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Intra-area : Packet reside in the same area  

Inter-area: Packet from different areas 

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OSPF use cost as the metric to choose the shortest path for each destination, this is true but it’s not entirely correct. OSPF will first look at the “type of path” to make a decision and secondly look at the metric. This is the prefered path list that OSPF uses:

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  • Intra-Area (O)

  • Inter-Area (O IA)

  • External Type 1 (E1)

  • NSSA Type 1 (N1)

  • External Type 2 (E2)

  • NSSA Type 2 (N2)

 

After the path selection it will look at the lowest cost path. To give a quick example…when Prefix “X” is learned as an intra-area route (O) and as an inter-area route (O IA) then OSPF will always select the intra-area route, even if the inter-area route has a lower cost.

OSPF prevents inter-area routing loops by implementing a split-horizon mechanism, allowing ABRs to inject into the backbone only Summary-LSAs derived from the intra-area routes, and limiting ABRs’ SPF calculation to consider only Summary-LSAs in the backbone area’s link-state database

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…Routing in the Autonomous System takes place on two levels, depending on whether the source and destination of a packet reside in the same area (intra-area routing is used) or different areas (inter-area routing is used). In intra-area routing, the packet is routed solely on information obtained within the area; no routing information obtained from outside the area can be used.   This protects intra-area routing from the injection of bad routing information.…

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Inter-area routingWhen routing a packet between two non-backbone areas the backbone is used. The path that the packet will travel can be broken up into three contiguous pieces: an intra-area path from the source to an area border router, a backbone path between the source and destination areas, and then another intra-area path to the destination. The algorithm finds the set of such paths that have the smallest cost.…The topology of the backbone dictates the backbone paths used between areas.…

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There are four possible types of paths used to route traffic to the destination, listed here in decreasing order of preference:
intra-area, inter-area, type 1 external or type 2 external.

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