Lab 27 - OSPF Path Selection with Transit Capability

Lab 27 - OSPF Path Selection with Transit Capability
Di Posting Oleh : Simple Learning
Kategori : Core Technologies OSPF Routing

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

Pic. 1 - OSPF Topology.

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

!

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

!

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

!

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

!

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

!

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

!

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

!

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.

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