Note!
Routers use OSPF configuration from the lab 6.
There are a few filtering methods:
- Ingress filtering using a ‘distribute-list’.
- Ingress filtering using a ‘distribute-list’ with a ‘route-map’.
- Ingress filtering by changing the Administrative Distance of the prefixes to UNKNOWN (255).
- Type 3 LSA filtering using ‘area area-number range’ command (applied on ABR).
- Type 3 LSA filtering using ‘filter-list’ command.
- LSA Flooding Filtering.
The first three methods (1-3) prevent prefixes from entering the routing table. The LSAs are still going to be present in the LSDB since all routers in OSPF area must be synchronized (the same LSDB). These methods are the intra-area filters.
The last two methods (4-6) are inter-area filters preventing LSAs from entering LSDB.
Topology
Task 1
Check the current routing table on R2. Make sure that it receives both 172.16.104.0/24 and 172.16.144.0/24.
Task 2
Configure R2 so the subnet 172.16.104.0/24 is no longer listed in the routing table. Make sure R2 still has connectivity to 172.16.144.0/24. Use route-map to accomplish this. Do not use an access-list to match the subnet in question.
Task 3
Check the results. R2 should have prefix 172.16.104.0/24 in its LSDB but not in the routing table.
Lab Solution
Task 1
Configure R2 so the subnet 172.16.104.0/24 is no longer listed in the routing table. Make sure R2 still has connectivity to 172.16.144.0/24. Use route-map to accomplish this. Do not use an access-list to match the subnet in question.
R2 Configuration:
match ip address prefix-list R4_L4
!
route-map FILTER_PREFIX permit 20
!
router-id 2.2.2.2
log-adjacency-changes
network 10.1.23.2 0.0.0.0 area 0
network 10.1.124.2 0.0.0.0 area 1
network 172.16.102.2 0.0.0.0 area 0
distribute-list route-map FILTER_PREFIX in
!
Task 3
R2 Check:
