The router level capability can be configured globally and on a per-interface basis. The interface-level parameters specify the interface routing level. The neighbor capability and parameters define the adjacencies that are established.
IS-IS is not enabled by default. When IS-IS is enabled, the global default level capability is Level 1/2, which enables the router to operate as either a Level 1 and/or a Level 2 router with the associated databases. The router runs separate shortest path first (SPF) calculations for the Level 1 area routing and for the Level 2 multi-area routing to create the IS-IS routing table.
The level value can be modified on both or either of the global and interface levels to be only Level 1-capable, only Level 2-capable or Level 1 and Level 2-capable.
If the default value is not modified on any routers in the area, then the routers try to form both Level 1 and Level 2 adjacencies on all IS-IS interfaces. If the default values are modified to Level 1 or Level 2, then the number of adjacencies formed are limited to that level only.