OSPF LSA Database Filtering - Learn Cisco

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OSPF LSA Database Filtering


Pic. 1 - Topology Diagram.

Task 1
Remove OSPF filtering applied in the previous lab.

Task2
Configure R4 such that it learns prefixes advertised by R5 via OSPF (Lo0: 10.0.5.5/32), but R5 does not learn any OSPF prefixes from R4. OSPF state between R4 and R5 should be 'full'. R5 should use a static default route in order to get connectivity to the rest of OSPF domain. Do not use any filtering mechanism to accomplish that (such as ACL, route-map etc.).

Questions
Try to answer the following questions:
  1. Can you enumerate  OSPF intra-area and inter-area filtering mechanisms?
  2. Apart from reducing the size of the routing table, what other goal can you accomplish with prefix filtering?
  3. What are two ways of using LSA Flood Filtering?
Lab Solution

Solution configuration can be accessed below (if you want to save it, click the link, then go to File-->Download):
https://drive.google.com/file/d/0BwE5C95tpjZOM3RQRVVjakNKUjA/edit?usp=sharing 

Task 1
Remove OSPF filtering applied in the previous lab.


R4 Config:
!
no ip prefix-list BLOCK deny 172.16.13.0/24
no ip prefix-list BLOCK permit 0.0.0.0/0 le 32 
router ospf 1
 no area 0 filter-list prefix BLOCK out
!

Task2
Configure R4 such that it learns prefixes advertised by R5 via OSPF (Lo0: 10.0.5.5/32), but R5 does not learn any OSPF prefixes from R4. OSPF state between R4 and R5 should be 'full'. R5 should use a static default route in order to get connectivity to the rest of OSPF domain. Do not use any filtering mechanism to accomplish that (such as ACL, route-map etc.).

R5 current OSPF configuration:
!
router ospf 1
 log-adjacency-changes
 network 10.0.5.5 0.0.0.0 area 45

 network 172.16.45.0 0.0.0.255 area 45
!

Pic. 2 - R5's OSPF Routing Table Before Filtering.

R4 Config:
!
interface Serial0/1
 ip address 172.16.45.4 255.255.255.0
 ip ospf database-filter all out
!

R5 Config:
!
ip route 0.0.0.0 0.0.0.0 s0/1
!

NOTICE!
Clear IP OSPF process on R5 for this change to take effect!

Pic. 3 - R5 After Applying Solution.

Study Drill

Filtering in OSPF can be applied as INTRA-AREA and INTER-AREA filtering mechanism. 
INTRA-AREA filtering cannot filter LSAs but can filter prefixes sent from LSDB into a routing table. 

INTRA-AREA (LSA1/2) Filtering Methods:

  • distribute-list (with ACL, or route-map referring to ACL or prefix-list).
  • Changing Administrative Distance to 255 (Unknown) on chosen prefixes.
INTER-AREA (LSA3) Filtering Methods (on ABR):
  • area number range pfx not-advertise.
  • area number prefix-list name in|out.
  • ip ospf database-filter all out command (LSA flood filtering)
Notice!
In OSPF point-to-multipoint mode, LSA Flood Filtering can be applied with the following command:

!
router ospf 1
 neighbor address database-filter all out
!

We must not forget that LSA and/or prefix filtering can help us influence the path selection (traffic engineering). 


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