How to filter lsa3 in ospf

New features in ospf have high requirements for ios versions:
The ospf abr Type 3 LSA Filtering feature is supported for the following platforms in Cisco IOS
Releasing 12.2 (4) T:
• Cisco 1400 series
• Cisco 1600 series
• Cisco 1600R series
• The Cisco 1720
• Cisco 1721 (Supported in Cisco IOS Release 12.2 (8) T and above .)
• The Cisco 1750
• The Cisco 1751
• Cisco 2500 series
• Cisco 2600 series
• The Cisco 3620
• Cisco 3631 (Supported in Cisco IOS Release 12.2 (8) T and above .)
• The Cisco 3640
• Cisco 3745 (Supported in Cisco IOS Release 12.2 (8) T and above .)
• The Cisco 3660
• Cisco MC3810
• Cisco 7100 series
• Cisco 7200 series
• Cisco 7500 series (Supported in Cisco IOS Release 12.2 (4) T3 and above .)
• Cisco uBR7200 series
• Universal Router Module (Supported in Cisco IOS Release 12.2 (8) T .)

The following configurations are completed on the abr:

Configuration steps:

1. filter the LSA3 that enters an AREA.

Command Purpose
Router (config) # router ospf process-id
Configures the router to run an OSPF process.

Router (config-router) # area-id filter-list prefixprefix-list-name in
Configures the router to filter interarea routes into the specified area.

Router (config-router) # ip prefix-list-name [seq-value] deny | permit network/len [ge-value] [le-value]
Creates a prefix list with the name specified for thelist-name argument.

2. Filter LSA3 from an AREA

Router (config) # router ospf process-id
Configures the router to run an OSPF process.

Router (config-router) # area-id filter-list prefix-list-name out
Configures the router to filter interarea routes out of the specified area.

Router (config-router) # ip prefix-list-name [seq-value] deny | permit network/len [ge-value] [lele-value]
Creates a prefix list with the name specified for the list-name argument.

3. Check Configuration

Router # show ip ospf 1

Routing Process "ospf 1" with ID 172.16.0.1
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
10.0.0.0/8 Passive Advertise
Area-filter AREA_0_IN in
Area-filter AREA_0_OUT out
Number of LSA 5. Checksum Sum 0x29450

4. Configuration example

Router (config) # router ospf 1
Log-adjacency-changes
Area 1 filter-list prefix area_timed out
Area 3 filter-list prefix AREA_3_IN in
Network 10.0.0.0 0.20.255.255 area 3
Network 172.16.1.0 0.0.0.255 area 0
Network 192.168.0.0 0.20.255.255 area 1
!
Ip prefix-list area_timed out seq 10 permit 10.25.0.0/8 ge 16
Ip prefix-list area_timed out seq 20 permit 172.20.20.0/24
!
Ip prefix-list AREA_3_IN seq 10 permit 172.31.0.0/16

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