OSPF: Best Path Loop Prevention

OSPF uses link state logic for routes inside its own area, however Type 3 summary LSAs of networks that exist outside the area only define the destination subnet, metric, and the ABR through which the subnet is reachable.  These are all distant vector concepts.

OSPF does not use the traditional distance vectors loop avoidance features for inter-area routes of this nature, outside of split horizon.  OSPF actually uses split horizon for several types of LSAs so that information from an LSA is propagated in the proper order across the network.

To prevent inter-area routes from being propagated back into the backbone from other non areas receiving the type 3 LSA, areas with multiple ABRs will receive the Type 3 LSA from the other area, and generate its own type 3 LSA to advertise into its own area.  The re-propagated LSA advertised to the redundant ABR will drop the advertisement since the LSA did not come from the backbone area, and in turn would not compute that LSA in SPF for best path.

Review of the Rules for type 3 LSAs:

  • When an ABR originates type 3 LSAs on behalf of known routes it translates only intra-area routes from a nonbackbone area into type 3 LSAs and floods them into the backbone… ABRs translate both intra-area and inter-area routes from the backbone area into type 3 LSAs and floods them into nonbackbone areas
  • When an ABR runs an SPF algorithm it ignores all type 3 LSAs received over nonbackbone areas

The first rule makes sure that the only valid way of one area learning about routes in another area is through the backbone, and also ensures that routes learned over the backbone are not fed back into the backbone by other ABRs in the same area.

The second rule makes sure that ABRs do not traverse nonbackboneareas to reach a network that is located in the backbone or in some other nonbackbone area.  Basically ABRs never use nonbackbone areas to reach an inter-area network.

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