Editing IS-IS (section)

== IS-IS path selection ==
Depending on the configuration, the router can have either L1, L2 or both L1/L2 databases, against which it runs [[Dijkstra's algorithm|SPF]] algorithm. 

But there can be situations, when IS-IS router has exactly the same prefix in different level databases, or external and internal. In order to choose best path in this situations, there is a very specific order, in which the route goes from the most preferred to the least preferred: 

* L1 intra-area with internal metric,
* L1 external with internal metric,
* L2 intra-area with internal metric,
* L2 external with internal metric,
* Inter-area (from L1 to L2) with internal metric,
* Inter-area external (from L1 to L2) with internal metric,
* Inter-area (from L2 to L1) with internal metric,
* Inter-area external (from L2 to L1) with internal metric,
* L1 external with external metric,
* L2 external with external metric,
* Inter-area external (from L1 to L2) with external metric,
* Inter-area external (from L2 to L1) with external metric.