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
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Step 1 |
enable
Router> enable |
Enables privileged EXEC mode. •Enter your password if prompted. |
Step 2 |
configure terminal
Router# configure terminal |
Enters global configuration mode. |
Step 3 |
router ospf process-id
Router(config)# router ospf 23 |
Configures an OSPFv2 routing process and enters router configuration mode. |
Step 4 |
prefix-suppression
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
Router(config-router)# end |
Returns to privileged EXEC mode. |
Step 6 |
show ip ospf
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:
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 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
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
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Step 1 |
enable
Router> enable |
Enables privileged EXEC mode. •Enter your password if prompted. |
Step 2 |
configure terminal
Router# configure terminal |
Enters global configuration mode. |
Step 3 |
interface type number
Router(config)# interface gigabitethernet 0/0/1 |
Configures an interface type and enters interface configuration mode. |
Step 4 |
ip ospf prefix-suppression [disable]
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
Router(config-if)# end |
Returns to privileged EXEC mode. |
Step 6 |
show ip ospf interface
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
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Step 1 |
enable
Router> enable |
Enables privileged EXEC mode. •Enter your password if prompted. |
Step 2 |
debug ip ospf lsa-generation
Router# debug ip ospf lsa-generation |
Displays informations about each OSPF LSA generated. |
Step 3 |
debug condition interface type number
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
Router# show debugging |
Displays information about the types of debugging that are enabled for your router. |
Step 5 |
show logging
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
OSPF summary lsa generation debugging is on
Condition 1: interface Et0/0 (1 flags triggered)
*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:
When the show ip ospf command is entered, the displayed output verifies that IP prefix suppression has been enabled for OSPF process 23:
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 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
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
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
Standards
MIBs
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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
Technical Assistance
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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
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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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