How To Configure OSPF Virtual Links:

If you studied Cisco’s CCNA you have learned that when you use OSPF all the areas have to be directly connected to the backbone area. Is this really true? Areas have to be connected to the backbone area but if they aren’t we can fix it with a virtual link. Let me show you an example:

 

Look at my picture above. We have three areas and on the left side is area 0. Area 2 is behind area 1. Normally this is not going to work since area 2 has to be directly connected to area 0. We can make this work by using a virtual link. By using a virtual link we can extend area 0 through area 1 so area 2 will be “directly connected” to area 0. Let’s take a look at how a virtual link can solve this problem:

 

This is basically how a virtual link works. It’s like a tunnel through area 1 to reach area 2. This way area 2 will be directly connected. Now let me show you how to configure a virtual link:

 

In the example above area 2 is not directly connected to area 0 so we’ll have to use a virtual link between router Nancy and Donna, here’s how we do it:

Nancy(config)#router ospf 1
Nancy(config-router)#network 1.1.1.0 0.0.0.255 area 0
Nancy(config-router)#network 192.168.12.0 0.0.0.255 area 1 

Donna(config)#router ospf 1
Donna(config-router)#network 192.168.12.0 0.0.0.255 area 1
Donna(config-router)#network 192.168.23.0 0.0.0.255 area 2

Susan(config)#router ospf 1
Susan(config-router)#network 192.168.23.0 0.0.0.255 area 2

I’ll start with a default OSPF configuration.

Nancy(config)#router ospf 1
Nancy(config-router)#area 1 virtual-link 192.168.23.2

Donna(config)#router ospf 1
Donna(config-router)#area 1 virtual-link 1.1.1.1

We configure the virtual-link between ABRs and we use the area virtual-link command. First you need to specify the area 1 where we need the virtual-link which is area 1 in my example. Second step is to configure the OSPF router ID of the other ABR. Keep this in mind…you need to configure the OSPF router ID and NOT the IP address of the ABR. If everything is OK area 2 will have be directly connected to area 0 through our virtual-link.

Nancy# %OSPF-5-ADJCHG  Process 1, Nbr 192.168.23.2 on OSPF_VL0 from LOADING to FULL, Loading Done

Donna# %OSPF-5-ADJCHG: Process 1, Nbr 1.1.1.1 on OSPF_VL0 from LOADING to FULL, Loading Done

You will see the message above that tells us the virtual link is established.

Nancy#show ip ospf virtual-links 
Virtual Link OSPF_VL0 to router 192.168.23.2 is up
  Run as demand circuit
  DoNotAge LSA allowed.
  Transit area 1, via interface FastEthernet0/0, 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 1/2, 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

Donna#show ip ospf virtual-links 
Virtual Link OSPF_VL0 to router 1.1.1.1 is up
  Run as demand circuit
  DoNotAge LSA allowed.
  Transit area 1, via interface FastEthernet0/0, 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 00:00:05
    Adjacency State FULL (Hello suppressed)
    Index 1/3, 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

You can use the show ip ospf virtual-links command to check if your virtual-link is working.

Nancy#show ip ospf database 

            OSPF Router with ID (1.1.1.1) (Process ID 1)

  Router Link States (Area 0)

Link ID         ADV Router      Age         Seq#       Checksum Link count
1.1.1.1         1.1.1.1         189         0x80000004 0x00E333 2
192.168.23.2    192.168.23.2    1     (DNA) 0x80000002 0x009816 1

Donna#show ip ospf database 

            OSPF Router with ID (192.168.23.2) (Process ID 1)

  Router Link States (Area 0)

Link ID         ADV Router      Age         Seq#       Checksum Link count
1.1.1.1         1.1.1.1         1     (DNA) 0x80000004 0x00E333 2
192.168.23.2    192.168.23.2    159         0x80000002 0x009816 1

If you look at the LSDB you will see that the virtual-link shows up as a type 1 router LSA. You can also see DNA which means do not age.

Any other situation where we need a virtual-link? What about a discontinuous backbone area? Let me show you an example:

 

Imagine the router on top was in area 0. Unfortunately this router crashed and the result is that area 0 is now split in two pieces. We call this a Discontiguous area 0. We can use a virtual-link through area 1 to solve this problem. Let me show you how to fix this problem. This is the topoloy that we will use:

 

The topology above has a broken area 0. We’ll configure a virtual link between router Nancy and Susan to fix it.

Nancy(config)#router ospf 1
Nancy(config-router)#network 1.1.1.0 0.0.0.255 area 0
Nancy(config-router)#network 192.168.12.0 0.0.0.255 area 1

Donna(config)#router ospf 1
Donna(config-router)#network 192.168.12.0 0.0.0.255 area 1
Donna(config-router)#network 192.168.23.0 0.0.0.255 area 1

Susan(config)#router ospf 1
Susan(config-router)#network 192.168.23.0 0.0.0.255 area 1
Susan(config-router)#network 3.3.3.0 0.0.0.255 area 0

First I’ll advertise all the networks, nothing special here.

Nancy#show ip ospf | include ID
 Routing Process "ospf 1" with ID 1.1.1.1

Susan#show ip ospf | include ID
 Routing Process "ospf 1" with ID 192.168.23.3

I need to configure a virtual link between router Nancy and Susan and I’ll need to use the router IDs for this.

Nancy(config)#router ospf 1
Nancy(config-router)#area 1 virtual-link 192.168.23.3

Susan(config)#router ospf 1
Susan(config-router)#area 1 virtual-link 1.1.1.1

This is how we do it.

Nancy# %OSPF-5-ADJCHG: Process 1, Nbr 192.168.23.3 on OSPF_VL1 from LOADING to FULL, Loading Done

Susan# %OSPF-5-ADJCHG: Process 1, Nbr 1.1.1.1 on OSPF_VL0 from LOADING to FULL, Loading Done

The configuration is the same as my previous example. Just make sure to configure the area you have to get through and the OSPF router ID of the other ABR.

Nancy#show ip ospf virtual-links 
Virtual Link OSPF_VL1 to router 192.168.23.3 is up
  Run as demand circuit
  DoNotAge LSA allowed.
  Transit area 1, via interface FastEthernet0/0, Cost of using 2
  Transmit Delay is 1 sec, State POINT_TO_POINT,
  Timer intervals configured, Hello 10, Dead 40, Wait 40, Retransmit 5
    Hello due in 00:00:04
    Adjacency State FULL (Hello suppressed)
    Index 1/2, 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

Susan#show ip ospf virtual-links 
Virtual Link OSPF_VL0 to router 1.1.1.1 is up
  Run as demand circuit
  DoNotAge LSA allowed.
  Transit area 1, via interface FastEthernet0/0, Cost of using 2
  Transmit Delay is 1 sec, State POINT_TO_POINT,
  Timer intervals configured, Hello 10, Dead 40, Wait 40, Retransmit 5
    Hello due in 00:00:07
    Adjacency State FULL (Hello suppressed)
    Index 1/2, 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

This is how we can verify if the virtual link is up and running.

Nancy#show ip route  ospf | include 3.3.3.3
     3.0.0.0/32 is subnetted, 1 subnets
O       3.3.3.3 [110/3] via 192.168.12.2, 00:05:13, FastEthernet0/0

Susan#show ip route ospf | include 1.1.1.1
O       1.1.1.1 [110/3] via 192.168.23.2, 00:05:52, FastEthernet0/0

We can also verify that 1.1.1.1 and 3.3.3.3 show up as intra-area prefixes on router Nancy and Susan. This is because area 0 is extended through area 1.