Lab 9 - OSPF Filtering with Route-Map

Prerequisites: CCNP level skills.

Note!
Routers use OSPF configuration from the lab 6.

One thing to remember is that all routers within the same OSPF area share the EXACT same LSA database! This will affect how we can filter OSPF updates.

There are a few filtering methods:

1. Ingress filtering using a 'distribute-list'.
2. Ingress filtering using a 'distribute-list' with a 'route-map'.
3. Ingress filtering by changing the Administrative Distance of the prefixes to UNKNOWN (255).
4. Type 3 LSA filtering using 'area area-number range' command (applied on ABR).
5. Type 3 LSA filtering using 'filter-list' command.
6. 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

Pic. 1 - OSPF Multi-Area Topology.

Icons designed by: Andrzej Szoblik - http://www.newo.pl

Task List

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

Check the current routing table on R2. Make sure that it receives both 172.15.104.0/24 and 172.16.144.0/24.

Pic. 2 - R2's Routing Table Before Applying Route-Map.

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.


R2 Configuration:

!

ip prefix-list R4_L4 seq 5 permit 172.16.104.0/24

!

route-map FILTER_PREFIX deny 10
 match ip address prefix-list R4_L4
!
route-map FILTER_PREFIX permit 20
!

router ospf 1
 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

Check the results. R2 should have prefix 172.16.104.0/24 in its LSDB but not in the routing table.

R2 Check:

R2#show ip ospf database router adv-router 4.4.4.4

Pic. 3 - LSDB after Applying the Route-Map.

Pic. 4 - R2's Routing Table.