OSPF Virtual Link

1. Introduction to Virtual link

OSPF design requires all areas must be physically connected to backbone area (Area 0) to communicate with each other. In some cases, the backbone area is separated by non-backbone area or a non-backbone area can not directly connect to backbone area - see figure below.

Virtual link is a connection from Area 2 to Area 0 through Area 29. Now, Area 29 is called Transit area. Transit area must have following requirements:

• Have full routing information.
• Not a stub area.

2. How it works

When virtual link configured, all routers know how to reach each other through virtual link. At this time, packets are transmitted through two ends of virtual link are not multicast packets, they are unicast packets on virtual link.
OSPF uses IP protocol 89 to transport, so if there is a firewall between virtual link, firewall should allow OSPF packets.
When routers become adjacent via virtual link, R3 now thinks itself as an ABR because it has a connection with Area 0, R3 then creates a LSA type 3 for its networks and advertises to Area 0 and Area 29. SPF processes calculate their best routes normally, using virtual link as a point-to-point link in Area 0.

3. Configuring OSPF Virtual link

Virtual link will be configured on R2 and R3, Area 29 will be transit area.

OSPF Configuration on R2:

R2#show run | section ospf
router ospf 1
 log-adjacency-changes
 area 29 virtual-link 10.0.2.1 <<
 network 1.1.1.0 0.0.0.255 area 0
 network 2.2.2.0 0.0.0.255 area 29 

R2#show ip ospf virtual-links
Virtual Link OSPF_VL0 to router 10.0.2.1 is up
  Run as demand circuit
  DoNotAge LSA allowed.
  Transit area 29, via interface FastEthernet0/1, Cost of using 10
  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 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

OSPF Configuration on R3:

R3#show run | section ospf
router ospf 1
 log-adjacency-changes
 area 29 virtual-link 2.2.2.1 <<< R2's Router ID
 network 2.2.2.0 0.0.0.255 area 29
 network 10.0.0.0 0.0.0.255 area 2
 network 10.0.1.0 0.0.0.255 area 2
 network 10.0.2.0 0.0.0.255 area 2

You can configure authentication for virtual link either.
Another post about Virtual link ( by VietNamese)