• OSPF filtering allows you to control which inter-area routes are advertised between different OSPF areas.

    Why would you need this?

    In many enterprise networks, security or design policies require that certain networks must not be reachable from specific areas. For example, a subnet used for internal services in Area 1 may not be allowed to appear in Area 0.

    Assume that the network 10.1.12.8/30 must not be learned in Area 0.

    OSPF ABR advertising all Area 1 networks into Area 0 without filtering

    Figure 1 – All routes advertised

    Since inter-area routes are advertised by the Area Border Router (ABR), filtering is performed on the ABR. In this topology, R2 acts as the ABR and therefore controls which Type 3 LSAs are generated and flooded between Area 1 and Area 0.

    The ABR as the Decision Point

    The ABR determines whether a Type 3 LSA is advertised into another area. If the LSA is suppressed, routers in the destination area will never receive it and therefore will not install the corresponding route in their routing tables.

    OSPF ABR filtering specific Area 1 route before advertising into Area 0

    Figure 2 - Route blocked at ABR

    This is the fundamental principle of OSPF inter-area filtering: control the generation and flooding of Type 3 LSAs at the ABR.

    But how does OSPF filtering actually work?

    It may seem simple, you block a route and it disappears.
    To understand what really happens, we need to look at OSPF's internal behavior.

    Answer the question below

    On which router is OSPF inter-area filtering performed?