Lab 11 - OSPF Filtering with Area Range

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 three methods (4-5) 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
On R4, check the routing table. Make sure that it shows prefixes: 172.16.103.0/24 and 172.16.133.0/24.

Task 2
Configure router(s) so the 172.16.103.0/24 and 172.16.133.0/24 are not advertised to R4. Do not use 'filter-list' command.

Task 3
Check the results. R4's LSDB and the routing table should not contain the two prefixes in question.

Lab Solution

Task 1
On R4, check the routing table. Make sure that it shows prefixes: 172.16.103.0/24 and 172.16.133.0/24.

Pic. 2 -  R4's LSDB before Filter (172.16.103.0/24).

Note!

172.16.133.0/24 is also advertised by the two ABRs (R1 and R2) but not shown here. 

Pic. 3 - R4's Routing Table.

Task 2
Configure router(s) so the 172.16.103.0/24 and 172.16.133.0/24 are not advertised to R4. Do not use 'filter-list' command.

R1 Configuration:

!

router ospf 1
 router-id 1.1.1.1
 log-adjacency-changes
 area 0 range 172.16.103.0 255.255.255.0 not-advertise
 area 0 range 172.16.133.0 255.255.255.0 not-advertise
 network 10.1.13.1 0.0.0.0 area 0
 network 10.1.124.1 0.0.0.0 area 1
 network 172.16.101.1 0.0.0.0 area 0

!

R2 Configuration

!

router ospf 1
 router-id 2.2.2.2
 log-adjacency-changes
 area 0 range 172.16.103.0 255.255.255.0 not-advertise
 area 0 range 172.16.133.0 255.255.255.0 not-advertise
 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

!

Task 3

Check the results. R4's LSDB and the routing table should not contain the two prefixes in question.

Pic. 4 - R4's Routing Table with the Filter.

Here, just a quick check of the routing table (LSDB checked but not shown here). Filtering has worked as expected.