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. |
