This section describes how to configure two alternative methods to exclude 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 Connected IP Prefixes on an OSPF Process Basis

•Excluding Connected IP Prefixes on an Interface Basis

•Troubleshooting Connected IP Prefix Suppression

Excluding Connected IP Prefixes on an OSPF Process Basis

SUMMARY STEPS

1. enable

2. configure terminal

3. router ospf [process-id]

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

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, and therefore IP prefixes from connected networks are excluded from OSPF LSAs: 

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

Excluding Connected IP Prefixes on an Interface Basis

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 gigabitethernet 0/0/1

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.

Note 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.

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

Troubleshooting Connected IP Prefix Suppression

SUMMARY STEPS

1. enable

2. debug ip ospf lsa-generation

3. debug condition interface type number

4. show debugging

5. 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 type number

Example:

Router# debug interface gigabitethernet 0/0/1

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

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 gigabitethernet 0/0/1

 
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

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

This section contains the following examples:

•Example: 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

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

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

The following example configures the suppression of all IP prefixes that are associated with GigabitEthernet interface 0/0/1: 

interface gigabitethernet 0/0/1

 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: complete command syntax, command mode, command history, command defaults, usage guidelines, and examples

Cisco IOS IP Routing: OSPF Command Reference


Standards

Standard
Title

None

—


MIBs

MIB
MIBs Link

None

To locate and download MIBs for selected platforms, Cisco IOS XE software 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 and Documentation website provides online resources to download documentation, software, and tools. Use these resources to install and configure the software and to troubleshoot and resolve technical issues with Cisco products and technologies. Access to most tools on the Cisco Support and Documentation website requires a Cisco.com user ID and password.

http://www.cisco.com/cisco/web/support/index.html


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

Table 1 lists the release history for this feature.

Use Cisco Feature Navigator to find information about platform support and software image support. Cisco Feature Navigator enables you to determine which Cisco IOS XE 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.

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

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

Feature Name
Releases
Feature Information

The OSPF Mechanism to Exclude Connected IP Prefixes from LSA Advertisements

Cisco IOS XE
Release 2.6

The Open Shortest Path First (OSPF) mechanism allows the network administrator to limit the number of IP prefixes that are carried in the OSPF LSAs, which can speed up OSPF convergence. It also allows the network administrator to prevent IP routing towards internal nodes, which enhances the security of the OSPF network.

The following commands were introduced or modified by this feature: ip ospf prefix-suppression, prefix-suppression.


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