Prerequisites: CCNP level skills.
Note!
OSPF prefers path according to the following rule: prefer intra area to inter area to external routes. The traffic between the areas should take a path through the backbone area (area 0). However, OSPFv2 allows to use non-backbone area as a transit area in case the cost is better since the flag called the 'transit capability' is set to TRUE now.
Topology
OSPF prefers path according to the following rule: prefer intra area to inter area to external routes. The traffic between the areas should take a path through the backbone area (area 0). However, OSPFv2 allows to use non-backbone area as a transit area in case the cost is better since the flag called the 'transit capability' is set to TRUE now.
Topology
Pic. 1 - OSPF Topology.
Lab Solution
Task 1
R2 OSPF Configuration:
Note!
R4 also considers R2 as the gateway through transit area 1 despite of the fact that there is a direct connection with area 0 through the virtual link between R4 and R1. Below I will use just R2 to be the best path which is possible because of the 'transit capability' flag set to true.
R2 Configuration:
R1 OSPF Configuration:
Icons designed by: Andrzej Szoblik - http://www.newo.pl
Task 1
Configure OSPF according to pic 1. Create a virtual link between R1 and R4 allowing area 2 connection to the backbone area.
Task 2
On R4 check the path towards 172.16.103.0/24 and 172.16.133.0/24. Influence the path so that R4 chooses R2 towards these two subnets.
Task 3
Modify the configuration (without changing the cost or bandwidth) so that R4 chooses R1 as the gateway towards 172.16.103.0/24 and 172.16.133.0/24.
Lab Solution
Task 1
Configure OSPF according to pic 1. Create a virtual link between R1 and R4 allowing area 2 connection to the backbone area.
R1 OSPF Configuration:
!
router ospf 1
router-id 1.1.1.1
log-adjacency-changes
area 1 virtual-link 4.4.4.4
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
router-id 1.1.1.1
log-adjacency-changes
area 1 virtual-link 4.4.4.4
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 OSPF Configuration:
!
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
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
!
R3 OSPF Configuration:
!
router ospf 1
router-id 3.3.3.3
log-adjacency-changes
network 10.1.13.3 0.0.0.0 area 0
network 10.1.23.3 0.0.0.0 area 0
network 172.16.103.3 0.0.0.0 area 0
network 172.16.133.3 0.0.0.0 area 0
router-id 3.3.3.3
log-adjacency-changes
network 10.1.13.3 0.0.0.0 area 0
network 10.1.23.3 0.0.0.0 area 0
network 172.16.103.3 0.0.0.0 area 0
network 172.16.133.3 0.0.0.0 area 0
!
R4 OSPF Configuration:
!
router ospf 1
router-id 4.4.4.4
log-adjacency-changes
area 1 virtual-link 1.1.1.1
network 10.1.124.4 0.0.0.0 area 1
network 172.16.104.4 0.0.0.0 area 2
network 172.16.144.4 0.0.0.0 area 2
router-id 4.4.4.4
log-adjacency-changes
area 1 virtual-link 1.1.1.1
network 10.1.124.4 0.0.0.0 area 1
network 172.16.104.4 0.0.0.0 area 2
network 172.16.144.4 0.0.0.0 area 2
!
Task 2
On R4 check the path towards 172.16.103.0/24 and 172.16.133.0/24. Influence the path so that R4 chooses R2 towards these two subnets.
Pic. 2 - R4's Routing Table Before Path Modification.
Note!
R4 also considers R2 as the gateway through transit area 1 despite of the fact that there is a direct connection with area 0 through the virtual link between R4 and R1. Below I will use just R2 to be the best path which is possible because of the 'transit capability' flag set to true.
R2 Configuration:
!
interface Serial0/2
ip address 10.1.23.2 255.255.255.0
ip ospf cost 1
ip address 10.1.23.2 255.255.255.0
ip ospf cost 1
!
Pic. 3 - R4's Routing Table After Path Modification.
Notice!
R2 is the best gateway towards the two subnets despite of the fact that there is the virtual link (area 0 connection) between R1 and R4. R4 chooses transit area to backbone area as far as the subnets in question. This is due to the Transit Capability flag turned on by default. Instead of using the virtual link (area 0), R4 picks the path through the transit area since the cost is better.
Task 3
Modify the configuration (without changing the cost or bandwidth) so that R4 chooses R1 as the gateway towards 172.16.103.0/24 and 172.16.133.0/24.
R1 OSPF Configuration:
!
router ospf 1
router-id 1.1.1.1
log-adjacency-changes
no capability transit
area 1 virtual-link 4.4.4.4
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
router-id 1.1.1.1
log-adjacency-changes
no capability transit
area 1 virtual-link 4.4.4.4
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 OSPF Configuration:
!
router ospf 1
router-id 4.4.4.4
log-adjacency-changes
no capability transit
area 1 virtual-link 1.1.1.1
network 10.1.124.4 0.0.0.0 area 1
network 172.16.104.4 0.0.0.0 area 2
network 172.16.144.4 0.0.0.0 area 2
router-id 4.4.4.4
log-adjacency-changes
no capability transit
area 1 virtual-link 1.1.1.1
network 10.1.124.4 0.0.0.0 area 1
network 172.16.104.4 0.0.0.0 area 2
network 172.16.144.4 0.0.0.0 area 2
!
Pic. 4 - R4's Routing Table.
Note!
Even though R2 has a lower cost towards the subnets, R1 is chosen as the gateway this time. With another IOS version I had to disable the transit capability on R4 as well.