How to configure OSPF Virtual Links

Introduction:

OSPF protocol requires all areas to be directly connected to the backbone area (area 0). However, it is not always possible to have a physical link to a backbone area. In this case, you can use a virtual link to connect to the backbone through a non-backbone area.  The area through which you configure the virtual link, is called a transit area, which must have all the routing information.  

Keep in mind that this is typically a far more complex design consideration when applied to a real-world network. At least one end of a virtual link must be terminated on a backbone (area 0) router.

The below topology has three areas (area 0, 1, 2) and area 0 and 2 are not directly connected to each other. In order to reach each other, virtual link is configured for area 1

We configure the virtual-link between ABRs by referencing the peer router's RID(Router ID), not an IP address. Here, we need to configure the virtual links on R1 and R2 for Area 1 which will help the Area 2 to be directly connected to Area 0.

Configuration:

R1:

set protocols ospf area 0 network '10.118.1.0/24'
set protocols ospf area 1 network '192.168.0.0/30'
set protocols ospf area 1 virtual-link 2.2.2.2
set protocols ospf log-adjacency-changes
set protocols ospf parameters router-id '1.1.1.1'

R2:

set protocols ospf area 1 network '192.168.0.0/30'
set protocols ospf area 1 virtual-link 1.1.1.1
set protocols ospf area 2 network '10.1.3.0/24'
set protocols ospf log-adjacency-changes
set protocols ospf parameters router-id '2.2.2.2'

R3:

set protocols ospf area 2 network '10.1.3.0/24'
set protocols ospf parameters router-id '3.3.3.3'
set protocols ospf log-adjacency-changes

Verification:

R1:

Log: vyos ospfd[777]: AdjChg: Nbr 2.2.2.2(default) on VLINK0: Loading -> Full (LoadingDone)

vyos@R1:~$ show ip ospf neighbor
Neighbor ID     Pri State           Dead Time Address         Interface        L
2.2.2.2           1 Full/DR           30.303s 192.168.0.2     eth0:192.168.0.1 0
2.2.2.2           1 Full/DROther      30.508s 192.168.0.2     VLINK0           0
4.4.4.4           1 Full/DR           30.557s 10.118.1.2      eth1:10.118.1.1  0

vyos@R1:~$ show ip ospf interface
VLINK0 is up
  ifindex 0, MTU 1500 bytes, BW 0 Mbit 
  Internet Address 192.168.0.1/30, Peer 192.168.0.2, Area 0.0.0.0
  MTU mismatch detection: enabled
  Router ID 1.1.1.1, Network Type VIRTUALLINK, Cost: 1
  Transmit Delay is 1 sec, State Point-To-Point, Priority 1
  No backup designated router on this network
  Multicast group memberships: 
  Timer intervals configured, Hello 10s, Dead 40s, Wait 40s, Retransmit 5
    Hello due in 3.994s
  Neighbor Count is 1, Adjacent neighbor count is 1

 vyos@R1:~$ show ip route
C>* 1.1.1.1/32 is directly connected, lo, 09:29:23
O>* 10.1.3.0/24 [110/2] via 192.168.0.2, eth0, weight 1, 09:28:42
  *                     via 192.168.0.2, eth1 inactive, weight 1, 09:28:42
O   10.118.1.0/24 [110/1] is directly connected, eth1, weight 1, 09:29:19
C>* 10.118.1.0/24 is directly connected, eth1, 09:29:23
O   192.168.0.0/30 [110/1] is directly connected, eth0, weight 1, 09:29:19
C>* 192.168.0.0/30 is directly connected, eth0, 09:29:22

 R2:

Log: ospfd[775]: AdjChg: Nbr 1.1.1.1(default) on VLINK0: Loading -> Full (LoadingDone)

vyos@R2:~$ show ip ospf neighbor
Neighbor ID     Pri State           Dead Time Address         Interface        L
3.3.3.3           1 Full/DR           39.287s 10.1.3.2        eth1:10.1.3.1    0
1.1.1.1           1 Full/Backup       33.257s 192.168.0.1     eth0:192.168.0.2 0
1.1.1.1           1 Full/DROther      36.944s 192.168.0.1     VLINK0           0

vyos@R2:~$ show ip ospf interface
VLINK0 is up
  ifindex 0, MTU 1500 bytes, BW 0 Mbit 
  Internet Address 192.168.0.2/30, Peer 192.168.0.1, Area 0.0.0.0
  MTU mismatch detection: enabled
  Router ID 2.2.2.2, Network Type VIRTUALLINK, Cost: 1
  Transmit Delay is 1 sec, State Point-To-Point, Priority 1
  No backup designated router on this network
  Multicast group memberships: 
  Timer intervals configured, Hello 10s, Dead 40s, Wait 40s, Retransmit 5
    Hello due in 6.847s
  Neighbor Count is 1, Adjacent neighbor count is 1

vyos@R2:~$ show ip route
C>* 2.2.2.2/32 is directly connected, lo, 09:32:26
O   10.1.3.0/24 [110/1] is directly connected, eth1, weight 1, 09:32:22
C>* 10.1.3.0/24 is directly connected, eth1, 09:32:26
O>* 10.118.1.0/24 [110/2] via 192.168.0.1, eth0, weight 1, 09:30:12
                          via 192.168.0.1, eth0, weight 1, 09:30:12
O   192.168.0.0/30 [110/1] is directly connected, eth0, weight 1, 09:31:05
C>* 192.168.0.0/30 is directly connected, eth0, 09:32:25

R3:

vyos@R3:~$ show ip ospf neighbor
Neighbor ID     Pri State           Dead Time Address         Interface                        RXmtL RqstL DBsmL
2.2.2.2           1 Full/Backup       34.684s 10.1.3.1        eth1:10.1.3.2                        0     0     0

vyos@R3:~$ show ip route

C>* 3.3.3.3/32 is directly connected, lo, 10:09:44
O   10.1.3.0/24 [110/1] is directly connected, eth1, 10:09:43
C>* 10.1.3.0/24 is directly connected, eth1, 10:09:43
O>* 10.118.1.0/24 [110/3] via 10.1.3.1, eth1, 09:26:47
O>* 192.168.0.0/30 [110/2] via 10.1.3.1, eth1, 09:27:07