OSPF Inter-Area Filtering Using Area Range

Previous | Intermediate Level | Next

Pic. 1 - Topology Diagram.

Task 1

Remove OSPF filtering applied in the previous lab.

Task 2

Configure OSPF Area 45 between R4 and R5.

Task 3
Ensure that R5 learns all OSPF prefixes except for 172.16.12.0/24. Use 'area range' command to accomplish that.

Questions

Try to answer the following questions:

1. What type of OSPF routers allow LSA filtering?
2. What LSA 1 through 5 (and LSA 7) represent?
3. What types of routers generate which LSAs?

Lab Solution

Solution configuration below: 

https://docs.google.com/file/d/0BwE5C95tpjZOU083MDNwcDg4eFk/edit?usp=sharing

Task 1

Remove OSPF filtering applied in the previous lab.

R4 Config:
!
R4(config)#
R4(config)#no access-list 1
R4(config)#router ospf 1
R4(config-router)#no distance 255 0.0.0.0 255.255.255.255 1
R4(config-router)#

!

Task 2

Configure OSPF Area 45 between R4 and R5.

R4 Config:
!
router ospf 1
 network 172.16.45.0 0.0.0.255 area 45
!

R5 Config:
!
router ospf 1
 network 172.16.45.0 0.0.0.255 area 45
!

Verification:

Pic. 2 - R5 Learns All OSPF Prefixes.

Task 3
Ensure that R5 learns all OSPF prefixes except for 172.16.12.0/24. Use 'area range' command to accomplish that.

Before filtering has been applied:

Pic. 3 - R5 (show ip ospf database summary).

R4 Config:
!
router ospf 1
 area 0 range 172.16.12.0 255.255.255.0 not-advertise
!

Verification:

Pic. 4 - R5's Routing Table.

Pic. 5 - R5's LSDB.

Let's make sure R4 still has 172.16.12.0/24 in its LSDB. Since, the link 172.16.12.0/24 belongs to broadcast network (link state = transit), either R1 (10.0.1.1) or R2 (10.0.2.2) must be the DR for that segment.

Pic. 6 - R4 (ABR) Keeps LSA in its LSDB.

Although it is completely redundant step here (since everything went as planned), let's humor ourselves by checking 172.16.12.2/24 (DR). 

Pic. 7 - R2's Information about 172.16.12.0/24.

Study Drill

Looking at pic. 7, we learn that:

• DR is 10.0.2.2 and its IP address connected to the subnet in question is 172.16.12.2.
• There are only two attached routers on the subnet: 10.0.1.1 and 10.0.2.2.

What do you think would happen if you used the same method of filtering on R4 trying to filter out the subnet 172.16.13.0/24 (LSA from area 13)? Try it out! Why did it work/not work?

Is LSA about 172.16.12.0 still present on R5 (give it a few seconds after you have applied the filter)?

What is the implication of using this method as far as the routing table on R6 regarding 172.16.12.0? Check its routing table before and after you have applied the filtering.

Previous | Intermediate Level | Next