• Wait.

    In the previous lesson, you configured a multi-area OSPF network without even knowing what an area is.
    Let's fix that.

    OSPF works very well in a single-area design.

    But imagine this:
    What happens if you place many routers inside Area 0 (the backbone)?

    OSPF Area 0 backbone topology showing centralized LSDB database

    Figure 1 - Area 0 LSDB overview

    Scalability issues start to appear.
    Let's understand why.

    In the example above, you can see six routers inside Area 0.

    Shared LSDB in Area 0

    All of them are backbone routers.
    And they all share the same Link-State Database (LSDB).

    This means every router stores the same topology information.

    OSPF backbone router concept illustrating central Area 0 connectivity

    Figure 2 - Backbone router role

    Now think about it:
    The more routers you add, the bigger the LSDB becomes.

    Answer the question below

    If you add many routers inside Area 0, what becomes larger?

    What Happens During a Topology Change?

    If we follow the OSPF concept strictly, you already know that when a topology change occurs,
    LSAs are distributed inside Area 0 so that all routers are informed about the change.

    That is exactly what is happening below.
    R5 experiences a link failure, and this triggers an OSPF reaction.

    OSPF single area link failure causing LSA flooding across backbone

    Figure 3 - Global LSA flooding

    LSAs are sent across the topology to indicate the change.
    All routers inside Area 0 take this new information into account, and the SPF algorithm is executed on every router.

    The Scalability Problem

    If all routers must execute SPF every time a small change happens in Area 0, CPU usage increases significantly.
    The more routers you have in Area 0, the greater the impact.

    OSPF topology change triggering SPF recalculation on all routers

    Figure 4 - Full SPF recalculation

    The size of the LSDB increases with the number of routers.
    It is clear that single-area OSPF does not scale well when there are many routers.

    This is where OSPF areas come into play.

    Answer the question below

    When a topology change occurs, which algorithm is executed on every router?