Technical Tip: OSPF path selection

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Description

This article shows the OSPF path selection preference to select the best route.

Scope

FortiGate.

Solution

In case of multiple routes to the same network with different route types, the FortiGate follows the below order of preference from highest to lowest to select the best route.

1. OSPF Intra-area routes (O).
2. OSPF Inter-area routes (O IA).
3. Type 1 External routes (O E1) and Type 1 NSSA (O N1).
4. Type 2 External routes (O E2) and Type 2 NSSA (O N2).

• When multiple routes with the same route type are present for the network, then the route with the lowest cost is selected as the best route.
• When multiple routes with the same route type and cost are present for the network, then all the routes would be installed in the routing table to perform equal-cost load balancing among the paths.
• With E1/N1 and E1/N2, the lowest cost will be preferred for the same route Type, and if the cost is equal then the NSSA route is preferred if the RFC1583Compatibility is disabled, if RFC1583Compatibility is enabled then all the equal cost routes would be installed in the routing table to perform equal-cost load balancing among the paths.