OSPF: Path Selection Process

OSPF has rules for selecting paths to subnets that cross areas.  Here are the following rules that have to be followed:

  • OSPF always chooses an intra-area route over an inter-area route for the same subnet regardless of metric
  • ABRs ignore type 3 LSAs learned from nonbackbone area during SPF calculation, which prevents ABrs from choosing a route that goes into a nonbackbone area and then back into the backbone.

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