What is an OSPF virtual link and when would you use one?

Example

Area 2 attaches only to R3. R3 connects to area 1. R2 is the ABR between area 0 and area 1.

! R2 (router ID 2.2.2.2)
router ospf 1
 area 1 virtual-link 3.3.3.3
!
! R3 (router ID 3.3.3.3)
router ospf 1
 area 1 virtual-link 2.2.2.2

The arguments are the transit area and the router ID of the other end, not an IP address.

R3# show ip ospf virtual-links
Virtual Link OSPF_VL0 to router 2.2.2.2 is up
  Run as demand circuit
  DoNotAge LSA allowed.
  Transit area 1, via interface GigabitEthernet0/0/0
  Topology-MTID    Cost    Disabled     Shutdown      Topology Name
        0           1         no           no            Base
  Transmit Delay is 1 sec, State POINT_TO_POINT,
  Timer intervals configured, Hello 10, Dead 40, Wait 40, Retransmit 5
    Adjacency State FULL (Hello suppressed)

Rules

• The transit area cannot be a stub or NSSA, and cannot be area 0.
• The virtual link is part of area 0, so area 0 authentication applies to it.
• It runs as a demand circuit: hellos are suppressed once Full, and its LSAs do not age.
• Its cost is the intra-area cost through the transit area.

Why it is a stopgap

Traffic between the two areas still crosses the transit area, often on links not sized for it, and troubleshooting gets harder. The real fix is a physical link into area 0 or a renumbered area design.