OSPF Virtual links

OSPF Virtual links are mainly used to avoid partitionned areas.


r1 and r2 belongs to area 0.
r1 and r3 to area 13, r3's loopback0 belongs to area 13.
r2, r3 and r4 to area 234.


If the link between r1 and r3 goes down, r3's loopback 0 becomes unreachable because area 13 has no connectivity with area 0 to reach other areas.
To avoid this, a virtual-link is established betwen the ABR, r3 and r2. This virtual-link belong to area 0.


Configuration:


r1:
!

interface Loopback0
 ip address 1.1.1.1 255.255.255.0
 ip ospf network point-to-point

!

!
interface Serial1/0
 ip address 12.0.0.1 255.255.255.0

!
interface Serial1/1
 ip address 13.0.0.1 255.255.255.0

!
router ospf 1
 router-id 1.1.1.1
 log-adjacency-changes
 redistribute connected subnets
 network 12.0.0.1 0.0.0.0 area 0
 network 13.0.0.1 0.0.0.0 area 13
!


On r2:

interface Loopback0
 ip address 2.2.2.2 255.255.255.0
 ip ospf network point-to-point
!

interface Serial1/0
 ip address 12.0.0.2 255.255.255.0
!

interface Serial1/1
 ip address 24.0.0.2 255.255.255.0
!

!
router ospf 1
 router-id 2.2.2.2
 log-adjacency-changes
 redistribute connected subnets
 network 12.0.0.2 0.0.0.0 area 0
 network 24.0.0.2 0.0.0.0 area 234
!






On r3:
!

interface Loopback0
 ip address 3.3.3.3 255.255.255.0
 ip ospf network point-to-point


!
interface Serial1/0
 ip address 34.0.0.3 255.255.255.0
! interface Serial1/1
 ip address 13.0.0.3 255.255.255.0!
router ospf 1

 router-id 3.3.3.3
 log-adjacency-changes
 redistribute connected subnets
 network 3.3.3.3 0.0.0.0 area 13
 network 13.0.0.3 0.0.0.0 area 13
 network 34.0.0.3 0.0.0.0 area 234
!




r4:
!
interface Loopback0
 ip address 4.4.4.4
 ip ospf network point-to-point
!
interface Serial1/0
 ip address 24.0.0.4 255.255.255.0
!
interface Serial1/1
 ip address 34.0.0.4 255.255.255.0
!

router ospf 1
 router-id 4.4.4.4
 log-adjacency-changes
 redistribute connected subnets
 network 24.0.0.4 0.0.0.0 area 234
 network 34.0.0.4 0.0.0.0 area 234
!



To create the virtual-link through area 234 (using OSPF router-id), configure the ABR:
On r2:
!
router ospf 1
 area 234 virtual-link 3.3.3.3
!


On r3:
!

router ospf 1

 area 234 virtual-link 2.2.2.2

!

r3#show ip ospf neighbor 

Neighbor ID     Pri   State           Dead Time   Address         Interface

2.2.2.2           0   FULL/  -           -        24.0.0.2        OSPF_VL1

1.1.1.1           0   FULL/  -        00:00:33    13.0.0.1        Serial1/1

4.4.4.4           0   FULL/  -        00:00:38    34.0.0.4        Serial1/0

r3#show ip ospf interface brief 

Interface    PID   Area            IP Address/Mask    Cost  State Nbrs F/C

Vl1          1     0               34.0.0.3/24        128   P2P   1/1

Lo0          1     13              3.3.3.3/24         1     P2P   0/0

Se1/1        1     13              13.0.0.3/24        64    P2P   1/1

Se1/0        1     234             34.0.0.3/24        64    P2P   1/1