OSPF Virtual Links

OSPF Virtual Links:
===============
1- When a area don't have connectivity with the backbone area ospf use virtual links to connect this isolated area to backbone.
2- The area through which you configure the virtual link, known as a transit area, in our case we are connecting Area 2 to area 0 via area 1 so for us area 1 is transit area.
3- The transit area cannot be a stub area.

4- Configs shown below to get the virtual links up and working
5- The OSPF packets between the two ends of the virtual link are not multicast packets. They are tunneled packets .
6-Once the routers become adjacent on the virtual link, Router3 considers itself an area border router (ABR), because it now has a link in Area 0. As a result, Router3 creates a summary LSA for 141.1.0.0/16 in Area 0 and in Area 1.


R1:-

router ospf 12
router-id 1.1.1.1
log-adjacency-changes
network 1.1.0.0 0.0.255.255 area 0
network 121.1.1.1 0.0.0.0 area 0

R2:-

router ospf 12
router-id 2.2.2.2
log-adjacency-changes
area 1 virtual-link 3.3.3.3 ---->virtual link to the router(3.3.3.3 is the Router iD)
network 1.1.0.0 0.0.255.255 area 0
network 2.2.2.1 0.0.0.0 area 1


R3:-

router ospf 12
router-id 3.3.3.3
log-adjacency-changes
area 1 virtual-link 2.2.2.2 -----> virtual Link 2.2.2.2 ( is the router id)
network 2.2.2.2 0.0.0.0 area 1
network 3.3.3.1 0.0.0.0 area 2

R4:-

router ospf 12
router-id 4.4.4.4
log-adjacency-changes
network 3.3.3.2 0.0.0.0 area 2
network 141.1.1.1 0.0.0.0 area 2


Few show CLI'S
=============

R2#sh ip ospf neighbor

Neighbor ID Pri State Dead Time Address Interface
3.3.3.3 0 FULL/ - - 2.2.2.2 OSPF_VL2
1.1.1.1 1 FULL/DR 00:00:35 1.1.1.1 GigabitEthernet0/3
3.3.3.3 1 FULL/DR 00:00:36 2.2.2.2 GigabitEthernet0/1
R2#
R2#
R2#sh ip os
R2#sh ip ospf vir
R2#sh ip ospf virtual-links
Virtual Link OSPF_VL2 to router 3.3.3.3 is up
Run as demand circuit
DoNotAge LSA allowed.
Transit area 1, via interface GigabitEthernet0/1, Cost of using 1
Transmit Delay is 1 sec, State POINT_TO_POINT,
Timer intervals configured, Hello 10, Dead 40, Wait 40, Retransmit 5
Hello due in 00:00:06
Adjacency State FULL (Hello suppressed)
Index 2/3, retransmission queue length 0, number of retransmission 0
First 0x0(0)/0x0(0) Next 0x0(0)/0x0(0)
Last retransmission scan length is 0, maximum is 0
Last retransmission scan time is 0 msec, maximum is 0 msec
R2#



R3#sh ip ospf neighbor

Neighbor ID Pri State Dead Time Address Interface
2.2.2.2 0 FULL/ - - 2.2.2.1 OSPF_VL3
2.2.2.2 1 FULL/BDR 00:00:31 2.2.2.1 FastEthernet3/1
4.4.4.4 1 FULL/DR 00:00:38 3.3.3.2 FastEthernet6/0
R3#
R3#
R3#sh ip os
R3#sh ip ospf vir
R3#sh ip ospf virtual-links
Virtual Link OSPF_VL3 to router 2.2.2.2 is up
Run as demand circuit
DoNotAge LSA allowed.
Transit area 1, via interface FastEthernet3/1, Cost of using 1
Transmit Delay is 1 sec, State POINT_TO_POINT,
Timer intervals configured, Hello 10, Dead 40, Wait 40, Retransmit 5
Hello due in 00:00:02
Adjacency State FULL (Hello suppressed)
Index 1/2, retransmission queue length 0, number of retransmission 1
First 0x0(0)/0x0(0) Next 0x0(0)/0x0(0)
Last retransmission scan length is 1, maximum is 1
Last retransmission scan time is 0 msec, maximum is 0 msec
R3#


R1#sh ip route
Codes: C - connected, S - static, R - RIP, M - mobile, B - BGP
D - EIGRP, EX - EIGRP external, O - OSPF, IA - OSPF inter area
N1 - OSPF NSSA external type 1, N2 - OSPF NSSA external type 2
E1 - OSPF external type 1, E2 - OSPF external type 2
i - IS-IS, su - IS-IS summary, L1 - IS-IS level-1, L2 - IS-IS level-2
ia - IS-IS inter area, * - candidate default, U - per-user static route
o - ODR, P - periodic downloaded static route

Gateway of last resort is not set

1.0.0.0/16 is subnetted, 1 subnets
C 1.1.0.0 is directly connected, GigabitEthernet0/3
2.0.0.0/16 is subnetted, 1 subnets
O IA 2.2.0.0 [110/2] via 1.1.1.2, 00:42:04, GigabitEthernet0/3
3.0.0.0/16 is subnetted, 1 subnets
O IA 3.3.0.0 [110/3] via 1.1.1.2, 00:32:50, GigabitEthernet0/3
141.1.0.0/32 is subnetted, 1 subnets
O IA 141.1.1.1 [110/4] via 1.1.1.2, 00:32:50, GigabitEthernet0/3
121.0.0.0/32 is subnetted, 1 subnets
C 121.1.1.1 is directly connected, Loopback12
R1#

above we can see R1 got the routes from R4 via virtual links.

Using a GRE Tunnel Instead of a Virtual Link

You can also build a generic routing encapsulation (GRE) tunnel between Router2 and Router3 and put the tunnel in Area 0. The main differences between a GRE tunnel and a virtual link are described in this table:

GRE Tunnel	Virtual Link
All traffic in the tunnel is encapsulated and decapsulated by the tunnel endpoints.	The routing updates are tunneled, but the data traffic is sent natively.
Tunnel headers in every packet cause overhead.	Data traffic is not subject to any tunnel overhead.
The tunnel can go through a stub area.	The transit area cannot be a stub area, because routers in the stub area do not have routes for external destinations. Because data is sent natively, if a packet destined for an external destination is sent into a stub area which is also a transit area, then the packet is not routed correctly. The routers in the stub area do not have routes for specific external destinations.