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Related Features and Technologies
Configuring OSPF ABR Type 3 LSA Filtering
Verifying OSPF ABR Type 3 LSA Filtering
Monitoring and Maintaining OSPF ABR Type 3 LSA Filtering
OSPF ABR Type 3 LSA Filtering
First Published: 12.0(15)SLast Updated: February Day, 2006The OSPF ABR Type 3 LSA Filtering feature extends the ability of an ABR that is running the OSPF protocol to filter type 3 link-state advertisements (LSAs) that are sent between different OSPF areas. This feature allows only packets with specified prefixes to be sent from one area to another area and restricts all packets with other prefixes. This type of area filtering can be applied out of a specific OSPF area, into a specific OSPF area, or into and out of the same OSPF areas at the same time.
History for the OSPF ABR Type 3 LSA Filtering Feature
Finding Support Information for Platforms and Cisco IOS Software Images
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Contents
Benefits
The OSPF ABR Type 3 LSA Filtering feature gives the administrator improved control of route distribution between OSPF areas.
Restrictions
Only type 3 LSAs that originate from an ABR are filtered.
Related Features and Technologies
This feature is an extension of the OSPF routing protocol. For more information about configuring OSPF and configuring route summarization and filtering, refer to the "OSPF" chapter of the Cisco IOS IP Configuration Guide, Release 12.4 and the Cisco IOS IP Routing Protocols Command Reference, Release 12.4T.
Configuration Tasks
See the following sections for configuration tasks for the OSPF ABR Type 3 LSA Filtering feature. Each task in the list is identified as either required or optional:
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Configuring OSPF ABR Type 3 LSA Filtering (required)
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Configuring OSPF ABR Type 3 LSA Filtering
To filter interarea routes into a specified area, use the following commands beginning in router configuration mode:
To filter interarea routes out of a specified area, use the following commands beginning in router configuration mode:
Verifying OSPF ABR Type 3 LSA Filtering
To verify that the OSPF ABR Type 3 LSA Filtering feature has been configured, use the show ip ospf command in the EXEC mode. The show ip ospf command will show that this feature has been enabled by listing the area filter as "in" or "out." The following is sample output from the show ip ospf command:
router# show ip ospf 1Routing Process "ospf 1" with ID 172.16.0.1Supports only single TOS(TOS0) routesSupports opaque LSAIt is an area border routerSPF schedule delay 5 secs, Hold time between two SPFs 10 secsMinimum LSA interval 5 secs. Minimum LSA arrival 1 secsNumber of external LSA 0. Checksum Sum 0x0Number of opaque AS LSA 0. Checksum Sum 0x0Number of DCbitless external and opaque AS LSA 0Number of DoNotAge external and opaque AS LSA 0Number of areas in this router is 2. 2 normal 0 stub 0 nssaExternal flood list length 0Area BACKBONE(0)Number of interfaces in this area is 2Area has no authenticationSPF algorithm executed 6 timesArea ranges are10.0.0.0/8 Passive AdvertiseArea-filter AREA_0_IN inArea-filter AREA_0_OUT outNumber of LSA 5. Checksum Sum 0x29450Number of opaque link LSA 0. Checksum Sum 0x0Number of DCbitless LSA 0Number of indication LSA 0Number of DoNotAge LSA 0Flood list length 0Area 1Number of interfaces in this area is 1Area has no authenticationSPF algorithm executed 4 timesArea ranges areArea-filter AREA_1_IN inArea-filter AREA_1_OUT outNumber of LSA 6. Checksum Sum 0x30100Number of opaque link LSA 0. Checksum Sum 0x0Number of DCbitless LSA 0Number of indication LSA 0Number of DoNotAge LSA 0Flood list length 0Monitoring and Maintaining OSPF ABR Type 3 LSA Filtering
Router# show ip prefix-list
Displays information about a prefix list or prefix list entries.
Configuration Examples
The following configuration example output shows interarea filtering that is applied to both incoming and outgoing routes:
Router(config)# router ospf 1log-adjacency-changesarea 1 filter-list prefix AREA_1_OUT outarea 3 filter-list prefix AREA_3_IN innetwork 10.0.0.0 0.255.255.255 area 3network 172.16.1.0 0.0.0.255 area 0network 192.168.0.0 0.255.255.255 area 1!ip prefix-list AREA_1_OUT seq 10 permit 10.25.0.0/8 ge 16ip prefix-list AREA_1_OUT seq 20 permit 172.20.20.0/24!ip prefix-list AREA_3_IN seq 10 permit 172.31.0.0/16!Additional References
The following sections provide references related to OSPF ABR Type 3 LSA Filtering.
Related Documents
Configuring OSPF ABR Type 3 LSA Filtering
"OSPF ABR Type 3 LSA Filtering" Chapter in the Cisco IOS IP Routing Protocols Configuration Guide, Release 12.4
OSPF Commands
"OSPF Commands" Chapter in the Cisco IOS IP Routing Protocols Command Reference, Release 12.4T
Standards
MIBs
None
To locate and download MIBs for selected platforms, Cisco IOS releases, and feature sets, use Cisco MIB Locator found at the following URL:
RFCs
Technical Assistance
Command Reference
This section documents modified commands only.
area filter-list
To filter prefixes advertised in type 3 link-state advertisements (LSAs) between Open Shortest Path First (OSPF) areas of an Area Border Router (ABR), use the area filter-list command in router configuration mode. To change or cancel the filter, use the no form of this command.
area area-id filter-list prefix {prefix-list-name in | out}
no area area-id filter-list prefix {prefix-list-name in | out}
Syntax Description
Defaults
This command has no default behavior.
Command Modes
Router configuration
Command History
Usage Guidelines
With this feature enabled in the "in" direction, all type 3 LSAs originated by the ABR to this area, based on information from all other areas, are filtered by the prefix list. Type 3 LSAs that were originated as a result of the area range command in another area are treated like any other type 3 LSA that was originated individually. Any prefix that does not match an entry in the prefix list is implicitly denied.
With this feature enabled in the "out" direction, all type 3 LSAs advertised by the ABR, based on information from this area to all other areas, are filtered by the prefix list. If the area range command has been configured for this area, type 3 LSAs that correspond to the area range are sent to all other areas, only if at least one prefix in the area range matches an entry in the prefix list.
If all specific prefixes are denied by the prefix list, type 3 LSAs that correspond to the area range command will not be sent to any other area. Prefixes that are not permitted by the prefix list are implicitly denied.
Examples
The following example filters prefixes that are sent from all other areas to area 1:
area 1 filter-list prefix AREA_1 inRelated Commands
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.
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