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
- Can you enumerate OSPF intra-area and inter-area filtering mechanisms?
- Apart from reducing the size of the routing table, what other goal can you accomplish with prefix filtering?
- 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
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).
