OSPF Path selection
RFC 1587 states that OSPF used the following path selection order when choosing a path:
- Intra-Area (O)
- Inter-Area (O IA)
- External Type 1 (E1)
- NSSA Type 1 (N1)
- External Type 2 (E2)
- NSSA Type 2 (N2)
RFC 3101 has since revised this order to:
- Intra-Area (O)
- Inter-Area (O IA)
- External Type 1 (E1) Paths / NSSA Type 1 (N1) Paths
- External Type 2 (E2) Paths / NSSA Type 2 (N2) Paths
Within the same path-type category (like E1/N1 or E2/N2), OSPF will prefer the path with the lower metric. Only when the metric is the same will type E1 be preferred over N1, and type E2 be preferred over N2.
Cisco IOS version 15.1(2)S and later adheres to the RFC3101 order while earlier versions adhere to the RFC1587 order
Links:
https://forum.networklessons.com/t/ospf-path-selection-explained/969/86?u=lagapidis
https://networklessons.com/ospf/ospf-path-selection-explained/