ABR Type 3 LSA filtering
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Table Of Contents
Related Features and Technologies
Supported Standards, MIBs, and RFCs
Configuring ABR Type 3 LSA Filtering
Verifying ABR Type 3 LSA Filtering
Monitoring and Maintaining ABR Type 3 LSA Filtering
ABR Type 3 LSA Filtering
This feature module describes filtering interarea routes on an Area Border Router (ABR) with the Open Shortest Path First (OSPF) protocol and includes the following sections:
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Related Features and Technologies
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Feature Overview
The ABR Type 3 link-state advertisement (LSA) Filtering feature extends the capability of an ABR that is running the OSPF protocol to filter type 3 LSAs between different OSPF areas. This feature allows only specified prefixes to be sent from one area to another area and restricts all 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. This feature is supported by the addition of the area filter-list command.
Benefits
The OSPF ABR Type 3 LSA Filtering feature gives the user improved control of route distribution between OSPF areas.
Restrictions
Only type-3 LSAs that originate from an ABR are filtered.
Related Features and Technologies
The OSPF ABR Type 3 LSA Filtering 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 Release 12.0 Network Protocols Configuration Guide, Part 1 and Cisco IOS Release 12.0 Network Protocols Command Reference, Part 1.
Supported Platforms
The OSPF ABR Type 3 LSA Filtering feature is supported by the following platforms in Cisco IOS Release 12.0(15)S:
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Note
Supported Standards, MIBs, and RFCs
Standards
No new or modified standards are supported by this feature.
MIBs
No new or modified MIBs are supported by this feature.
To obtain lists of MIBs supported by platform and Cisco IOS release, and to download MIB modules, go to the Cisco MIB web site on Cisco Connection Online (CCO) at http://www.cisco.com/public/sw-center/netmgmt/cmtk/mibs.shtml.
RFCs
No new or modified standards are supported by this feature.
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 optional or required:
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Configuring 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 ABR Type 3 LSA Filtering
To verify that the OSPF ABR Type 3 LSA Filtering feature has been configured, use the show ip ospf EXEC command. 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 177.0.0.1
Supports only single TOS(TOS0) routes
Supports opaque LSA
It is an area border router
SPF schedule delay 5 secs, Hold time between two SPFs 10 secs
Minimum LSA interval 5 secs. Minimum LSA arrival 1 secs
Number of external LSA 0. Checksum Sum 0x0
Number of opaque AS LSA 0. Checksum Sum 0x0
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 2. 2 normal 0 stub 0 nssa
External flood list length 0
Area BACKBONE(0)
Number of interfaces in this area is 2
Area has no authentication
SPF algorithm executed 6 times
Area ranges are
39.0.0.0/8 Passive Advertise
===> Area-filter AREA_0_IN in
===> Area-filter AREA_0_OUT out
Number of LSA 5. Checksum Sum 0x29450
Number of opaque link LSA 0. Checksum Sum 0x0
Number of DCbitless LSA 0
Number of indication LSA 0
Number of DoNotAge LSA 0
Flood list length 0
Area 1
Number of interfaces in this area is 1
Area has no authentication
SPF algorithm executed 4 times
Area ranges are
===> Area-filter AREA_1_IN in
===> Area-filter AREA_1_OUT out
Number of LSA 6. Checksum Sum 0x30100
Number of opaque link LSA 0. Checksum Sum 0x0
Number of DCbitless LSA 0
Number of indication LSA 0
Number of DoNotAge LSA 0
Flood list length 0
Monitoring and Maintaining 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-changes
area 1 filter-list prefix AREA_1_OUT out
area 3 filter-list prefix AREA_3_IN in
network 11.0.0.0 0.255.255.255 area 3
network 20.1.1.0 0.0.0.255 area 0
network 55.0.0.0 0.255.255.255 area 1
!
ip prefix-list AREA_1_OUT seq 10 permit 35.25.0.0/8 ge 16
ip prefix-list AREA_1_OUT seq 20 permit 192.20.20.0/24
!
ip prefix-list AREA_3_IN seq 10 permit 172.17.0.0/16
!
Command Reference
This section documents the area filter-list command that configures the OSPF ABR Type 3 LSA Filtering feature. All other commands used with this feature are documented in the Cisco IOS Release 12.0 command reference publications.
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. 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 there is at least one prefix in the area range that 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-list AREA_1 inThe following example filters prefixes that are sent from area 2 to all other areas:
area 2 filter-list prefix-list AREA_2 out
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