• You already know the first job of a virtual link: attaching an area that never touched Area 0. There is a second case, and this one hits the backbone itself. Here, Area 0 is not missing, it is broken in two.

    The Healthy Backbone

    Start from a working design. Area 0 is present on both sides of the topology, connected through R10 at the top:

    Contiguous OSPF Area 0 backbone forming a ring around Area 1 through router R10

    Figure 1 - The backbone is contiguous

    As you can see, the topology forms a ring: R1, R2, R10, R5, and R4 are all connected inside Area 0.

    The backbone is contiguous. Contiguous simply means in one piece: starting from any Area 0 router, you can reach every other Area 0 router using only Area 0 links. As long as this is true, everything works.

    Answer the question below

    A backbone that is in one piece is called _______.

    The Backbone Partition Event

    Now the link between R10 and R5 goes down:

    OSPF backbone link failure between R10 and R5 breaking the Area 0 ring

    Figure 2 - The backbone link fails

    One cable, and Area 0 is cut in two pieces: the backbone is discontiguous.

    Discontiguous means the opposite: Area 0 is no longer in one piece. The left half and the right half are separated by Area 1, and no Area 0 link connects them anymore.

    Discontiguous OSPF backbone with Area 0 split in two pieces separated by Area 1

    Figure 3 - Area 0 is split in two

    Each half still floods its LSAs and converges on its own. But the two halves never exchange their routes again: the routers on the left can no longer reach the networks on the right.

    Why this confuses engineers: Area 1 is still up and forwarding traffic. The physical path exists, but the OSPF path does not because OSPF strictly forbids inter-area routing through a non-backbone area to reach another Area 0 segment.

    Answer the question below

    When Area 0 is split in two pieces, the backbone is called _______.