OSPF Inter-Area Filtering using Area Filter-List

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Pic. 1 - Topology Diagram.

Task 1

Remove OSPF filtering applied in the previous lab.

Task2

Ensure that R5 learns all OSPF prefixes except for 172.16.12.0/24. Use area filter-list command to accomplish that.

Questions

Try to answer the following questions:

1. What is the difference in operation between 'area number range', and 'area number filter-list prefix' command?
2. What type of OSPF router do both of these command work on?
3. Do they filter LSAs or just a prefix?

Lab Solution

Solution configuration below: 

https://docs.google.com/file/d/0BwE5C95tpjZOQ1gwU1NVaE5wY28/edit?usp=sharing

Task 1

Remove OSPF filtering applied in the previous lab.

R4 Config:
!

router ospf 1

  no area 0 range 172.16.12.0 255.255.255.0 not-advertise

!

Task2
Ensure that R5 learns all OSPF prefixes except for 172.16.12.0/24. Use area filter-list command to accomplish that.

Pic. 2 - R5 Status Before Filter-List.

 R4 Config:
!
ip prefix-list BLOCK deny 172.16.13.0/24
ip prefix-list BLOCK permit 0.0.0.0/0 le 32
!
router ospf 1
 area 0 filter-list prefix BLOCK out
!

Verification:

Pic. 3 - R5 LSDB and Routing Table.

NOTICE!

LSDB entry will be removed after few seconds (wait at least 30-40 seconds for this to take effect)! 

The conclusion is that both commands ('area range' and 'area filter-list'), prevent ABR from sending LSAs into the area(s) as well. In this instance, R4 has filtered out LSA from the area for which it is not ABR (R4 is not directly connected to area 13).

  

Study Drill

If you have not tried my suggestion in the previous lab, try to use the same prefix (172.16.13.0) to filter it using 'area number range' command. Filtering will not work.

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