OSPF E1 E2 N1 N2 routes

These route types are all external routes. In other words, routes we redistributed into OSPF. Here is the difference between type 1 and type 2:

Type 1 routes add the cost of each OSPF link to the route.
Type 2 routes only use the redistributed cost and don't add the cost of OSPF links. In other words, the cost remains the same throughout the OSPF domain.

And the difference between "E" and "N" routes:

This means we have:

External Type 1 (E1):
- Routes redistributed into OSPF from another routing protocol or static route.
- The cost of this route is the cost of the redistributed route plus the cost to reach the ASBR (Autonomous System Boundary Router).
NSSA Type 1 (N1):
- Similar to E1 routes but specific to NSSA areas.
- Routes redistributed into OSPF from another routing protocol or static route.
- The cost of this route is the cost of the redistributed route plus the cost to reach the ASBR (Autonomous System Boundary Router).
External Type 2 (E2):
- Routes redistributed into OSPF from another routing protocol (or static route).
- The cost of the route remains the same throughout the OSPF domain.
NSSA Type 2 (N2):
- Similar to E2 routes but specific to NSSA areas.
- Routes redistributed into OSPF from another routing protocol (or static route).
- The cost of the route remains the same throughout the OSPF domain.

Here is an explanation in what order OSPF prefers to use these different routes:

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