OSPF Path Selection Between Areas

The OSPF routing table that exists on a router depends on the following factors:

• The position that the router has in the area and the status of the network

• The type of area that the router is located in

• Whether there are multiple areas in the domain

• Whether there are communications outside the autonomous system

Remember the sequence of events. The router receives LSAs. It builds the topological database. Then it runs the Dijkstra algorithm, from which the shortest path first is chosen and entered into the routing table. The routing table is therefore the conclusion of the decision-making process. It holds information on how that decision was made by including the metric for each link. This enables the network administrator to view the operation of the network.

Different LSAs hold different weighting in the decision-making process. It is preferable to take an internal route (within the area) to a remote network rather than to traverse multiple areas just to arrive at the same place. Not only does multiple-area traveling create unnecessary traffic, but it also can create a loop within the network.

The routing table reflects the network topology information and indicates where the remote network sits in relation to the local router.

The router will process the LSAs in this order:

Step 1 The internal LSA (Type 1 and 2).

Step 2 The LSAs of the AS (Type 3 and 4). If there is a route to the chosen network within the area (Type 1 or 2), this path will be kept.

Step 3 The external LSAs (Type 5).

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