27.1.6 Route selection criteria

The OSPFv3 protocol performs SPF calculation whenever LSAs are updated by LSA generation or learning. In SPF calculation, routes are selected based on the SPF algorithm. When a destination becomes unreachable by the SPF algorithm, the protocol deletes the route to that destination.

An area border router performs route selection separately via the SPF algorithm for each area it serves.

Route selection criteria for OSPFv3 are listed below in order of precedence. You cannot change this priority.

Type of routing information

OSPFv3 internal AS routes (intra-area routes or inter-area routes) take precedence over external AS routes.
Learning source domain

If routes exist in more than one domain, the protocol selects the route with the smallest distance. When the distances are equal, the protocol selects the route with the smallest OSPFv3 domain number.
Route destination type
- Internal AS route
  
  The intra-area route has priority over the inter-area route.
- External AS route
  
  Routes advertised by AS border router in an area take precedence over routes advertised by AS border router in another area.
External AS route type

The external AS route whose metric type is Type1 has precedence over the external AS route of Type2.
Areas traversed by external AS route

For a router located at an area border, when a destination AS boundary router connects to more than one area, the area with the smallest cost to the AS boundary router is selected. When the cost values are equal, the area with the largest area ID is selected.
Cost
- Internal AS route
  
  The route with the smallest cost to the destination takes precedence.
- Type1 external AS route
  
  Selects the route that has the smallest sum of the metric of the external AS route and the cost to AS border router.
- Type2 external AS route
  
  Selects the route with the smallest metric for the external AS route. If the metrics are equal, the route with the smallest cost to the AS boundary router is selected.
Router ID

The protocol selects the route with the smallest router ID of the router used as the next hop.
Interface ID

The protocol selects the interface that learned the smallest interface ID in the hello packet from the router used as the next hop.

<Structure of this section>

(1) Distance value

(1) Distance value

When the Switch learns more than one route to the same destination via a number of protocols, it compares the distance of each route and applies the route with the highest priority.

In OSPFv3, you can set default distances on a per-domain basis. You can assign different distances to external AS routes, intra-area routes, and inter-area routes.